Omics might provide biomarkers of high sensitivity and specificity for radiation research.

Omics can provide a qualitative and quantitative overview of the global perturbations induced by IR in cells and biological fluids.

Proteomics provides a comprehensive analytical platform to study the molecular mechanisms of the biological effects of radiation on normal tissues and tumours.

Proteomic profiling is used to identify biomarkers of radiation exposure, radiation-induced damage, individual sensitivity, and biodosimetry.

Proteome analyses of cancers deliver information on the outcomes of cancer radiotherapy.

Metabolomics is used to detect small molecules (<1000 Da) in biological samples.

To obtain metabolic information, different methodologies can be used as nuclear magnetic resonance (NMR) or mass spectrometry (MS).

Ionizing radiation triggers a complex response influencing molecular and cellular processes that can be characterized using metabolomics.


Omics might provide biomarkers of high sensitivity and specificity for radiation research.


Omics can provide a qualitative and quantitative overview of the global perturbations induced by IR in cells and biological fluids.


Proteomics provides a comprehensive analytical platform to study the molecular mechanisms of the biological effects of radiation on normal tissues and tumours.


Proteomic profiling is used to identify biomarkers of radiation exposure, radiation-induced damage, individual sensitivity, and biodosimetry.


Proteome analyses of cancers deliver information on the outcomes of cancer radiotherapy.


Metabolomics is used to detect small molecules (<1000 Da) in biological samples.


To obtain metabolic information, different methodologies can be used as nuclear magnetic resonance (NMR) or mass spectrometry (MS).


Ionizing radiation triggers a complex response influencing molecular and cellular processes that can be characterized using metabolomics.


Molecular signatures provide a composite inventory of radiation-specific responses as changes in the composition of these molecules and their abundance. Transcript levels of signature genes in various tissues, notably blood, saliva, skin, and tumor samples, can discriminate and, in some instances, quantify irradiated from unexposed samples [239]. Expression of coding genes and noncoding miRNAs, and lncRNAs, has been implicated in radiation responses, in some cases, which can be precisely calibrated to dose. For individual genes, e.g., FDXR, alternative splice isoforms have also been demonstrated to arise in a radiation-specific manner.


Transcriptomic approaches to classify radiation response can be based on the evaluation of many known genes, which minimizes technical bias in the selection of optimal combinations of gene subsets. Detection and quantification of ionizing radiation have been based on changes in the expression of a set of genes, primarily in blood from multiple individuals. Generally, signatures selected genes with largest average differences and combined changes in gene expression levels, to predict ionizing radiation exposure in humans and mice [240]. Genes previously implicated or established from genetic evidence and biochemical pathways that are altered in response to these exposures can be used to predict radiation exposure by supervised machine learning (ML) [241] (Fig. 3.51). Termed biochemically inspired ML, diagnostic gene signatures for radiation and chemotherapy have been proven accurate on clinical samples. However, typical sample sizes of typical datasets have limited the effective ML methods for deriving gene signatures.

A flow diagram depicts the gene in models and not in models that enhance or inhibit D N A damage, apoptosis, actin nucleation, keratinocyte differentiation, regulation of proliferation, survival, and differentiation.

Fig. 3.51
Graphical depiction of major cellular functions containing the most frequently appearing genes of the highest performing human signatures adapted with permission (CCBY) from Zhao et al. [241]. Genes common among these signatures (white lettering) are indicated in pathways which contain products that these genes interact with (black lettering)


A flow diagram depicts the gene in models and not in models that enhance or inhibit D N A damage, apoptosis, actin nucleation, keratinocyte differentiation, regulation of proliferation, survival, and differentiation.


To establish the reproducibility of these signatures, radiation exposures are predicted with data from independently exposed individuals. Consistent performance on independent dataset depends on the composition of the genes, how genes are ranked, how they validate the response, and how they account for the amplitude of the radiation response. Other important variables include how well signatures differentiate irradiated from unirradiated samples, or even different levels of absorbed radiation from each, or different radiation qualities (energy levels and source, particle types). Transcriptomics can integrate different genes/transcripts in the induced and repressed biochemical pathways that constitute these responses. There is an enormous range of accurate gene signatures that can be derived in many independently derived datasets. Interestingly, there are some core sets of genes and pathways that are present from different studies.


Many radiation response genes were frequently selected for multiple blood signatures, which include genes with roles in DNA damage response (CDKN1A, DDB2, GADD45A, LIG1, PCNA), apoptosis (AEN, CCNG1, LY9, PPM1D, TNFRSF10B), metabolism (FDXR), cell proliferation (PTP4A1), and immune system (LY9 and TRIM22). While biochemical pathways comprising the best performing radiation gene signatures are often shared between human and other species, there are some distinctive differences, notably genes associated with immune cell communication (mice) and redox response (humans).


Expression of gene combinations that detect radiation exposure can also exhibit similar expression patterns in infectious diseases and other blood disorders [242]. Underlying pathways activated by radiation effects, for example DNA damage response and apoptosis, appear to be activated in some individuals affected with other conditions The genes involved are commonly present in multiple published radiation gene signatures and assays. For example, a 74-gene radiation signature comprised of 16 genes present in the human signatures was developed as reported in Zhao et al. [241], including CDKN1A, DDB2, and PCNA. Misclassification of radiation exposures in unexposed individuals with other blood disorders might be mitigated by reevaluating false-positive predictions with signatures containing radiation-responsive genes explicitly derived from other unrelated biochemical pathways, including those encoding secreted proteins.


An important factor in the cellular response to radiation is the ability to repair DNA damage. In some individuals, hereditary mutations in genes involved in DNA repair result in a high sensitivity to irradiation [243]. The A-T syndrome is an example of one such mutation and is described in detail below. However, even with intact DNA repair pathways, it turns out that for small doses, cells refrain from the cell cycle arrest that would give time for repair. This results in a much higher cell kill per unit dose for small doses than higher doses, a phenomenon called low-dose hyper-radiosensitivity (HRS). An explanation to the presence of HRS could be that sacrificing a few cells may be advantageous to the risk of misrepair.


Ataxia-telangiectasia mutated (ATM) is a serine/threonine protein kinase with a key role in repairing double-strand DNA breaks (DSBs) and is also involved in the regulation of oxidative stress, metabolic syndrome, and neurodegeneration, among others reported [244].


The human ATM gene is located at 11q22-23, contains 66 exons, covers 160 kb of genomic DNA, and encodes the 370 kDa ATM protein. Germline mutation in ATM gene, either loss or inactivation of both copies, leads to the autosomal recessive ataxia-telangiectasia (A-T) syndrome, a devastating childhood condition characterized by chromosomal instability, neurological degeneration, immune dysfunction, premature aging, and high cancer risk. In fact, ATM has a critical role in the activation of the cell cycle progression and checkpoint activation in response to DSBs, as well as in the repair of these lesions through homologous recombination (HR) and nonhomologous end joining (NHEJ) pathway [245]. Thus, these mechanisms need to be active during DNA replication to maintain genome integrity in cells. When disruptions occur in these mechanisms, the cells are more susceptible to damage induced by the exposure to endogenous or exogenous agents, such as radiation exposure. Considering the well-known fact that exposure to IR activates ATM kinases to mediate the cellular response, individuals with these defective mechanisms, like A-T patients, will be more sensitive to the same IR exposure than non-A-T patients. Several published studies have corroborated these hypotheses, showing that failures in cell cycle checkpoints lead to a failure in the arrest in G1/S, S, or G2/M allowing the cells to escape from the proper DSB repair, among other signaling pathways including apoptosis and chromatin remodeling process [243]. Therefore, these non-repaired or misrepaired DBSs are responsible for chromosomal instability also in daughter cells, increasing the radiation sensitivity in these individuals as well as the carcinogenesis risk. It is important to elucidate that this cellular radiosensitivity is not unique for A-T syndrome being also observed in other individuals carrying mutations in other genes related to DNA damage repair and response pathways, namely in FANC, BRCA 1/2, MRE11, and DNA Lig4 genes, among others (Box 3.39).


Individuals carrying mutations in genes related to DNA damage repair are hypersensitive to radiation.

A-T syndrome is connected to mutations in ATM, which has a critical role in checkpoint activation and DNA damage recognition and repair.

HRS/IRR has been attributed to the early G2 checkpoint, which is only activated at a certain level of phosphorylated ATM.


Individuals carrying mutations in genes related to DNA damage repair are hypersensitive to radiation.


A-T syndrome is connected to mutations in ATM, which has a critical role in checkpoint activation and DNA damage recognition and repair.


HRS/IRR has been attributed to the early G2 checkpoint, which is only activated at a certain level of phosphorylated ATM.


As described in Sect. 3.19.1, cells with defects in DNA DSB repair pathways are very sensitive to radiation. However, even repair-competent cell lines can be hypersensitive to radiation within a certain low-dose range. This is called low-dose hyper-radiosensitivity (HRS) and is characterized by a high sensitivity to radiation doses below about 0.5 Gy (depending on the cell line and radiation quality) [246], which is followed by a more radioresistant response per unit dose in the dose range of ~0.5–1 Gy. This transition towards radioresistance is described by the term induced radioresistance (IRR) (Fig. 3.52) and occurs at doses corresponding to about ten double-strand breaks.

Two line graphs of surviving fraction versus dose in gray units plot the falling curves for I R R and H R S. The H R S indicates about 10 D S B per cell on the right graph.

Fig. 3.52
Low-dose hyper-radiosensitivity (HRS) and increased radioresistance (IRR) in T-47D breast cancer cells. The left panel shows a full dose-response curve. The right panel shows the low-dose region. Below about 0.3 Gy, the cells appear to proceed from G2 to mitosis without repair of DNA damage, leading to a steep decrease in survival with dose. For doses above a threshold around 0.3 Gy, the damage is repaired increasingly with dose until the surviving fraction follows the linear-quadratic response curve. The transition dose corresponds to approximately 8–10 double-strand breaks. The dashed line shows a curve fit by the linear-quadratic model, and the solid line by the induced repair model (see Chap. 4)


Two line graphs of surviving fraction versus dose in gray units plot the falling curves for I R R and H R S. The H R S indicates about 10 D S B per cell on the right graph.


HRS [247] was first identified in vitro in 1993 after having been observed in mouse skin in 1986 and in mouse kidney in 1988. HRS has been observed in cells given acute proton and pi-meson irradiation as well as in cells given high-LET neutrons at a low dose rate and appears to be the default response for all radiation qualities in both tumor and normal cell lines. IRR, on the other hand, is only observed after low-LET irradiation and only in repair-competent cell lines [247].


In 2003, a mechanism explaining the basis for the HRS effect was proposed [248]. A second radiation-induced G2 checkpoint was discovered to be associated with HRS/IRR [249]. The radiation-induced G2 checkpoint that was known earlier, often denoted the Sinclair checkpoint, does not arrest cells irradiated while in G2, because it takes some time for the checkpoint to be activated. Only cells irradiated while in G1 or S phase are accumulated in G2 by the Sinclair mechanism. The newly discovered G2 checkpoint, also called the early G2 checkpoint, is induced immediately after irradiation and therefore arrests cells that were irradiated while in G2.


Contrary to the dose-dependent mechanisms by which cells irradiated in G1 or S phase accumulate in the Sinclair checkpoint, the “early” G2 checkpoint is ATM dependent [249] and independent of dose over the range of 1–10 Gy. Activation of the critical damage sensor molecule ATM by an autophosphorylation event at serine 1981 is detectable at doses of 0.1 Gy with a gradual increase until phosphorylation of more than 50% of the ATM molecules in the cell after a dose of about 0.5 Gy and saturated expression at larger doses. Thus, the HRS/IRR transition appears to be coincident with both the induction of the early G2-phase checkpoint and activation of ATM.


The hyper-radiosensitivity is thus a result of the progress into mitosis with repairable but unrepaired DNA damages for cells that were irradiated while in G2 but received doses below the threshold for checkpoint activation (see Fig. 3.53).

A flowchart starts with 0 to 1 gray that leads to dose-dependent activation of A T M and is divided into 2 paths. Path 1 includes 0 to 0.3 gray, no arrest, mitosis with unrepaired damage, cell death, H R S. Path 2 includes 0.3 to 1 gray, early G 2 arrest, repair before mitosis, cell survival, I R R.

Fig. 3.53
Path to “increased radioresistance” or “hyper-radiosensitivity.” Cells irradiated with doses below about 0.3 Gy while in G2 will not have enough ATM activated by serine 1981-phosphorylation to reach the threshold level for activation of the early G2 checkpoint. They therefore follow the alternative in the left column, which does not give extra time for repair before mitosis resulting in “hyper-radiosensitivity” (HRS). Cells irradiated with doses above 0.3 Gy while in G2 follow the alternative in the right column and thereby are given more time for repair before mitosis resulting in “increased radioresistance” (IRR)


A flowchart starts with 0 to 1 gray that leads to dose-dependent activation of A T M and is divided into 2 paths. Path 1 includes 0 to 0.3 gray, no arrest, mitosis with unrepaired damage, cell death, H R S. Path 2 includes 0.3 to 1 gray, early G 2 arrest, repair before mitosis, cell survival, I R R.


It has been demonstrated that HRS-negative cell lines have the same ATM activation pattern as cells with HRS, whereas they show an early G2 arrest even after low radiation doses that produce insufficient damage to induce full ATM activation. It has therefore been suggested that the dose-dependent ATM regulatory control is evaded by aberrant early G2-checkpoint response in HRS-negative cell lines caused by dissociation between ATM activity and early G2-checkpoint function [250].


The existence of HRS appears in some cell lines to be associated with an elevated level of caspase-3-mediated apoptosis after low-dose exposures [250], suggesting that the radiation-damaged G2-phase cells that evade the early G2 checkpoint are disposed of by this mechanism when entering mitosis. However, also cell lines deficient in TP53 induction after irradiation or with mutated TP53 have been shown to display HRS/IRR, and no increase in apoptosis in response to HRS-inducing doses was observed in BMG-1 cells with wild-type TP53. In addition, no connection between HRS and apoptosis in the six HRS-competent cell lines investigated measured by DNA-PKcs as early apoptosis marker was found. This corroborates the hypothesis that the transition from HRS to IRR primarily is related to induction of the “early” G2 checkpoint and that the death process of the cells entering mitosis with damages is cell line dependent.


Early G2-checkpoint activation as the underlying mechanism for HRS/IRR is supported by priming experiments where a dose of 0.2–0.5 Gy removes the HRS response to subsequent irradiation given within 6–8 h after the priming. This timing corresponds to the duration of the early G2-checkpoint activation. Surprisingly, the HRS/IRR response can be permanently eliminated by priming irradiation if it is given with a low dose rate of 0.1–0.3 Gy/h for 1 h (shown in T98G glioblastoma and T-47D breast cancer cells). The low dose rate primed cells activated the early G2 checkpoint for all doses. The response was shown to involve transforming growth factor beta 3 (TGF-β3) and inducible nitric oxide synthase (iNOS) activation. The HRS phenotype could be reinstated by inhibition of iNOS [251].


It has been suggested that HRS is a protective mechanism and that it is advantageous to sacrifice a small fraction of cells rather than risking the development of genomic instability and mutations. While cells without the IRR are deficient in DNA repair or checkpoint activation, cells without HRS may have been “turned off” by mechanisms similar to the ones induced by low dose rate priming (Box 3.40).


Cells with HRS are very sensitive to doses below about 0.5 Gy.

Above 0.5 Gy, the survival per dose increases until it reaches the LQ curve.

HRS/IRR has been attributed to the early G2 checkpoint, which is only activated at a certain level of phosphorylated ATM.


Cells with HRS are very sensitive to doses below about 0.5 Gy.


Above 0.5 Gy, the survival per dose increases until it reaches the LQ curve.


As described in Chap. 2, irradiated (donor) cells can secrete signals which, when transferred to unirradiated (reporter) cells, make these respond as if they had been irradiated themselves. This is called the bystander effect. For a while, it was believed that HRS and cell kill by the bystander effect were mutually exclusive, but when doses in the HRS dose range were tested on HRS-proficient cells, these were able to induce strong bystander signals. However, when doses reached the level where IRR was dominating, there was no bystander signaling.


Bystander signals may also be protective. Transfer of medium from HRS-deficient HaCaT normal human epithelial cells to HRS-proficient T98G glioblastoma cells increased the survival above the normal plating efficiency for these cells [252]. In addition to the effect on cell survival, bystander signals can also moderate the HRS/IRR response to subsequent irradiation. Medium transferred from cells, in which the HRS/IRR response was permanently eliminated by priming with 0.1–0.3 Gy for 1 h, has been seen to remove the HRS response in recipient cells for 8–12 h [251] (Box 3.41).


HRS-proficient cells irradiated with doses in the HRS range produce bystander signals that reduce the survival of reporter cells.

HRS-proficient cells, in which the HRS response has been removed by low dose rate priming, produce bystander signals that remove the HRS response to subsequent irradiation in recipient cells.


HRS-proficient cells irradiated with doses in the HRS range produce bystander signals that reduce the survival of reporter cells.


HRS-proficient cells, in which the HRS response has been removed by low dose rate priming, produce bystander signals that remove the HRS response to subsequent irradiation in recipient cells.


The presence of HRS may have implications for cancer radiotherapy in which the aim is to control the eradication of tumor tissue while minimizing the damage to normal tissue. The introduction of intensity-modulated radiation therapy (IMRT) in cancer treatment results in irradiation of a larger proportion of normal tissue but at lower doses when compared to conventional treatment. In some situations, one could fear that HRS will tend to increase the effect of low doses in normal tissue and thus negate the benefits of using IMRT, in particular in tissues with a pronounced volume effect [253].


Since HRS is related to the fraction of cells in G2 phase, it may be of more consequence for early-responding proliferating tissues, such as skin, than for slowly proliferating normal tissues with a small fraction of cells in G2. In support, evidence of HRS has been demonstrated in studies with human skin using basal cell density or skin erythema as endpoint. On the other hand, with the HRS effect being more pronounced in fast-dividing tumor cells than slowly or nondividing normal tissues, it may be possible to exploit HRS clinically using dose fractions within the HRS dose range. However, to obtain the same cell kill as with 2 Gy fractions, more fractions are needed. Increasing treatment time would give the tumor more time to grow; therefore, the time between fractions has to be decreased, but that could be a problem: Experiments with ultrafractionation of 0.4 Gy per fraction, three fractions per day in murine DDL1 lymphoma or in human A7 glioblastoma xenografts, did not show evidence of HRS. This could be because with three fractions per day, the timing between fractions would have been too short for the cell to be released from the early G2-checkpoint arrest induced by the previous dose. Since HRS affects cells in G2, another approach is to synchronize the cells to improve the therapeutic potential of ultrafractionation. A protocol using a taxane (paclitaxel), which synchronizes cells in G2 phase, in combination with carboplatin and low-dose fractionated radiation, was extremely well tolerated by the patients and showed a synergistic effect in patients with squamous cell cancer of the head and neck [254] (Box 3.42).


Attempts to exploit HRS in the clinic using hyperfractionation have not been successful.

Combination of low radiation doses with chemotherapeutics synchronizing cells in G2 phase has shown promise.


Attempts to exploit HRS in the clinic using hyperfractionation have not been successful.


Combination of low radiation doses with chemotherapeutics synchronizing cells in G2 phase has shown promise.


By the term radiation resistance, we refer to the inherent ability of specific types of cells and tissues (usually of malignant origin) to show a differential response to ionizing radiation overcoming its damaging effects like cell killing or inactivation. The amount of energy (for example level of dose in Gy) and consecutive damage that each organism can withstand is a characteristic of the organism’s ability to respond to radiation by a variety of mechanisms often called as DNA damage response (DDR) mechanisms. At the organism level, usually humans are more sensitive compared to other primates or mammals. In nature, there is a great variety of resistance to radiation with the extreme case of certain extremophiles, such as the bacteria Deinococcus radiodurans and the tardigrades, to be able to withstand large doses of ionizing radiation on the order of 5000 Gy. Although none of the strategies discussed in various studies on extreme radioresistance appear to be universal against ionizing radiation, a general trend was found. There are two cellular mechanisms by which radioresistance is accomplished: (a) protection of the proteome and DNA from damage by scavenging and regeneration strategies and (b) recruitment of advanced and highly sophisticated DNA repair mechanisms, in order to reconstruct a fully functional genome [255].


While normal (nonmalignant) mammalian cells compared to tumor ones are usually less radioresistant, one cannot exclude the opposite possibility. Elucidation of the molecular mechanisms and pathways related to radioresistance of tumor cells is of major importance in order to develop strategies maximizing tumor control during chemo- or radiation therapies. Many studies using a wide range of in vitro, ex vivo, and in vivo models as well as bioinformatics have fingerprinted the main pathways leading to cellular radioresistance, and these are primarily implicated in DNA damage repair, oxidative stress, cell pro-survival, hypoxia, cell cycle control, and apoptotic pathways [231].


Another important factor contributing to resistance of tumors is the existence of cancer stem cells (CSCs) as a distinct subpopulation within a tumor. CSCs are able to self-renew and differentiate while showing a high proficiency to repair DNA damage, reveal low levels of reactive oxygen species (ROS), and proliferate at a slower rate compared to other tumor cell populations. These features render CSCs resistant to various therapies, including radiation therapy (RT) [256]. The results of such studies can serve as potential diagnostic/prognostic markers of cancer cell resistance to radiation treatment, as well as for therapy outcome and increase of cancer patient survival.


The radiation-induced adaptive response was first described by Olivieri et al. in 1984 [257] as the reduced sensitivity to a challenge irradiation induced by a previous small priming dose. Radio-adaptive responses have been observed in vitro and in vivo using various endpoints, such as cell lethality, chromosomal aberrations, mutation induction, radiosensitivity, and DNA repair [258]. Adaptation is most efficiently induced by doses of 0.01–0.5 Gy at dose rates from 0.01 to 1.0 Gy/min (Tapio und Jacob 2007) with challenge doses in the range of 0.5–2 Gy. The protective effect has been reported to last for about three generations following the priming irradiation. The molecular mechanisms underlying the adaptive response are not well understood, but data indicate involvement of DNA damage repair, antioxidant production (NRF2 pathway), NF-κB inflammatory pathway, MAPK pathway, autophagy, cell cycle regulation, apoptosis, and bystander signaling [258] (Box 3.43).


Cellular radioresistance can be modulated either through protection against DNA damage or through DNA repair.

Pre-exposure to a low dose can induce protection against a subsequent high dose.


Cellular radioresistance can be modulated either through protection against DNA damage or through DNA repair.


Pre-exposure to a low dose can induce protection against a subsequent high dose.


The continuous advances and improvements in anticancer therapies using IR has significantly increased the treatment efficacy and quality. However, radioresistance is still one of the major problems of radiation oncology, since it leads to tumor locoregional recurrence and disease progression. One plausible cause of tumor radioresistance is the failure of the current treatments in eradicating a subpopulation of cells intrinsically more resistant to multiples therapies, the cancer stem cells (CSCs). The biological characteristics and radioresistance mechanisms of CSCs are shown in Table 3.16. Targeting CSCs and controlling their behavior is an approach to overcome radioresistance and to improve on the efficacy of cancer treatments (Box 3.44).Table 3.16
Biological characteristics and radioresistance mechanisms of cancer stem cells

Biological characteristics

Radioresistance mechanisms

Are long-lived and have tumorigenic abilities

To activate pro-survival pathways

Are able to proliferate, maintain their growth indefinitely

To improve DNA repair ability through the activation of DNA damage checkpoint proteins, such as ATM, Chk1, Chk2, SMC1, and TP53

Differentiate, generating different cell populations inside the tumor

To defend against oxidative stress, since CSCs present lower levels of ROS and overexpress ROS scavengers that protect them from ROS produced in response to radiation

Have long-term repopulation potential

To indefinitely self-renew, through the activation of cell signaling pathways, such as Wnt/β-catenin, notch, TGF-β, and PI3K/AKT/mTOR

Have a flexible phenotype (plasticity), since a conversion of a CSC into a non-CSC phenotype can be reversed, a process highly dependent on the epithelial-mesenchymal transition (EMT)

To overcome the cell cycle control by the abnormal expression of cell cycle-related proteins

Can adapt to the tumor microenvironment

To inhibit cell death pathways after radiation exposure, through the upregulation of anti-apoptotic proteins (like BCL-2 and survivin) and the inhibition of autophagy-related proteins (like Beclin-1 and ATG-5)


Cancer stem cells are a radioresistant tumor subpopulation due to high DNA repair proficiency, low ROS generation and high ROS scavenging, and slow proliferation (giving time for repair).


Hypoxia refers to conditions with low oxygen. Hypoxia induces radiation resistance by preventing the sensitizing effect of the presence of oxygen during or within microseconds of radiation exposure. Oxygen has high affinity for electrons. It may therefore react with radiation-damaged biomolecules as well as with radiation-induced water radicals (see Chap. 1).


Radiation creates radicals either directly in a biomolecule (e.g., DNA) or indirectly through water radicals: MH + radiation → M° + H°, where MH is an intact biomolecule while M° is the radical after loss of one hydrogen atom. Oxygen can sensitize by a direct interference with the primary radiation process. This can take place because deposit of radiation energy not always completes the dissociation. Often, the large biomolecule is just polarized as follows: MH + radiation →+MH−


This process is however reversible. Thus, a spontaneous restitution can take place by the electron falling back to its normal position in the molecule and losing the excitation energy. Due to its great affinity for electrons, oxygen may however “steal” the excited electron before it gives away the excitation energy:


By this process, oxygen creates a biomolecule radical after the following dissociation:


Due to its great affinity for electrons, oxygen will easily react with both radiation-damaged biomolecules and radiation-induced water radicals. When oxygen reacts with the biomolecule radical, it forms a stable bond as follows:


Thereby, oxygen fixates the damage and prevents restitution by hydrogen donors (antioxidants), which is a natural protective means of the cells:


where RSH represents hydrogen donors, of which glutathione is one example.


In cells, the concentration of SH compounds is normally high, and they represent a fundamental protective means against harmful radicals if not outcompeted by oxygen. Hypoxia thus protects against radiation damage through restitution by hydrogen donors because oxygen is not present to outcompete restitution and fixate the damage (Box 3.45).


Hypoxia induces radioresistance by preventing the radiosensitizing effect of oxygen.

Oxygen radiosensitizes by fixating DNA damage and thus preventing restitution by hydrogen donors.

Oxygen can also increase the amount of radicals through the direct effect.


Oxygen radiosensitizes by fixating DNA damage and thus preventing restitution by hydrogen donors.


Oxygen can also increase the amount of radicals through the direct effect.


Q1.
Which of the following effects of IR produce free radicals within the cell, which can damage the cellular macromolecules?(a)
Double ionization
 (b)
Direct action
 (c)
Indirect action
 (d)
Single ionization

 Q2.
What are the most significant differences between the two repair patterns of the base excision repair (BER) repair mechanism?
 Q3.
Which repair pathway provides a “backup” to the replicative proofreading carried out by most (but not all) DNA polymerases during DNA replication?(a)
Base excision repair
 (b)
Nucleotide excision repair
 (c)
Nonhomologous end joining
 (d)
Mismatch repair

 Q4.
Compare the two principal DNA DSB pathways. What is similar and what is different in these?
 Q5.
Complex translocation to some extent depends on the radiation quality. Please indicate when they most often occur. Complex translocation types are characteristic especially for cellular exposure to:(a)
Low-LET radiation
 (b)
High-LET radiation
 (c)
Photonic radiation
 (d)
LET of radiation has no effect on the character of chromosomal translocation

 Q6.
Pick one incorrect statement for completing the sentence “The superoxide anion is …”(a)
Produced by mitochondria
 (b)
A free radical reactive oxygen species
 (c)
Converted to water by superoxide dismutase
 (d)
Able to react with hydrogen peroxide producing hydroxyl radicals
 (e)
Less lipid soluble than hydrogen peroxide

 Q7.
For cell transition in the cell cycle, in which phase do the CDK1/cyclin B complex plays a significant role?(a)
G2 into M
 (b)
G1 into S
 (c)
S into G2
 (d)
G0 into G1
 (e)
M into G1

 Q8.
What will the cycling cells do when They get a “go-ahead” indication at the checkpoint?(a)
Directly progress into the telophase
 (b)
Finish the cell cycle and finally divide
 (c)
Leave the cell cycle and modify to a nondividing state
 (d)
Demonstrate a fall in M phase-promoting factor
 (e)
Finish cytokinesis and generate new cell membranes

 Q9.
Which is the most radiosensitive cell cycle phase, and which is the most resistant one?
 Q10.
Why is immortalization so important for cancer cells?
 Q11.
Cells may execute cell death in different ways in response to IR. Please discuss the factors that may influence the pathway elicited.
 Q12.
What is the main reason for activation of cell death in response to IR in solid tumor cells?(a)
DNA damage-induced apoptosis
 (b)
Initiation of senescence as a result of DNA damage
 (c)
Mitotic catastrophe following improper segregation of genetic material
 (d)
Oxidation-triggered damage to proteins
 (e)
Generation of ceramide at the plasma membrane via sphingomyelinase

 Q13.
Which of the following pathways has been implicated in cellular response to IR:(a)
Autophagy
 (b)
Apoptosis
 (c)
Necrosis
 (d)
Mitotic catastrophe
 (e)
All of a–d

 Q14.
Apoptosis can proceed by two main routes, intrinsic and extrinsic signaling. Describe the initial triggers for these two pathways and how they lead to apoptosis.
 Q15.
Cite the different steps of the autophagy process.
 Q16.
Considering that 800 colonies have grown at 0 Gy for 1200 cells seeded, and that 126 colonies are counted at 2 Gy for 2000 cells seeded, which of the following statements are correct?(a)
The plating efficiency at 0 Gy is 66.6%.
 (b)
The plating efficiency at 2 Gy is 6.4%.
 (c)
The plating efficiency at 2 Gy is 9.5%.
 (d)
The surviving fraction at 2 Gy is 9.5%.
 (e)
The surviving fraction at 1 Gy is 100%.

 Q17.
Which alteration is more likely to lead to the death of an embryo? Alteration of the function of an oncogene or a tumor suppressor gene?
 Q18.
Which cells are mainly involved in inflammation and modulated by low to medium doses of IR?
 Q19.
Are the following statements regarding epigenetic DNA alterations true or false?(a)
5-Methylcytosine is a common DNA modification.
 (b)
DNA methylation is equally common in all four nucleotides.
 (c)
Histone variants are only synthesized during S phase.
 (d)
The amino acid lysine in a histone protein is a target for acetylation.

 Q20.
Is this statement true?
 (a)
One miRNA regulates only one mRNA target.
 Q21.
Is the following statement true or false: ARS involves a total dose of over 0.7 Gy (70 rad) from an external source, administered in a few minutes.
 Q22.
Is the following statement true or false: PTEN is a central positive regulator of the PI3-K/AKT pathway.
 Q23.
Which of the following statements are correct about lncRNAs?(a)
lncRNAs are translated into regulatory proteins.
 (b)
lncRNAs are short RNA transcripts of around 20 nucleotides.
 (c)
lncRNAs can interact with other RNA subtypes to regulate gene expression.

 Q24.
Which of the following statements are correct about extracellular vesicles?(a)
Extracellular vesicles have a size range of 40 nm to several μm.
 (b)
Extracellular vesicles cargo only proteins.
 (c)
Extracellular vesicles can indicate cell death.
 (d)
Extracellular vesicles are only formed by cells and tissue undergoing cell death.
 (e)
Radiation effects on cells and tissue only generate extracellular vesicles to protect against radiation-induced cell death.

 Q25.
In the field of lipidomics or metabolomics, what is the accurate method to achieve the comprehensive metabolite of a sample using LC-MS/MS?(a)
MRM method
 (b)
PCR methods
 (c)
Elisa methos
 (d)
Dicentric assay

 Q26.
What happens to cells irradiated while in G2 with (a) 0.1 Gy and (b) 1Gy?
 Q27.
What is the challenge when exploiting HRS in radiotherapy?
 Q28.
What is the radiation adaptive response?
 Q29.
Please refer two mechanisms responsible for the increased radioresistance of cancer stem cells.
 Q30.
Explain why hypoxic cells are more radioresistant than oxygenated cells.


Which of the following effects of IR produce free radicals within the cell, which can damage the cellular macromolecules?(a)
Double ionization
 (b)
Direct action
 (c)
Indirect action
 (d)
Single ionization


What are the most significant differences between the two repair patterns of the base excision repair (BER) repair mechanism?


Which repair pathway provides a “backup” to the replicative proofreading carried out by most (but not all) DNA polymerases during DNA replication?(a)
Base excision repair
 (b)
Nucleotide excision repair
 (c)
Nonhomologous end joining
 (d)
Mismatch repair


Compare the two principal DNA DSB pathways. What is similar and what is different in these?


Complex translocation to some extent depends on the radiation quality. Please indicate when they most often occur. Complex translocation types are characteristic especially for cellular exposure to:(a)
Low-LET radiation
 (b)
High-LET radiation
 (c)
Photonic radiation
 (d)
LET of radiation has no effect on the character of chromosomal translocation


LET of radiation has no effect on the character of chromosomal translocation


Pick one incorrect statement for completing the sentence “The superoxide anion is …”(a)
Produced by mitochondria
 (b)
A free radical reactive oxygen species
 (c)
Converted to water by superoxide dismutase
 (d)
Able to react with hydrogen peroxide producing hydroxyl radicals
 (e)
Less lipid soluble than hydrogen peroxide


For cell transition in the cell cycle, in which phase do the CDK1/cyclin B complex plays a significant role?(a)
G2 into M
 (b)
G1 into S
 (c)
S into G2
 (d)
G0 into G1
 (e)
M into G1


What will the cycling cells do when They get a “go-ahead” indication at the checkpoint?(a)
Directly progress into the telophase
 (b)
Finish the cell cycle and finally divide
 (c)
Leave the cell cycle and modify to a nondividing state
 (d)
Demonstrate a fall in M phase-promoting factor
 (e)
Finish cytokinesis and generate new cell membranes


Which is the most radiosensitive cell cycle phase, and which is the most resistant one?


Cells may execute cell death in different ways in response to IR. Please discuss the factors that may influence the pathway elicited.


What is the main reason for activation of cell death in response to IR in solid tumor cells?(a)
DNA damage-induced apoptosis
 (b)
Initiation of senescence as a result of DNA damage
 (c)
Mitotic catastrophe following improper segregation of genetic material
 (d)
Oxidation-triggered damage to proteins
 (e)
Generation of ceramide at the plasma membrane via sphingomyelinase


Which of the following pathways has been implicated in cellular response to IR:(a)
Autophagy
 (b)
Apoptosis
 (c)
Necrosis
 (d)
Mitotic catastrophe
 (e)
All of a–d


Apoptosis can proceed by two main routes, intrinsic and extrinsic signaling. Describe the initial triggers for these two pathways and how they lead to apoptosis.


Considering that 800 colonies have grown at 0 Gy for 1200 cells seeded, and that 126 colonies are counted at 2 Gy for 2000 cells seeded, which of the following statements are correct?(a)
The plating efficiency at 0 Gy is 66.6%.
 (b)
The plating efficiency at 2 Gy is 6.4%.
 (c)
The plating efficiency at 2 Gy is 9.5%.
 (d)
The surviving fraction at 2 Gy is 9.5%.
 (e)
The surviving fraction at 1 Gy is 100%.


The plating efficiency at 0 Gy is 66.6%.


The plating efficiency at 2 Gy is 6.4%.


The plating efficiency at 2 Gy is 9.5%.


The surviving fraction at 2 Gy is 9.5%.


Which alteration is more likely to lead to the death of an embryo? Alteration of the function of an oncogene or a tumor suppressor gene?


Which cells are mainly involved in inflammation and modulated by low to medium doses of IR?


Are the following statements regarding epigenetic DNA alterations true or false?(a)
5-Methylcytosine is a common DNA modification.
 (b)
DNA methylation is equally common in all four nucleotides.
 (c)
Histone variants are only synthesized during S phase.
 (d)
The amino acid lysine in a histone protein is a target for acetylation.


The amino acid lysine in a histone protein is a target for acetylation.


Is the following statement true or false: ARS involves a total dose of over 0.7 Gy (70 rad) from an external source, administered in a few minutes.


Is the following statement true or false: PTEN is a central positive regulator of the PI3-K/AKT pathway.


Which of the following statements are correct about lncRNAs?(a)
lncRNAs are translated into regulatory proteins.
 (b)
lncRNAs are short RNA transcripts of around 20 nucleotides.
 (c)
lncRNAs can interact with other RNA subtypes to regulate gene expression.


lncRNAs can interact with other RNA subtypes to regulate gene expression.


Which of the following statements are correct about extracellular vesicles?(a)
Extracellular vesicles have a size range of 40 nm to several μm.
 (b)
Extracellular vesicles cargo only proteins.
 (c)
Extracellular vesicles can indicate cell death.
 (d)
Extracellular vesicles are only formed by cells and tissue undergoing cell death.
 (e)
Radiation effects on cells and tissue only generate extracellular vesicles to protect against radiation-induced cell death.


Extracellular vesicles have a size range of 40 nm to several μm.


Extracellular vesicles are only formed by cells and tissue undergoing cell death.


Radiation effects on cells and tissue only generate extracellular vesicles to protect against radiation-induced cell death.


In the field of lipidomics or metabolomics, what is the accurate method to achieve the comprehensive metabolite of a sample using LC-MS/MS?(a)
MRM method
 (b)
PCR methods
 (c)
Elisa methos
 (d)
Dicentric assay


What happens to cells irradiated while in G2 with (a) 0.1 Gy and (b) 1Gy?


Please refer two mechanisms responsible for the increased radioresistance of cancer stem cells.


SQ1.
Alternative (c). Free radicals are formed after IR by indirect action.
 SQ2.
The two BER mechanisms are SP-BER and LP-BER. SP-BER involves replacing the damaged base only. It requires DNA synthesis to replace the missing bases by DNA polymerase β, and to finalize the process, it uses ligase 3. In LP-BER, up to ten nucleotides are cut out and replaced, and the polymerases used are DNA polymerases δ and ε and ligase 1 to finalize the process.
 SQ3.
Alternative (d). Mismatch repair.
 SQ4.
Common: End termini protection is used to avoid extensive exonuclease activity, but different proteins are important for HR and NHEJ for this purpose. Differences: HR is operative only when there is an undamaged chromosome to work with, i.e., in late S or G2, while NHEJ can operate on DNA DSBs in all cell cycle phases. The fidelity in repair is higher in HR, while NHEJ may cause alterations in DNA sequence as a consequence of the repair which can result in mutations/chromosomal aberrations and which may cause oncogenic transformation of cells.
 SQ5.
Alternative (b). High-LET radiation.
 SQ6.
Alternative (c). Superoxide dismutase converts superoxide anions to hydrogen peroxide.
 SQ7.
Alternative (a). It works in the G2 into M transition.
 SQ8.
Alternative (b). Complete the cycle and divide.
 SQ9.
The mitosis is most sensitive, and early G1 and late S are most resistant.
 SQ10.
Because otherwise they would reach their Hayflick limit and proceed to senescence, and thus they would not be able to divide continuously to form large tumors.
 SQ11.
The type of radiation quality, dose, and dose rate as well as the cellular threshold for DNA damage and repair largely influence the cell death route. The position in the cell cycle when the damage is inflicted as well as functionality of DNA damage sensors, e.g., TP53, influence the decision.
 SQ12.
Alternative (c). Mitotic catastrophe.
 SQ13.
Alternative (e). Cell death after IR can take place via several routes, including mitotic catastrophe, autophagy, apoptosis, and necrosis.
 SQ14.
The triggers and execution of the two pathways, intrinsic and extrinsic, are depicted in Fig. 3.38. The answer can be found in the legend of the figure.
 SQ15.
Initiation and phagophore nucleation-phagophore elongation-cargo sequestration-autophagosome maturation-fusion of the autophagosome with the lysosome.
 SQ16.
Alternatives (a, b, d, and e). The plating efficiency at 0 Gy is 66.6%, the plating efficiency at 2 Gy is 6.4%, the surviving fraction at 2 Gy is 9.5%, and the surviving fraction at 1 Gy is 100%.
 SQ17.
The alteration of an oncogene because it then affects the normal embryonic development and causes embryonic lethality. The alteration of a tumor suppressor gene does not affect the embryogenesis; it increases the probability of cancer during life.
 SQ18.
From the table, it can be seen that the radiosensitivity is correlated to the existence of TNTs and their density and the complexity of networks formed. If all other properties are the same, the hypothesis which can be formulated is the following: The ability of cells to avoid death after irradiation is connected to the ability of the cells to communicate in a direct and fast manner through TNTs. This might be linked to the rescue effect, where less damaged cells are able to send components needed for the damaged cells to survive.
 SQ19.
Macrophages, endothelial cells, lymphocytes, and PMN.
 SQ20.
Answers:(a)
True, 5-mC accounts for about 1% of all bases within DNA.
 (b)
False, guanine is the predominantly modified base.
 (c)
False, histone variants are synthesized throughout the cell cycle.
 (d)
True, lysine and arginine are the most frequently acetylated amino acids.

 SQ21.
No, each miRNA can act on multiple different target genes, and one target gene can be regulated by many different miRNAs.
 SQ22.
True.
 SQ23.
False: PTEN is a central negative regulator of the PI3K/AKT pathway.
 SQ24.
a. Wrong, lncRNAs lack protein-coding sequences and they are not translated.

b. Wrong, lncRNAs are defined as transcripts longer than 200 bp; microRNAs around 20 nucleotides in size.

c. Correct, for example lncRNAs can interact with mRNAs and microRNAs for regulatory purposes.
SQ25.
a. correct. The different sizes of extracellular vesicles are given in Table 3.14.

b. Wrong. Extracellular vesicles cargo in addition to proteins also mRNA/miRNA, long noncoding RNAs, DNA fragments, and lipids.

c.  Correct. In particular apoptotic bodies; see Table 3.14.

d. Wrong. Exosomes are generated by viable cells (Fig. 3.52) albeit they may act as communicators in cell death.

e. Wrong. Extracellular vesicles may via their cargo participate in both cell pro-survival and pro-death signals.
SQ26.
(a) MRM method.
 SQ27.
(a) The low dose will not phosphorylate enough ATM to activate the early G2 checkpoint, and the cells will proceed to mitosis with unrepaired damage and die. (b) The cells will be arrested in G2, and the DNA damage that is repairable will be repaired before the cells enter mitosis.
 SQ28.
The timing between doses may coincide with the duration of early G2 arrest.
 SQ29.
A protection against high radiation doses induced by a low “priming” dose.
 SQ30.
For example, to activate pro-survival pathways and to improve DNA repair ability through the activation of DNA damage checkpoint proteins.
 SQ31.
Oxygen fixates DNA damage and sensitizes the cells to radiation.


Alternative (c). Free radicals are formed after IR by indirect action.


The two BER mechanisms are SP-BER and LP-BER. SP-BER involves replacing the damaged base only. It requires DNA synthesis to replace the missing bases by DNA polymerase β, and to finalize the process, it uses ligase 3. In LP-BER, up to ten nucleotides are cut out and replaced, and the polymerases used are DNA polymerases δ and ε and ligase 1 to finalize the process.


Alternative (d). Mismatch repair.


Common: End termini protection is used to avoid extensive exonuclease activity, but different proteins are important for HR and NHEJ for this purpose. Differences: HR is operative only when there is an undamaged chromosome to work with, i.e., in late S or G2, while NHEJ can operate on DNA DSBs in all cell cycle phases. The fidelity in repair is higher in HR, while NHEJ may cause alterations in DNA sequence as a consequence of the repair which can result in mutations/chromosomal aberrations and which may cause oncogenic transformation of cells.


Alternative (b). High-LET radiation.


Alternative (c). Superoxide dismutase converts superoxide anions to hydrogen peroxide.


Alternative (a). It works in the G2 into M transition.


Alternative (b). Complete the cycle and divide.


The mitosis is most sensitive, and early G1 and late S are most resistant.


Because otherwise they would reach their Hayflick limit and proceed to senescence, and thus they would not be able to divide continuously to form large tumors.


The type of radiation quality, dose, and dose rate as well as the cellular threshold for DNA damage and repair largely influence the cell death route. The position in the cell cycle when the damage is inflicted as well as functionality of DNA damage sensors, e.g., TP53, influence the decision.


Alternative (c). Mitotic catastrophe.


Alternative (e). Cell death after IR can take place via several routes, including mitotic catastrophe, autophagy, apoptosis, and necrosis.


The triggers and execution of the two pathways, intrinsic and extrinsic, are depicted in Fig. 3.38. The answer can be found in the legend of the figure.


Initiation and phagophore nucleation-phagophore elongation-cargo sequestration-autophagosome maturation-fusion of the autophagosome with the lysosome.


Alternatives (a, b, d, and e). The plating efficiency at 0 Gy is 66.6%, the plating efficiency at 2 Gy is 6.4%, the surviving fraction at 2 Gy is 9.5%, and the surviving fraction at 1 Gy is 100%.


The alteration of an oncogene because it then affects the normal embryonic development and causes embryonic lethality. The alteration of a tumor suppressor gene does not affect the embryogenesis; it increases the probability of cancer during life.


From the table, it can be seen that the radiosensitivity is correlated to the existence of TNTs and their density and the complexity of networks formed. If all other properties are the same, the hypothesis which can be formulated is the following: The ability of cells to avoid death after irradiation is connected to the ability of the cells to communicate in a direct and fast manner through TNTs. This might be linked to the rescue effect, where less damaged cells are able to send components needed for the damaged cells to survive.


Answers:(a)
True, 5-mC accounts for about 1% of all bases within DNA.
 (b)
False, guanine is the predominantly modified base.
 (c)
False, histone variants are synthesized throughout the cell cycle.
 (d)
True, lysine and arginine are the most frequently acetylated amino acids.


True, 5-mC accounts for about 1% of all bases within DNA.


True, lysine and arginine are the most frequently acetylated amino acids.


No, each miRNA can act on multiple different target genes, and one target gene can be regulated by many different miRNAs.


False: PTEN is a central negative regulator of the PI3K/AKT pathway.


a. Wrong, lncRNAs lack protein-coding sequences and they are not translated.


b. Wrong, lncRNAs are defined as transcripts longer than 200 bp; microRNAs around 20 nucleotides in size.


c. Correct, for example lncRNAs can interact with mRNAs and microRNAs for regulatory purposes.


a. correct. The different sizes of extracellular vesicles are given in Table 3.14.


b. Wrong. Extracellular vesicles cargo in addition to proteins also mRNA/miRNA, long noncoding RNAs, DNA fragments, and lipids.


c.  Correct. In particular apoptotic bodies; see Table 3.14.


d. Wrong. Exosomes are generated by viable cells (Fig. 3.52) albeit they may act as communicators in cell death.


e. Wrong. Extracellular vesicles may via their cargo participate in both cell pro-survival and pro-death signals.


(a) The low dose will not phosphorylate enough ATM to activate the early G2 checkpoint, and the cells will proceed to mitosis with unrepaired damage and die. (b) The cells will be arrested in G2, and the DNA damage that is repairable will be repaired before the cells enter mitosis.


The timing between doses may coincide with the duration of early G2 arrest.


A protection against high radiation doses induced by a low “priming” dose.


For example, to activate pro-survival pathways and to improve DNA repair ability through the activation of DNA damage checkpoint proteins.


Particle fluence, or planar fluence, Φ, is defined as the number of ionizing particles which traverse a finite plane in space some distance from the source. If dN particles are incident on a planar surface of area, dA, then the fluence is Φ = dN/dA [1, 2].


We may also define the energy fluence, Ψ, which is the radiant energy, dR, which crosses a plane of area, dA, as Ψ = dR/dA. The radiant energy, R, of a radiation field is defined as the total energy of the particles that cross the plane, excluding their rest mass energy. The fluence rate may be defined in terms of energy fluence or planar fluence and is simply the rate at which either energy fluence or planar fluence cross unit area. In the context of the amount of radiant energy absorbed in matter, these concepts provide the basis from which all the remaining dosimetric quantities are defined [1, 2].


Exposure, X, is defined as the total charge which is liberated per unit mass in air by ionizing radiation [1, 2]. Its unit is the Roentgen, R, where one Roentgen is 2.58 × 10−4 C kg−1. Exposure is related to energy fluence, Ψ, by the following equation:


where (μen/ρ)air is the mass energy absorption coefficient of air which defines the fraction of the energy of a beam of particles which is absorbed per unit mass of air at a particular beam energy. Wair = 33.97 eV is the energy required to produce an ion pair in air and e is the charge of the electron.


Kerma, K, is defined as the kinetic energy released per unit mass of material by a specific combination of an incident radiation field and an absorbing material. Kerma is related to energy fluence, Ψ, by the following equation:


where μtr/ρ is the mass energy transfer coefficient, which defines the fraction of the incident radiant energy which is released as kinetic energy in charged particles in a given volume of material. More strictly, the Kerma is the amount of energy liberated through ionization in the volume encompassed by a unit mass of an absorbing material [1, 2]. This energy is transferred through ionization of the material at the atomic level and is ultimately manifested in the kinetic energy of ionization electrons in the material. As may be seen from Fig. 4.1, kinetically charged particles or photons, created in collisions between incident ionizing particles and the material, may not deposit their energy in the mass volume. Therefore, Kerma is a measure of the amount of ionizing energy offered for absorption in the material, which in this case is the initial kinetic energy of the primary electron.

A circular diagram indicates interactions between photons, ionization, primary electron, scattered photon, and absorbing volume, mass m.

Fig. 4.1
Kerma in relation to interactions between ionizing photons and matter in a unit mass volume


A circular diagram indicates interactions between photons, ionization, primary electron, scattered photon, and absorbing volume, mass m.


The energy imparted, ϵ, by ionizing radiation to the matter in a volume is given by the following equation [3]:


where the first and second terms in the equation, respectively, describe the sums of all the radiant energies of all ionizing radiations entering and leaving a particular volume. The third term denotes the sum of all the mass energies of all the particles produced during the interactions of the ionizing radiations with the matter to which it is imparting energy. In diagnostic radiology, the photon energy is not sufficient to instigate pair production (production of positrons and electrons in the vicinity of a strongly positive nucleus), and therefore particle production does not occur. Thus, for diagnostic energies, the third term on the right-hand side of Eq. (4.3) is zero [1, 4]. Energy Imparted is quoted in the units of energy, the Joule, J.


A distinction must be made between the term “Energy Imparted” and the term “Imparted Energy.” Energy Imparted is the term for a gross quantity or concept, where the energy is imparted to matter that has a macroscopic size. Imparted Energy is the energy that is imparted in a single interaction between any particle and the matter in a given volume. The Imparted Energy, dϵ, in an interaction is a stochastic quantity, and is difficult to measure, and impossible to infer with any great accuracy [3, 5]. Thus, the Energy Imparted is also a stochastic quantity. However, repeated measurements can establish mean energy imparted,

, which is a non-stochastic quantity (Box 4.1).


Kerma, K, is defined as the kinetic energy released per unit mass of material by a specific radiation field and it is related to energy fluence, ψ

Exposure, X, is defined as the total charge which is liberated per unit mass in air by ionizing radiation. Its unit is the Roentgen, R.


Kerma, K, is defined as the kinetic energy released per unit mass of material by a specific radiation field and it is related to energy fluence, ψ


Exposure, X, is defined as the total charge which is liberated per unit mass in air by ionizing radiation. Its unit is the Roentgen, R.


The absorbed dose (sometimes referred to simply as “dose”) is the radiant ionizing energy absorbed per unit mass of absorbing material. It is therefore defined as:


The quantity ε/m is sometimes referred to as the specific energy. It is stochastic in the same way that the imparted energy in a given interaction is stochastic, but with repeated measurements, and on macroscopic scales involving many single particle interactions, it becomes a measurable quantity ([1], p. 86; [2, 4]). The unit of Absorbed dose is the Gray, Gy, which is equal to J kg−1.


The Kerma in Eq. (4.2) may be split into two parts depending on the ways in which the energy of the photon is lost through interactions with the material [1]. Photons may either release their energy through collision interactions in which excitation and ionization of the stopping material occur or through radiative processes in which their energy is radiated through the release of photons. Thus, the Kerma can be expressed as:


where Kc is the portion transferred through collisions, and Kr is the portion transferred through radiative interactions [1]. Radiative interactions generally occur in situations in which charged particles are incident on a material [1]. In the case of diagnostic radiology, Kerma is released through collision interactions, with Collision Kerma therefore given by:


In diagnostic radiology, a simple relationship between Kerma and Absorbed dose may be derived. When charged particle equilibrium (CPE) exists in a medium, the number of charged particles leaving a unit mass volume is replaced by an equal number entering from other mass volumes. In such a situation, which occurs at the photon energies in diagnostic radiology, all Kerma is absorbed in the unit mass volume. It has been shown by Attix, that for a medium of uniform density and atomic composition, such a situation does indeed exist for a field of X-ray photons and a uniformly irradiated medium [1, 2]. In this case the Absorbed dose, D, and Collision Kerma, Kc, are equal, such that

 (4.7)


The Absorbed Dose is the energy absorbed per unit mass of material. The unit of Absorbed Dose is the Gray, Gy, which is equal to J kg−1


In general, radiation detectors  operate by providing the means to measure the energy deposited over time in the detector absorbing material from exposure to a source of ionizing radiation. This is typically measured as the quantity of charge, Q, over time elicited from an absorbing medium forming the main component of the detecting element. An ideal radiation detector is one that gives spatial resolution, temporal resolution, information regarding the energy of the particle, and information regarding the identity of the radiation. In reality, single detectors of this type are difficult to construct such that practical detectors that are used in the field have a focused range of capabilities which should be taken into consideration when a detector is chosen for a particular application [6, 7].


Ionization chambers are designed to measure the number and/or total energy deposited as a result of the ionizations produced when a charged particle or ionizing photon traverses the detector medium. Therefore, they are not suitable for the detection or analysis of neutral particles. Ionization detectors consist of an isolated detection medium, generally a gas such as air that can be easily ionized (i.e., has a low ionization potential), which is placed between two oppositely charged electrodes (Fig. 4.2). The medium should be chosen such that it does not respond adversely to ionization such that its characteristics will not change with use.

A schematic diagram exhibits the ionization detector of the dashed arrow of the radiation track with both positive and negative ions placed between the rectangular board of high voltage source, bottom connected with the output signal.

Fig. 4.2
Schematic of the basic elements of an ionization detector


A schematic diagram exhibits the ionization detector of the dashed arrow of the radiation track with both positive and negative ions placed between the rectangular board of high voltage source, bottom connected with the output signal.


The charged particle will ionize the detector medium along its path and these ions will then be accelerated towards the detector electrodes. In general, a high electric field is applied between the electrodes to prevent the recombination of ions produced by the traversal of the charged particle. As the charged particle traverses the sensitive region of the detector (i.e., the gas) it produces multiple electron-ion pairs, which begin to drift along the electric field lines and reach the plates of the detector. These ions may produce further ionization of the neutral gas atoms via further collision, ultimately producing a small current that induces a voltage drop across the resistor. These chambers typically generate very low measurement currents per ionizing particle, and therefore require low noise amplifiers to improve their operating performance.


The amplified output signal from the detector may be used to trigger a counting mechanism to measure the number of incident charged particles or ionizing photons (i.e., exposure) or its pulse height may be analyzed to determine the total energy within the beam (i.e., dose). The amount of ionization that is detected is dependent on the nature of the gas used in the detector, the level of the applied electrical field, and the characteristics of the plates used in the detector. How the chambers operate, i.e., as a device for the measurement of absolute energy deposition or number of charged particle incident on the detector, depends on the HV level applied to the detector, as depicted in Fig. 4.3 [7].

An increasing waveform depicts the number of ion pairs versus V in volts at the different regions are the ionization chamber region, region of proportionality, Geiger region, and discharge region, waves are named alpha, and beta.

Fig. 4.3
Signal response to ionization as a function of the applied voltage for heavily ionizing (top curve) and weakly ionizing particles (lower curve). In the Geiger region, the output does neither depend on the voltage nor on the amount of deposited energy or initial ionization. [Adapted from Fig. 4.12 Martin and Shaw (2006). Copyright (2006), Wiley Publishers]


An increasing waveform depicts the number of ion pairs versus V in volts at the different regions are the ionization chamber region, region of proportionality, Geiger region, and discharge region, waves are named alpha, and beta.


When the applied voltage is small, the electrons and ions can recombine soon after they are produced and only a small fraction of the ions reach their respective plates in the detector (Fig. 4.3). As the applied voltage between the plates is increased, a region is reached where the output pulse reflects the amount of ionization seen in the chamber (Ionization region). When the voltage is increased still further, the electrons and positive ions released by the initial ionization can themselves cause further ionizations in the medium and thus amplify the ionization pulse (Proportional region). Increasing the applied field still further (Geiger region) creates an avalanche effect and a highly amplified output signal. Any further increases in the applied voltage lead to a continuous discharge of the detector [6].


It is possible to determine the typical output current that will be generated by an ionization chamber in the presence of a source of known activity. Consider the case of an in-air ionization chamber (where air has an ionization potential of 30 eV) which is exposed to alpha particles (Eα = 5.486 MeV) from a 10 MBq Am-241 source. The total number of ionizations produced by a single Am-241 α-particle will be the ratio of the energy of the alpha particle to the ionization potential of air:


In this case, the total number of ionizations produced will be the product of the activity of the source and the number of ionizations produced by a single alpha particle, or N = 1.829 × 1012 ionizations, which are observed in a single second (as the unit of activity, the Bq is s−1 in SI units). The final step is then to compute the product of the total number of ionizations with the charge on the electron such that the total current observed will be:


Frequently, ionization chambers are open to the air to allow for changes in ambient pressure which could collapse or expand a sealed chamber, damaging the thin chamber walls. As a consequence, chamber outputs must be adjusted for changes in ambient temperature, T, and pressure, P, from those at which the chamber was calibrated, Tn and Pn, respectively. In practice, we can multiply the chamber output by the following correction factor to adjust for ambient conditions (where all temperatures are expressed in Kelvin and pressures in Pascals [6]):


While the ionization chamber provides a device for the measurement of absolute energy deposition, it does not provide information on directionality. Proportional counters are ionization chambers that may be used for both measuring absolute energy deposition (through a measurement of the pulse height) in addition to giving directional information on the path of charged particle (through the output of a given anode wire, each of which is independently amplified).


Multi-wire proportional chambers (MWPCs) such as those shown in Fig. 4.4 are used in high-energy particle physics experiments as a means of tracking the path of charged particles. Anode wires (typically with a ~2 mm separation) are positioned between the cathode plates of the chamber (which have a typical separation of 1 mm) and the construct is sandwiched between thin mylar windows or some other superstructure, with an operating gas infused into the region between the plates. In practice, several individual chambers may then be joined together to provide fine detail on the direction of passage of individual charged particle, where pulses will be produced on the anode electrodes closest to the path of the charged particle through the detector as a result of ionization of the gas in the region closest to each anode. MWPCs can be used to infer further information on the momentum of beams of charged particles via the degree of their deflection in a magnetic field (which is typically how they have been used in collider experiments such as the LHC at CERN [6]).

A schematic diagram exhibits a multi-wire chamber of the anode placed between the cathode with positive and negative ions with H V, R, A, and O by P.

Fig. 4.4
Schematic of a multi-wire proportional chamber [6, 7]


A schematic diagram exhibits a multi-wire chamber of the anode placed between the cathode with positive and negative ions with H V, R, A, and O by P.


Scintillators are materials that react to the passage of a charged particle by the emission of a very small flux of photons of light. Charged particles may excite electrons within atoms of the scintillating material to a higher energy state; these atoms then emit photons as they de-excite to their ground state. Scintillators can be developed from organic (e.g., naphthalene or anthracene) or inorganic (including sodium iodide or cesium iodide) materials and have applications as the first detection element with gamma cameras used in nuclear medicine.


Scintillating materials typically need to be chosen to detect photons of a specific wavelength and may often be doped to achieve specific wavelength sensitivity. They are generally coupled to photomultiplier tubes (PMT) to amplify the intensity of the weak photon signal output from the scintillator, either for photon counting or imaging applications.


In Fig. 4.5, a schematic of a photomultiplier tube is shown. An incoming charged particle or ionizing photon impacts the scintillator, which emits a photon flux towards a photocathode material (constructed typically with a negatively charged plate covered by a photosensitive material such as gallium–arsenide or indium–gallium–arsenide). Here, the photon flux is converted to an electron flux as they enter the inner (evacuated) environment of the PMT tube. These electrons are accelerated towards the first of several dynodes by the high-voltage field between the photocathode and the anode. When each electron collides with a dynode, it causes the emission of several electrons (typically 5–10), which are then accelerated towards subsequent dynode and amplify the electron flux through collision and reemission. At the detector anode, a significant and measurable electric current is then generated as a result of the acquisition of a single photon. Apart from a small degree of signal fluctuation, the current seen at the anode is linearly proportional to the photon  flux seen at the photocathode [6, 7].

A schematic diagram exhibits a photomultiplier tube of incoming radiation, photon, photocathode, dynodes, output pulse, and scintillating material.

Fig. 4.5
Schematic diagram of a photomultiplier tube (PMT) (courtesy of Physics Libretexts, Fig. 31.2.3)


A schematic diagram exhibits a photomultiplier tube of incoming radiation, photon, photocathode, dynodes, output pulse, and scintillating material.


Semiconductor detectors  based upon p–n junction diodes offer a practical and robust option for detector construction, operating in a similar manner to an ionization chamber [6, 7]. Here a p–n junction is constructed through the joining together of a piece of p-type semiconductor (such as silicon or germanium) to a piece of n-type silicon. P-type material is doped with atoms of a material with one vacant outer-electron state, such as boron, B, while N-type material is doped with atoms of a material with an extra “free” electron in its outermost energy level, such as antimony, Sb. At the junction between the two materials a “depletion layer” is formed where electrons from the N-type material migrate to the P-type material to fill vacant energy states or “holes” leaving behind holes in the N-type material surrounding the junction. This creates a region where electrons and holes are depleted around the junction and creates a barrier to conduction. For the purposes of photon or particle detection, the depletion region is the sensitive portion of the electronic detector. When operated in reverse bias (Fig. 4.6a, b), this depletion region is larger and these detectors are typically operated in reverse bias with a voltage of 100 V to increase the depletion layer and therefore the sensitive region of the detector [6, 7].

A. A schematic diagram p n junction diode operated with both positive and negative ions. B. A graph of concave and convex curves of values approximately 30, and 0.6 volts, with an inflection point 0.

Fig. 4.6
(a) Schematic of a p–n junction diode operated in forward and reverse bias. (b) Operating characteristics of the diode in forward and reverse bias


A. A schematic diagram p n junction diode operated with both positive and negative ions. B. A graph of concave and convex curves of values approximately 30, and 0.6 volts, with an inflection point 0.


When the sensitive element of the detector is exposed to a charged particle or ionizing photon, this causes electrons within the depletion layer to be promoted from the valence band to the conduction band (Fig. 4.7), and their conduction through the junction towards the positive terminal of the detector. Here, the valence band is equivalent to the energy level of the outermost electron, while the conduction band is the energy level of the next vacant energy state above the valence band. A current is produced which is proportional to the energy loss by the charged particle or photon within the depletion layer [6, 7]

A. A schematic diagram p-n junction diode with both positive and negative ions. B. A schematic diagram of 4 horizontal bars of C B, and V B of E subscript gap of e V E subscript gap = h c by lambda.

Fig. 4.7
Operation of a semiconductor particle detector (a) where an incident proton causes the promotion of one or more electrons from the valence to the conduction band within the detector (b)


A. A schematic diagram p-n junction diode with both positive and negative ions. B. A schematic diagram of 4 horizontal bars of C B, and V B of E subscript gap of e V E subscript gap = h c by lambda.


The creation of an electron–hole pair in a silicon or germanium semiconductor requires as little as ~3 eV in comparison to the 30 eV required for in-air ionization chambers. Detectors can be constructed and tuned to radiation of a specific wavelength, λ, by altering the energy difference between the valence and conduction bands, or the band gap, Egap, as shown in Fig. 4.7 and Table 4.1.Table 4.1
Typical semiconducting materials used for radiation detection, including bandgaps, wavelength, and electro magnetic (EM) band sensitivity

Material

Egap (eV)

λ (nm)

Band

C (diamond)

5.65

220

UV

GaN

3.45

360

UV

AlGaN

3.45–5.64

360–260

UV

CdZnTe

1.4–2.12

870–580

Visible

Si

1.12

1100

Visible

GaAs

1.42

875

Visible

Ge

0.66

1800

NIR

PtSi

0.41–0.25

3000–5000

IR

HgCdTe

0.41–0.25 or 0.16–0.10

3000–5000 or 8000–12,000

IR

HgCd

0.7–0.1

1700–12,500

NIR-FIR


Semiconducting materials, when incorporated in radiation detectors can therefore produce a large signal in response to irradiation with a small photon flux. The detectors can be constructed very thinly (as little as 200–300 μm) for the detection of ionized particles, or larger for stopping of photons. Their performance is approximately linear if an electric field is applied that prevents the recombination of the electrons and holes formed by the radiation [6, 7].


The phenomenon of Cerenkov radiation was first observed and described by Pavel Cerenkov in 1934 and characterized by Franck and Tamm in 1937. This work resulted in all three being given the Nobel Prize in physics in 1958.


To understand the operation of Cerenkov detectors we must first describe the effect itself. Suppose that we have a charged particle traveling at a relatively low velocity though a static medium. As the particle travels slowly relative to the speed at which the ions/molecules of the material can orient and reorient themselves as it passes, the ions/molecules will orient themselves such that the part of the ion/molecule that is charged opposite to the charge of the ionizing projectile would be in the direction of the particle (Fig. 4.8a). The molecules are displaced in an isotropic conformation relative to the position and direction of movement of the charged particle, and therefore there is no overall change in the energy of the medium locally [6, 7].

A. 2 rectangular boxes full of circular-shaped charged particles. B. An overlapping circle with charged particles, along with angle theta at the bottom. C. A schematic diagram of cone formation of Cerenkov light by neutrino or antineutrino, and radionuclide. D. A cubic shape of negative refractive index at the center.

Fig. 4.8
(a) Passage of a charged particle through a medium of refractive index n at velocities that polarize the medium. (b) The generation of coherent light waves via the Cerenkov effect. (c) The formation of a cone of Cerenkov light along the path of the charged particle through a medium with positive and (d) negative refractive index. [Taken from Shaffer et al., Nature Nanotechnology, 12, 106–117 (2017). Copyright Springer Nature]


A. 2 rectangular boxes full of circular-shaped charged particles. B. An overlapping circle with charged particles, along with angle theta at the bottom. C. A schematic diagram of cone formation of Cerenkov light by neutrino or antineutrino, and radionuclide. D. A cubic shape of negative refractive index at the center.


However, in instances where the velocity of the particle, v ~ c/n or v > c/n, the molecules of the medium that are displaced by the passage of the charged particle are generally anisotropic relative to the position and diNurUhr (Fig. 4.8b). By Huygens principle of wavelets, each of the reoriented molecules of the medium can reradiate the energy delivered to them and do so as point wavelet sources. These sources will be coherent along a direction as shown in Fig. 4.8c. If ϑ is the angle at which the point sources reradiate, then it may be shown that

 (4.9)


where n is the refractive index of the medium and β = v/c, where v is the velocity of the particle and c is the speed of light.


It is therefore possible to discriminate the identity of high energy charged particles purely based on the angle of Cerenkov emission or the threshold value of n at which Cerenkov emission is observed [6, 7]. In particle physics, experiments materials of various refractive indices are typically used to provide several potential Cerenkov thresholds for the detection of a variety of radiation types. The weak Cerenkov photons can be detected using PMTs or electronic photodetectors. Cerenkov photons are also observable as a result of the passage of charged particles through human tissue and Cerenkov imaging has seen a recent application for in vivo dosimetry in radiotherapy [8].


Calorimeters allow the estimation of the total energy of a high-energy charged particle or ionizing photon through absorption of its total energy, via successive ionization of the material in the detector in a process that is termed a particle shower (Fig. 4.9), in a detector that is capable of absorbing all of the particles incident radiation. These devices may be ionization chambers as described earlier or semiconductor detectors, or a combination of the two. Depending on the nature and identity of the incident particle, it can create ionizing photons through bremsstrahlung or can produce further “hard” ionizing particles that may not be stopped easily in detectors with unsuitable absorbing characteristics, and therefore a single detector type will not achieve the experimental objectives (Box 4.3).

A. A schematic diagram of clusters of particles of the calorimeter. B. The branches of the particle shower are present in the crystal calorimeter.

Fig. 4.9
(a) A particle shower within a calorimeter; (b) a particle shower caused by the incidence of a photon on a calorimeter


A. A schematic diagram of clusters of particles of the calorimeter. B. The branches of the particle shower are present in the crystal calorimeter.


Radiation detectors measure the energy deposited over time in the detector-absorbing material

An ideal radiation detector provides spatial resolution, temporal resolution, information regarding the energy of the particle, and information regarding the identity of the radiation. No single detector can offer these simultaneously

Ionization chambers are common dosimeters that measure the ionizations produced when a charged particle or ionizing photon traverses the detector medium (generally a gas, requiring temperature and pressure corrections). An electric field is applied between the electrodes to prevent the recombination of ions produced.

Proportional counters are ionization chambers that also provide directional information on the path of charged particles

Scintillator materials are also used as dosimeters by relating the flux of photons emitted to the energy deposited. They are generally coupled to photomultiplier tubes (PMT) to amplify the intensity of the photon signal

Semiconductor detectors measure the number of charge carriers produced by the radiation in the detector material. Semiconductor materials are used due to the small energy required to produce electron-hole pairs

Cerenkov detectors record light produced by charged particles traveling through materials at a velocity greater than that at which light can travel through the material

Calorimeters quantify the absolute dose absorbed by measuring the increase in temperature produced by radiation


Radiation detectors measure the energy deposited over time in the detector-absorbing material


An ideal radiation detector provides spatial resolution, temporal resolution, information regarding the energy of the particle, and information regarding the identity of the radiation. No single detector can offer these simultaneously


Ionization chambers are common dosimeters that measure the ionizations produced when a charged particle or ionizing photon traverses the detector medium (generally a gas, requiring temperature and pressure corrections). An electric field is applied between the electrodes to prevent the recombination of ions produced.


Proportional counters are ionization chambers that also provide directional information on the path of charged particles


Scintillator materials are also used as dosimeters by relating the flux of photons emitted to the energy deposited. They are generally coupled to photomultiplier tubes (PMT) to amplify the intensity of the photon signal


Semiconductor detectors measure the number of charge carriers produced by the radiation in the detector material. Semiconductor materials are used due to the small energy required to produce electron-hole pairs


Cerenkov detectors record light produced by charged particles traveling through materials at a velocity greater than that at which light can travel through the material


Calorimeters quantify the absolute dose absorbed by measuring the increase in temperature produced by radiation


The Monte Carlo (MC) method is a numerical calculation method based on random draws. A succession of draws is carried out in order to sample the random variables of the treated problem to deduce a value of interest. Repeated several times, this procedure allows to obtain a distribution of the values of interest and thus an estimation of their mean and their associated confidence interval. However, the number of samples must be sufficiently large for the empirical mean of the results to be an unbiased estimator of the expectation of the quantity of interest and its distribution as predicted by the central-limit theorem. In this process, the quality of the random number generator is essential. However, only pseudo-random numbers (having a period) can be generated and each Monte Carlo calculation code uses a different mathematical algorithm for that purpose.


The Monte Carlo method is currently used in many fields of physics to model the interactions of particles in a medium. In particular, it is used in dosimetry to estimate the energy loss of the particles in the medium and thus the absorbed dose.


To simulate the course of the particles, MC codes use the notion of cross-sections expressed in barn (b) (1 barn = 10−22 cm2). This cross-section is a physical quantity representing the probability of collision between an incident particle and a target, as it is proportional to the ratio between the interaction rate (T) and the incoming particle fluence (φ):

 (4.10)


with Ntarget the number of target particles in the target volume, corresponding to the surface S of the beam intercepting the target and starget the number of target particles per surface unit.


Therefore, we can calculate the probability p for a particle to interact with the target in the following way:


With NA the Avogadro’s constant, ρ the target medium density, d the target thickness, and A the atomic mass of the target medium. Sigma?


We see that the probability of interaction depends directly on the quantity (ρ ⋅ d), which has the unit of g cm−2. Moreover, we see the unit of σ: p appear without dimension, σ has the dimensions of a surface. One can imagine σ as a geometrical surface: a particle striking the target in this area would interact, while outside this area it would cross the target without diffusion (Fig. 4.10).

A schematic representation of a cross-section view of a rectangle with 9 circles between fluence phi and S of target diffusion.

Fig. 4.10
Schematic representation of the cross-section for a target with Ntarget = 9 and an irradiated surface S


A schematic representation of a cross-section view of a rectangle with 9 circles between fluence phi and S of target diffusion.


From this concept of interaction cross-section, it is possible to define the mean free path (λ) of a particle by means of the equation:


This mean free path corresponds to the average value of a random variable representing the path traveled by a particle between two interactions (l). The probability density of this random variable is given by:

. This probability density allows to sample the distance traveled by a particle between two interactions using a random variable ξ0 uniformly distributed between 0 and 1 as follows l =  − λ ln ξ0.


Cross-sections used in the MC codes are obtained either experimentally or are calculated from theoretical diffusion models and then used to determine the probability distributions of the random variables related to a trajectory as mean free path but also the type of interaction or the energy loss.


A key point of these MC calculations is related to the simulation of the electron (and positron) interactions. Indeed, these particles lose a very small part of their energy at each interaction they undergo. Thus, they generate a very large number of events before being finally absorbed into the medium. The detailed simulation of this cascade of interactions and of these weak energy deposits is particularly slow. Thus, most Monte Carlo codes apply simplifying theories called “condensed histories” or “multiple scattering” that summarize a certain number of interactions in a single step, allowing to reduce the simulation time. The compromise between the detail of the simulation and the speed of the calculation conditions the performance of the calculation code. Among the most used MC codes in dosimetry, we can highlight PENELOPE « Penetration and ENErgy LOss of Positrons and Electrons, EGSnrc « Electron Gamma Shower », MCNP6 and MCNPX « Monte-Carlo N-Particle eXtended », or Geant4 « GEometry And Tracking » [9–13].


Thanks to their capacity to include a large part of the physical processes involved in radiation–matter interactions and the possibility of taking into account all the different components of the experimental geometry of the problems, MC codes have clear advantages since they can provide information on the values of certain quantities that cannot be determined experimentally.


In radiotherapy, it is required to deliver a dose to the tumor with an uncertainty equal to or less than 5% [14]. Prescribed dose metrology involves the determination of quantities characterizing the transfer and absorption of energy in the irradiated media. In principle, Monte Carlo simulations allow the dose calculation with the required accuracy using phantoms or even patient’s voxelized images and thus provide information on dose distribution in the organ volume. However, to do this it is necessary to have quite exact knowledge of the beam characteristics, which means the need for detailed consideration of each accelerator including head shielding and structural components, which is very time-consuming and often submitted to industrial secret. Therefore, up to now, Monte Carlo codes have been mainly used to calculate the correction factors, often close to unity, to be applied to the experimental values obtained at the hospital during this metrological control.


Nevertheless, The Monte Carlo technique is increasingly used for clinical treatment planning by implementing MC-based algorithms that are used in situations where conventional analytical methods used by the Treatment Planning Systems (TPS) of the machines are not enough. To decrease the computation time, most implementations for radiotherapy divide the calculation into two steps. The first one consists in simulating the head of the treatment machine. This part being fixed and independent of the ballistics associated with the treatment of patients, a phase space can be recorded at the output of the treatment head and be reused. The second step consists in tracking the particles previously recorded in the phase space in the specific geometry of a patient for a specific treatment. Both parts must be, of course, experimentally verified by comparisons with percentage depth dose curves (PDD) and absorbed dose profiles at various depths in water or with measurements in situations where electronic equilibrium is not respected, for example, at the interfaces of materials of different densities (Box 4.4).


The Monte Carlo (MC) method is a numerical calculation method used to estimate the dose deposited through simulation of the stochastic events through which radiation deposits energy


Microdosimetry was first introduced by H.H. Rossi in 1955 and is a fundamental and evolving research field in experimental radiation science [15, 16]. It studies the interaction between radiation and matter in micrometric volumes of cell-like dimensions taking into account the stochastic nature of the energy deposition process.


The interaction between radiation and matter is a stochastic process that manifests itself as energy deposition, δ-electron production, or nuclear reactions. The latter produce charged particles, called secondaries, which in turn interact with surrounding matter, releasing energy, as δ-electrons do. The fundamental quantity in microdosimetry is the lineal energy, y, which aims to quantify the individual energy deposition events. Energy deposition is a stochastic quantity defined as the energy deposited at the point of interaction:

 (4.13)


where Tin is the energy of the incident ionizing particle (exclusive of rest mass), Tout is the sum of the energies of all ionizing particles leaving the interaction site (exclusive of rest mass), and QΔm is the change of rest mass energy of the atom and all particles involved in the interaction. εi is usually expressed in eV. The lineal energy, y, is therefore defined by the ICRU report 36 ([17], p. 36) as the quotient of εtot by l, where εtot is the total energy imparted to a volume of matter by a single energy deposition event and l is the mean chord length in that volume:

 (4.14)


The lineal energy is usually expressed in keV/μm. A single energy deposition event denotes the energy imparted by correlated charged particles. Due to the stochastic nature of radiation interaction, each particle traversal gives rise to a different lineal energy value thus producing a probability distribution function. Such probability distribution functions fully characterize the irradiation at a given point. The individual energy deposition events (opportunely corrected for the detector charge collection efficiency and converted into energy to tissue equivalent material) are collected in a form of spectrum [f (ε) vs ε] where f (ε) is the probability of an energy deposition event ε. From these energy spectra, the lineal energy spectra [f (y) vs y; with y = lineal energy] can be calculated by dividing the energy events by the average chord length of the detector, which is the average distance that the particle will traverse in the detector. In the case of a spherical detector, this can be demonstrated to be 2/3 of the diameter, while for thin plate detectors in a unidirectional particle beam, this can be approximated to the detector thickness [18]. The probability density function f (y), also called lineal energy frequency distribution, is independent of the absorbed dose or dose rate. Its expectation value

F is called frequency mean lineal energy and, being a mean value, is no longer a stochastic quantity.


As the radiation biological damage is proportional to the dose delivered, it is useful to consider also the lineal energy dose distribution d (y), as it provides the fraction of the total absorbed dose in the interval [y, y + δy]. The dose-weighted lineal energy distribution d (y) is therefore given by:


By definition, this distribution is normalized and is generally plotted as d (y) vs log (y) to make it easier to appreciate the relative contribution of various energy deposition events (see Fig. 4.11).

4 waveforms. A and B are counts versus channel n, and lineal energy y, the peak value is (200, 1300), c and d, f of y, and y d of y versus lineal energy y of the fluctuating values.

Fig. 4.11
The same microdosimetric spectrum represented through the raw counts per channel acquired (a), counts as a function of the lineal energy (y) after a channel calibration (b), converted into lineal energy frequency (c) and dose (d) distributions


4 waveforms. A and B are counts versus channel n, and lineal energy y, the peak value is (200, 1300), c and d, f of y, and y d of y versus lineal energy y of the fluctuating values.


Similar to the frequency mean lineal energy

F, the dose-weighted mean lineal energy

D can be defined as

 (4.17)


This quantity provides the average lineal energy value when each energy deposition event is weighted based on its contribution to the total dose.


A crucial parameter for the calculation of the lineal energy is the mean cord length (

), as the energy lost by a charged particle traversing a finite volume is proportional to the path traveled (track length) in that volume. The cord length however is itself a random quantity and for microdosimetric calculations, its mean can be estimated through Monte Carlo simulations or, for convex volumes, using the Cauchy formula

 = 4 V/S where V is the body volume and S is its surface area.


The first microdosimeter detector was designed and developed by Rossi in 1955 [16]. It was a spherical proportional counter made of tissue-equivalent plastic walls and filled with low-pressure tissue-equivalent gas (TEPC—Tissue Equivalent Proportional Counter). The low pressure allows to simulate micrometer volumes using a millimeter-size chamber (10–150 mm diameter), which is easier to handle and manufacture. Methane or propane-based gases are typically used at a pressure of ~0.9 kPa to simulate volumes of a few micrometer in diameter. The electrons produced by the traversal of the radiation through the chamber are amplified and collected by an electric field. Every radiation traversal generates therefore a small current that gives rise to a pulse later processed by the acquisition electronics. This allows the quantification of the energy deposited in micrometer volumes by individual radiation events. TEPCs are still the most common detector for microdosimetry measurements. Their main limitation is related to the wall-effects, which are events generated by the interaction of the incoming radiation with the walls of the device, and to controlling the electron avalanche process caused by high electric fields, which are required to simulate very small volumes. New devices are addressing these limitations with wall-less TEPC where specially designed electrodes are aligned to generate an electric field within a confined volume and reduce electron avalanches (avalanche confinement TEPCs). The new devices are also less cumbersome than the first TEPC designed by Rossi and can be operated in clinically relevant radiation beams.


More recently, solid-state detectors have been employed as microdosimeters taking advantage of their unique characteristics including compact size, economic development, and low sensitivity to vibrations, which makes them particularly suitable for clinical environment. The working principle is based on the electron-hole pairs produced by the radiation as it crosses the sensitive volume of the semiconductor crystal. The number of electron-hole pairs is proportional to the total energy deposition (ΔE) and the crystal ionization energy (W; average energy required to produce an electron-hole pair) by


The ionization energy is specific for each crystal and in the order of a few eV for typical semiconductor materials, which is an order of magnitude lower than that required for gas detectors. Furthermore, it is largely independent of the energy of the incoming radiation. Similar to TEPCs, the current generated by the collection of the produced electrons is used to quantify the energy deposition events. As the sensitive volume of the detector can be of a few micrometers, the pulses generated provide a microdosimetric spectrum of the incident radiation. In order to serve as a microdosimeter, solid-state detectors need to have well-defined and micrometer-sized sensitive volumes coupled to an efficient charge collection mechanism, as the electrical signal generated can be very small. A potential drawback of solid-state microdosimeters is their non-tissue equivalence, which generally requires additional conversion calculations provided by Monte Carlo simulations.