Dose or absorbed dose is the mean energy imparted by ionizing radiation to a material. The SI unit of dose is gray (Gy).

Dose rate is defined as a dose of ionizing radiation absorbed or delivered per unit time. The SI unit of dose rate is gray/hour.


Dose or absorbed dose is the mean energy imparted by ionizing radiation to a material. The SI unit of dose is gray (Gy).


Dose rate is defined as a dose of ionizing radiation absorbed or delivered per unit time. The SI unit of dose rate is gray/hour.


The interaction of radiation with matter or tissue is also influenced by the type of radiation. Some types of radiation produce different effects than others for the same amount of energy. This is because the pattern of dose distribution and the density of ionization events will be different. To account for these variations when describing human biological harm from radiation exposure, the “equivalent dose” is used. For example, for equal absorbed doses, neutrons may be 20 times as damaging as X-rays. The equivalent dose is the product of the absorbed dose averaged over the tissue or organ and the radiation weighting factor WR particular for the type and energy of radiation involved. It is based on the absorbed dose to an organ, adjusted to account for the effectiveness of the type of radiation [85, 87]:


The SI unit of equivalent dose is sievert (Sv). The unit “rem” (roentgen equivalent in man) is also still used. One rem is equivalent to 0.01 Sv.


The radiation weighting factors recommended by the ICRP are shown in Table 2.6.Table 2.6
Radiation weighting factors (ICRP 103)

WR

X-γ-rays

1

β+–β−

1

Protons and charged particles

2

Neutrons

5–20

α-Particles

20


If a mixture of radiation types is used, the equivalent dose is the sum of the individual doses of the various types of radiation, each multiplied by the corresponding weighting factor:


The effective dose is the addition of equivalent doses to all organs, each adjusted to account for the sensitivity of the organ to radiation. If a body is uniformly exposed to radiation, the probability of biological effects is assumed to be proportional to the equivalent dose. However, various tissues react to ionizing radiation in different ways and have different sensitivity to radiation. The ICRP has introduced the tissue weighting factor (WT), which represents the relative contribution of each tissue or organ to the total damage or “effect” resulting from uniform irradiation of the whole body [85, 87, 88] (Table 2.7).Table 2.7
Tissue weighting factors (ICRP 103)

Tissue/organ

2007 WT

Bone marrow

0.12

Breast

0.12

Colon

0.12

Lung

0.12

Stomach

0.12

Bladder

0.04

Esophagus

0.04

Gonads

0.08

Liver

0.04

Thyroid

0.04

Bone surface

0.01

Brain

0.01

Salivary glands

0.01

Skin

0.01

Remainder tissues

0.12


The effective dose is the product of the equivalent dose and the tissue weighting factor:

 (2.34)


Despite differences in the sensitivity of tissue due to age and sex of the person, for the purpose of radiation protection, the values for tissue weighting factors are taken as constants and are applicable to the average population. The effective dose is a calculated quantity and not a physical, measurable quantity.


The effective dose is used to compare radiation exposure and risks between different radiation types and exposure modes and a total body exposure. According to the ICRP Publication 103, effective dose is to be used for “prospective dose assessment for planning and optimization in radiological protection, and retrospective demonstration of compliance for regulatory purposes.”


Annual dose limits for occupational and public exposure are given in terms of the annual effective dose.


In case of external irradiation, the absorbed dose is delivered at the time of exposure. In the case of internal irradiation, when radionuclides are taken into the body, the total absorbed dose is distributed over time as well as to different tissues in the body. The dose rate falls depending on the half-lives of the radionuclides. The committed equivalent dose considers the varying time distributions of dose delivery. The committed equivalent dose is calculated as the integral over 50 years of the equivalent dose in each tissue after intake of a radionuclide [85, 87].


This is the sum of the committed equivalent dose to the individual tissues or organs multiplied by their respective WT.


The radiation doses discussed above relate to exposures of individuals. The collective equivalent dose is used to measure the total impact of a radiation exposure to a group or population. The collective equivalent dose is the product of the average equivalent dose to a population and the number of persons exposed. It is measured in man-sievert (man-Sv).


The collective effective dose allows a rough estimation of the potential health risks to a population after exposure to radiation. It is the product of the average effective dose to a population and the number of persons exposed. It is measured in man-sievert (man-Sv).


If a population is exposed to internal exposure by radionuclides, the integral of the effective dose over 50 years is called the collective committed effective dose. It is measured in man-sievert (man-Sv) (Box 2.19).


The effective dose is the product of the equivalent dose and the tissue weighting factor. The SI unit of effective dose is sievert (Sv).

The equivalent dose is the product of the absorbed dose averaged over the tissue or organ and the radiation weighting factor WR. The SI unit of equivalent dose is sievert (Sv).


The effective dose is the product of the equivalent dose and the tissue weighting factor. The SI unit of effective dose is sievert (Sv).


The equivalent dose is the product of the absorbed dose averaged over the tissue or organ and the radiation weighting factor WR. The SI unit of equivalent dose is sievert (Sv).


Ionizing radiation causes significant physical and chemical modifications, which eventually lead to biological effects in the exposed tissue. The amount of energy absorbed by the tissue (absorbed dose) and the rate at which such energy is deposited (dose rate and fractionation for clinical applications) play a critical role in determining the type and extent of the effects. However, other physical parameters can also affect the biological response. It is therefore necessary to introduce a radiation quality term to discriminate between different radiation types. Radiobiological data and models clearly point to the spatial distribution of energy deposition as a key radiation quality parameter. However, the stochastic nature of the interaction of radiation with matter prevents a comprehensive and unique description and measurements of the ionization patterns produced by the pathway of charged particles in matter. The alternative is, therefore, to define a suitable but inevitably incomplete characterization of radiation quality that will enable radiobiological predictions with sufficient accuracy.


The concept of linear energy transfer (LET), the amount of energy transferred per unit length, was introduced by Zirkle et al. [89] to account for the density of energy transfer occurring along the track of charged particles, including excitations and ionizations, until the particles reach the end of their range. LET values are generally reported in keV/μm. The symbol LET∞ (unrestricted LET) is used when all possible energy transfers are included, and also the energy deposition by particles that in principle exit the volume of interest. The LET∞ is numerically equivalent to electronic stopping power, i.e., the energy loss by the incoming particle (which may be a primary or a secondary particle) without any restrictions in energy and range. The formula for the electronic stopping power contains a negative sign as it is seen as the slowing force acting on charged particles, due to interaction with matter, resulting in loss of particle energy:


where S(E) is the stopping power, dl is the distance traversed by the particle, and dE is the mean energy loss due to collisions with energy transfers.


There is however a conceptual difference: the stopping power deals with the energy loss of the particle, while the LET∞ focuses on the energy deposition in the medium, and thus, the LET generally has an opposite sign. For large volumes, the electronic stopping and the LET∞ coincide (same absolute value), as for large volumes all the energy loss by the impacting particles is well likely deposited in the sample.


In radiobiology, the concept of “restricted LET” is mostly used. This is the locally transferred energy per unit length, with “locally” restricting to only the energy fraction, which leads to ionizations and/or excitations within the considered site. The remaining kinetic energy of particles leaving the site is excluded. This is particularly relevant for electrons since they may possess considerably long ranges. For example, for ions with E > 1000 MeV/μ, these electrons can have energies higher than 1 MeV. The lateral spread of the track is usually 100 s of nm, but for higher energies of the ions such as 1000 MeV/μ, this lateral spread can even be 1 cm.


According to the ICRU 1970, the linear energy transfer of charged particles in a medium is the quotient of dE by dl. Here, dE is less than some specified value Δ. The definition includes an energy cutoff rather than a range cutoff as this is of more practical use:

 (2.36)


It has become customary to specify a limit of energy deposition below which the deposition is considered to be local (energy restriction); 100 eV has been widely accepted, which corresponds to an electron range of about 5 nm. Electrons of longer ranges are called “δ electrons” or “δ rays.”


X-rays and gamma rays are considered low LET (sparsely ionizing) radiation types, while high-energetic protons, neutrons, and heavy charged particles are considered as high LET (densely ionizing) radiation. A proton can have high or low LET, depending on its energy. Although commonly high-energy protons have been considered low LET radiation, recently this has been questioned, starting a new “paradigm in radiation biology” [90]. For indirectly ionizing neutrons, LET refers to that of the secondary charged particles they produce. The value which is generally considered to mark the distinction between low and high LET is about 10 keV/μm.


As ionizing particles decelerate along their track, the LET decreases, leading to a LET distribution, and consequently two different LET average concepts can be defined. The “track average LET” is calculated by performing a weighted average considering the proportion of the total track length that is within specified LET intervals and assigning equal statistical weight to each unit of the track length. On the other hand, the “dose average LET” is a weighted average of the LET values taking into account the proportion of the energy that is deposited for each LET interval so equal statistical weight is assigned to each unit of the energy deposition. In the first approximation, the dose-averaged LET is more suitable as the radiation quality factors are based on such quantity.


Apart from ionizations and excitations, among which ionizations bring the highest contribution to electronic stopping over a wide range of energies [91], other mechanisms cause energy loss of the impinging particle and thus induce deposited energy. At energies below some few keV/μm of the traveling ion, also nuclear collisions can occur. Such elastic nuclear collisions (described by the concept of nuclear stopping), which cause displacement of atoms, can induce alteration and breaking of bonds, and thus also contribute to biological damage. For particles with high energy, inelastic nuclear collisions, where the impacting particle causes fragmentation of the nuclei generating daughter nuclei with emission of several secondary particles, can also occur. These loss mechanisms are not described by the concept of stopping. A significant loss of primary beam fluence is caused by such nuclear reactions. The inelastic nuclear cross section determines the number of particles left at a certain depth. For instance, for protons hitting a water target with an energy of 160 MeV, at the Bragg peak position, approximately 20% of the incident protons will be lost [92].


The Bragg curve represents the energy loss, in this case electronic stopping or LET, as a function of the distance through a stopping medium. The energy loss is characterized primarily by the square of the nuclear charge, Z, and the inverse square of the projectile velocity, β. This gives the Bragg curve its familiar shape, peaking at very low energies (Bragg peak), just before the projectile stops (Fig. 2.19). The stopping of charged particles increases with decreasing ion energy; in particular, around the Bragg peak, the stopping (or the LET) is maximum, near the very end of the particle’s range. Ions of the same specific energy (energy per nucleon) have a similar range, typically on the order of 10 μm at ~1 MeV/μ up to 1 mm at ~100 MeV/μ [25].

A line graph plots dose in arbitrary units and linear energy transfer in kiloelectron volts per micrometer versus depth. 5 lines follow a left-skewed trend, with the peaks at 50, 100, 150, 200, and 250 mega-electron volts, respectively. 5 dashed lines follow an increasing trend along the peaks.

Fig. 2.19
Dose and LET distribution for proton beams of various energy in water (simulated using TOPAS MC)


A line graph plots dose in arbitrary units and linear energy transfer in kiloelectron volts per micrometer versus depth. 5 lines follow a left-skewed trend, with the peaks at 50, 100, 150, 200, and 250 mega-electron volts, respectively. 5 dashed lines follow an increasing trend along the peaks.


Sparse energy deposition events along the track of a particle per unit of energy deposited appear to be less biologically damaging than “dense” deposition. The value of the LET that seems “optimal” for cell killing is in the range of 100 keV/μm. This is linked to the fact that the average separation of ionization events at this LET is about the same as the diameter (2 nm) of the DNA double helix, implying a higher probability of DSB, from the passage of a single particle. Clusters of lesions in the DNA molecule play a key role in biological damage [93] (Box 2.20).


LET is a parameter that quantifies the amount of transferred energy per unit length.

LET is reported in units of keV/μm.

LET increases with the ion mass and with decreasing ion energy.


LET is a parameter that quantifies the amount of transferred energy per unit length.


LET increases with the ion mass and with decreasing ion energy.


There is an intrinsic relationship between the quantities in dosimetry, e.g., absorbed dose (see Sect. 2.5), linked to the electronic stopping power, and quantities at the microscale and down to the nanoscale.


The study of the pattern of energy deposition at micrometer length scale is called microdosimetry [94]. In particular, microdosimetry studies the fluctuations and pattern of energy deposition in a micrometer-sized target, providing a comprehensive view of the energy deposition more detailed than the one given just by the LET alone. The measured spectra are distributions of energy depositions in the microscopic volume, which are a combination of several stochastic processes including the LET distribution, the track length distribution, the energy loss straggling (statistical fluctuation of energy loss along the particle track) of the primary particles, and the transport of energy by δ-rays [95]. Microdosimetric quantities are stochastic and therefore given in terms of particle interaction probabilities [95, 96]. The relevant quantities in microdosimetry are as follows:
y: the lineal energy, which is defined as the energy imparted to matter in the microscopic volume by a single event divided by the mean chord length in that volume and the mean length of randomly oriented chords in a given convex volume

f (y): the probability distribution of linear energy

: the first moment of f (y), also called the frequency mean lineal energy

dy = yf (y)/

: the dose distribution, which is important for obtaining the dose components of the microdosimetric spectrum

•

: the first moment of d (y), also called the dose mean lineal energy


y: the lineal energy, which is defined as the energy imparted to matter in the microscopic volume by a single event divided by the mean chord length in that volume and the mean length of randomly oriented chords in a given convex volume


: the first moment of f (y), also called the frequency mean lineal energy


dy = yf (y)/

: the dose distribution, which is important for obtaining the dose components of the microdosimetric spectrum


•

: the first moment of d (y), also called the dose mean lineal energy


Relative biological effectiveness (RBE) is a method to quantify and compare the biological damage of different types of radiation [97]. The RBE is a dimensionless quantity and can be described as a radiation quality index with regard to biological damage. Quantitatively, RBE is the ratio between the absorbed dose of a reference radiation type and the absorbed dose of the radiation type of interest, such that both the absorbed doses compared produce the same amount of a biological effect, known as isoeffect. The reference radiation is defined as a low LET radiation. Previously, the standard radiation used was 250 keV of X-ray; however, nowadays, it is more common to use as standard 1 MeV photons (from a cobalt-60 source). This means that RBE is 1, when cobalt-60 biological effect is compared with itself.


RBE guides in the selection of the weighting factors, which are required to define the effective dose (E) (Sect. 2.5). RBE varies with several factors described in detail later, namely LET, radiation dose, fractionation, dose rate, biological system, endpoint measured, and radiation quality.


Over the past decades, radiobiology and nanodosimetry studies have pointed out that the characteristic spatial distribution of energy deposition at the subcellular scale induced by different particles at different speed is a key aspect at the origin of the RBE of different radiation qualities [91]. Localized clusters of energy deposition within the DNA molecule play a critical role. The frequency and topological distribution of clustered lesions determine the effectiveness of the DNA repair mechanisms. Isolated lesions are more efficiently repaired, while for complex lesions, errors are more likely to occur in the repair, often leading to permanent damage [98]. One of the main aims of the radiation community is to develop models for the radiation quality factors, the RBE and cell survival, which are consistent with nanodosimetry. Several efforts have been done recently to (a) develop biologically relevant quantities based on nanodosimetry [99], in order to overcome the simplistic description of the quality factor as a (continuous) function of the sole LET; (b) develop new quality factors incorporating a formula that relates to densely and sparsely ionizing components of the radiation tracks and core track contributions and penumbra contributions [13]; (c) develop an RBE based on a radiation quality descriptor depending on energy deposition clustering [100]; (d) develop a cell damage/survival model based on the interactions between lesions at both the nanometer and micrometer scale [101]; and (e) perform a detailed analysis of the radial distribution of ionization cluster size distribution [102].


Prediction of radiobiological response is a major challenge in radiotherapy. Survival curves allow to determine the radiosensitivity of a cell line to different types of radiation, as well as to compare the response of one different cell type to one type of radiation. The linear-quadratic (LQ) model has been best validated by experimental and clinical data and describes the surviving fraction (SF) of cells as a function of radiation dose D: SF (D) = e−α.D − βD2. It allows determining important biological parameters such as the survival fraction or the ratio α/β, which represents the intrinsic radiosensitivity. Cells with a higher α and β are more sensitive to radiation. The shape of the curves depends on the LET. Indeed, cells irradiated with the same dose of different LET induce different biological effects translated into different cell survivals. As the LET increases, the slope of the curve becomes steeper and straighter with less shoulder. This indicates a higher ratio of lethal to potentially lethal lesions or a less efficient repair of the high LET radiation damage. For the LQ representation, this is shown by a higher α/β ratio for high LET radiation. However, the lower the α/β ratio is (high β relative to α), the more curved the clonogenic curve is.


Although the LET is a common and useful parameter to quantify the distribution of absorbed radiation energy, there are considerable limitations, which need to be considered. The limitations in terms of using the LET for predicting biological effects are strongly related to the RBE models and have been discussed in previous sections. There are also caveats of more physical nature. In particular, LET measurements are complex, difficult to relate to clinical or radiobiological setups, and affected by several constraints particularly if LET distributions are to be reported rather than single LET values. Direct measurements of dE/dl can be attempted with very thin particle detectors (such that multiple interactions within the active volume rarely occur) with high-energy resolution and able to discriminate between secondary particles and photons. In this case, the energy loss (ΔE) by a particle passing through is related to the thickness of the detector (Δl). Ideally, detectors with different thickness would be employed and the energy detected plotted against the detector thickness from which the slope at the origin is extrapolated. The density of the sensitive material of the detector should also be considered to convert the measurements into water. This provides an estimation of the stopping power and therefore the LET∞. The development of several Monte Carlo-based codes has offered the possibility to quickly calculate LET values taking also into consideration the specific experimental settings.


The definition of the LET concept also implies that an average LET value may not always be adequate to describe the radiation quality to which biological samples are exposed. As mentioned, the LET changes significantly along the path of an individual charged particle and it is affected by the specific irradiation setup including any scattering conditions. Single LET values are suitable for “track segment” experiments where thin biological samples are exposed to mono-energetic charged particle beams. Even under such conditions, however, the energy loss by a charged particle over a cellular distance fluctuates and it can occasionally reach extreme high or low values, which are not well accounted for in an averaging process. Also, the angular deflection and the lateral extension of the particle tracks due to the finite range of δ-rays are in principle not taken into account in the LET concept. The restricted LET, which only includes energy transfer below a specified cutoff, can actually partially take into account the second point. However, a set of LET distributions that belong to different cutoff values would be needed, but still little information about the actual structure of particle tracks would be gained [103]. A quantitative evaluation has shown that the LET concept is quite inadequate for electrons; there are no sites sufficiently small to disregard the finite range of the electrons and simultaneously sufficiently large to disregard the lateral escape of δ-rays and the energy loss straggling [103].


Contrarily, for heavy ions, there are site sizes and particle energies for which the LET predicts adequately the energy deposition. LET increases approximately as the square of the ion charge, Z, and the inverse square of its velocity, v. On the other hand, the maximum range of the δ-ray electrons depends on the velocity of the particle but not its charge. Thus, the consideration of the sole LET of a particle is not sufficient for a description of the particle’s track structure, as two particles of identical LET but very different velocity and charge will have very different track structures [104].


RBE increases as LET increases, up to a maximum LET value of about 100 keV/μm, and then decreases as LET increases (Fig. 2.20—RBE and LET) [97]. In general, high LET radiations allow the deposition of a given amount of energy over a shorter distance, being more efficient in producing biological effects than low LET radiations. In other words, low LET radiation creates sparse ionization, requiring more than one radiation track to pass through the cell and induce lethal biological damage, while high LET radiation is more effective, since one radiation track through the cell is enough to induce lethal biological damage. However, over 100 keV/μm, there are many “wasted” ionizations due to the very high ionization densities (number of ions per unit of path length). This phenomenon is the so-called overkilling and reflects the RBE declining for further increases in LET, for which biological effect is reduced since most of the energy is wasted.

A line graph of R B E versus L E T plots a curve of low L E T, high L E T, and overkill. A line begins near the origin, increases gradually at the low L E T region, reaches a peak at 100 kiloelectron volts per micrometer at the high L E T region, and declines at the overkill region.

Fig. 2.20
RBE variation with LET. RBE increases as LET increases, up to a maximum LET value of about 100 keV/μm. An “overkilling” effect is observed for higher LET values (Created with BioRender)


A line graph of R B E versus L E T plots a curve of low L E T, high L E T, and overkill. A line begins near the origin, increases gradually at the low L E T region, reaches a peak at 100 kiloelectron volts per micrometer at the high L E T region, and declines at the overkill region.


When determining RBE, it is important to understand that RBE values also depend on the radiation dose and, consequently, on the isoeffect level chosen for the comparison between radiation types [97]. For small radiation doses, RBE is particularly variable tending to increase. This is explained by the fact that at low doses, the difference in the biological damage induced by low and high LET radiation is huge; that is, high LET radiation is very effective in killing cells, while low LET radiation is ineffective in doing so. For high radiation doses, the difference between the effects induced by low and high LET radiation becomes smaller, considering that low LET radiation becomes more lethal. At very high doses, the RBE no longer depends on the dose.


The shape of the cell survival curve determines the presence or absence of a fractionation effect. With repeated daily low-dose X-ray fractions, the shoulder curvature is repeated, and cell survival is increased relative to a single high-dose radiation fraction at equal total dose. As mentioned previously, the bending of the cell survival curve is described by the α/β ratio parameter of the LQ model equation. The principle of fractionation is the repeat of the shoulder of the cell survival curve. The broader the shoulder, the lower the α/β ratio and the higher the cell survival in fractionated irradiation, i.e., the higher the sparing effect. In other words, the straighter the curve is, the less the fractionation effect is. The cell survival curve of high LET irradiation such as alpha particles is a straight line (e.g., Fig. 2.22); hence, the effect of fractionation is lost. Fractionation of carbon ions does not influence its biological effectiveness.


The same effect is seen when the dose per fraction is reduced in vivo. While low LET X-ray irradiation shows—related to the α/β ratio of the LQ model—sparing effect with multiple low dose fractions, high LET irradiation does not show such typical fractionation sparing effect, as illustrated in Fig. 2.21 [105]. Figure 2.21 (left) shows large sparing, and thus an increased tolerance to low LET irradiation, for late-responding normal tissues (with a low α/β ratio such as the spinal cord and kidney) with decreasing dose per fraction, while early-responding normal tissues (e.g., jejunum) and tumors (e.g., fibrosarcoma), both characterized with a high α/β ratio in the LQ model, are marginally spared. With high LET neutron irradiation, very little normal tissue sparing of fractionation has been demonstrated (Fig. 2.21, right), neither for early-responding normal tissues and tumors nor for late-responding normal tissues. The current view is to use at least two high LET fractions to obtain some sparing and benefit from reoxygenation, but multiple fractions would not be further beneficial [106] (Box 2.21).

2 multi-line graphs plot total dose for several isoeffects versus dose per fraction for low L E T and high L E T. The values are plotted for late and acute responding tissue in various organs. 1. The late line for skin has the highest value. 2. The acute line for skin necrosis has the highest value.

Fig. 2.21
Isoeffect curves as a function of total dose and the number of fractions for low LET X-rays or gamma rays (left) and high LET neutrons (right). See insert for explanation of symbols and curves. [Redrawn from Withers et al. [105] with permission]


2 multi-line graphs plot total dose for several isoeffects versus dose per fraction for low L E T and high L E T. The values are plotted for late and acute responding tissue in various organs. 1. The late line for skin has the highest value. 2. The acute line for skin necrosis has the highest value.


The higher the LET, the straighter the radiation–cell survival relationship, and the lower the sensitivity to dose fractionation.

The RBE of high LET irradiation decreases with increasing dose or dose per fraction for both cells and tissues.

Little normal tissue sparing after fractionated high LET irradiation: Few fractions are sufficient.


The higher the LET, the straighter the radiation–cell survival relationship, and the lower the sensitivity to dose fractionation.


The RBE of high LET irradiation decreases with increasing dose or dose per fraction for both cells and tissues.


Little normal tissue sparing after fractionated high LET irradiation: Few fractions are sufficient.


The dose rate is defined as the ratio of the radiation dose [Gy] to the duration of the radiation exposure [hour]. The spectrum of dose rates used in radiation oncology is broad: from low dose rate (LDR < 2 Gy/h) to ultrahigh dose rate (FLASH, >144.000 Gy/h). The dose rate of radiation exposure largely determines its RBE. Lowering the dose rate reduces the effectiveness of radiation in many ways. In terms of the 6 Rs of radiobiology, the dose rate affects the induction and repair of DNA damage and related clonogenic cell survival, cell cycle (re-)distribution and activation of cell cycle checkpoints, and cell repopulation and reoxygenation and likely influences the immune response as well. For a particular equal biological effect, a biological endpoint, lowering the dose rate relative to a reference radiation quality (usually high-dose-rate 250 keV X-rays), the RBE decreases (Fig. 2.22). The dose rate effect could also be defined with the dose reduction factor (DRF), also termed the dose recovery factor. The DRF indicates the ratio of the radiation dose to achieve an equal biological effect at specified dose rate and the dose at high dose rate. The term DRF is used by analogy with the dose enhancement factor or sensitizer enhancement ratio, to quantify a change toward steeper cell survival curves. With increasing dose rate, the DRF value is >1.

A multi-line graph of surviving fraction versus dose plots the curves for no repair, 150 centigray per minute, 7.6 centigray per minute, 1.6 centigray per minute, and full repair. 5 lines begin at (0, 1) and follow decreasing trend.

Fig. 2.22
Effects of the dose rate on clonogenic cell survival for a human melanoma cell line irradiated at dose rates of 1.6, 7.6, and 150 cGy/min. At equal biological effectiveness, e.g., 0.01 cell survival (broken line), high-dose-rate irradiation has larger relative biological effect than low-dose irradiation, resulting in a dose reduction of approximately 5 Gy, i.e., a DRF of 1.6 (12.8/7.7). Dotted lines: (A) no repair; (B) condition of full repair at infinitely low dose rate. (Figure adapted from Steel [107], with permission)


A multi-line graph of surviving fraction versus dose plots the curves for no repair, 150 centigray per minute, 7.6 centigray per minute, 1.6 centigray per minute, and full repair. 5 lines begin at (0, 1) and follow decreasing trend.


The increase in biological effectiveness with increasing dose rate applies to all tissues and organs and, importantly, discriminates between early-responding tumors and normal tissues and late-responding normal tissues. In late-responding normal tissues, characterized by a low α/β ratio of the LQ model, the increase of dose rate is more detrimental than for tumors and early-responding normal tissues with a high α/β ratio. Literature data show that, at ultrahigh dose rate in FLASH radiotherapy, this differential effect could be inverted [108]. This inverse effect could be explained by the oxygen depletion hypothesis, the DNA damage hypothesis, and the immune response hypothesis.


Dose rate: radiation dose delivery per unit time (e.g., Gy/hour)

Dose rate effect: decrease in biological effectiveness with decreasing dose rate


Dose rate: radiation dose delivery per unit time (e.g., Gy/hour)


Dose rate effect: decrease in biological effectiveness with decreasing dose rate


During the last decades, many tissues and cells were characterized by survival curves in response to different types of radiation, especially X-rays. They underlined a great variation of the RBE for all the biological systems studied. Indeed, large variable shoulder regions were observed in response to X-rays, whereas less variation was observed with neutrons, explaining that the RBE is different for each cell line. In response to heavy ions, the depth of the irradiation has also to be considered and explains in part the different RBE calculated for one cell line compared with X-rays.


While the physical and dosimetric aspects of radiobiology are well understood, the biological aspects such as the complex biological endpoints induced need further attention. The current estimates of RBE listed above depend on the biological system, but also depend on the detection methods used as it has been demonstrated that DNA damage and the resulting apoptotic responses vary greatly depending on the radiation quality in a tissue- and dose-dependent manner. Experimental data emerging from recent studies suggest that, for several endpoints of clinical relevance, the biological response is differentially modulated by particles compared to photons. However, up to date, only few studies have been performed to understand the differential response on the molecular and cellular levels between different radiation qualities.


The biological effects of ionizing radiation relate strongly to the dose, dose rate, and quality of the radiation. To distinguish the different types of radiation, from low LET to high LET particle radiation, the quality factor Q (L) has been introduced. This factor reflects all cumulative knowledge on the dependence of the detrimental effects of radiation on physical characteristics and mainly LET (ionization density). Therefore, this factor can be used to multiply the absorbed dose (rad or gray) to obtain a quantity that expresses, on a common scale for all ionizing radiations, the biological damage (rem or sievert) to the exposed tissue. Although Q (L) has been superseded by the radiation weighting factor WR in the definition of equivalent dose, it is still being used in calculating the operational dose equivalent quantities used for example in monitoring [109].


In order to encompass the dependence of biological effects to LET, many studies have been performed in order to measure RBE for a specific biological endpoint (usually reproductive cell death) for radiations of different LET [110]. In most cases, survival curves are evaluated assuming a linear-quadratic dose dependence of the induction of reproductive death of cells. The linear term accounts for damage from single particle tracks and the quadratic term for damage due to interaction of lesions from independent tracks. Although for many years 250 kVp X-ray was considered the standard reference radiation for the determination of RBE, the International Commission on Radiation Protection (ICRP) recommended in their 92nd report to use gamma rays of 60Co as the reference radiation [111]. In both cases of low LET radiation, RBE is assumed to be equal to 1.0. When specific biological effects of high LET radiation (such as fast neutrons) on human cells are measured, the RBE ranges from about 3 to greater than 100 for various biological effects.


The oxygen effect is an important parameter in radiation therapy. Its influence on the tissue’s biological response (typically survival curves) will differ according to the radiation type used. This concept is represented by the oxygen enhancement ratio (OER).


The OER is a measure of the influence of the oxygen effect. It is defined as the ratio of radiation doses that produce the same biological effect in hypoxic compared to aerobic (well-oxygenated) conditions:


The OER varies with the LET (ionization density) (Fig. 2.23). The OER decreases as the LET increases and approaches OER = 1 at LET ≈ 150 keV/μm, meaning that the level of oxygenation has little or no influence on the cell survival in case of high LET radiation (α particles, neutrons, and heavily charged particles). This is explained by the fact that high LET radiation mostly induces direct damage, which is not oxygen dependent. Therefore, high LET radiation is expected to lead to a better tumor control of hypoxic tumors compared to low LET radiation.

A line graph of log surviving fraction versus absorbed dose plots the curves for normoxia and hypoxia. 2 lines follow a decreasing trend. O E R equals a dose of normoxia overdosed of hypoxia.

Fig. 2.23
OER as a function of LET (Created with BioRender)


A line graph of log surviving fraction versus absorbed dose plots the curves for normoxia and hypoxia. 2 lines follow a decreasing trend. O E R equals a dose of normoxia overdosed of hypoxia.


It should be noted that these OER values were originally derived from in vitro experiments. Recently, the oxygen effect during carbon ion therapy was questioned due to low LET values in the spread-out Bragg peak, giving rise to a possible impact of oxygen on carbon ion treatment outcome [112]. In case of low LET radiation (X- and γ-rays, electrons), the OER increases and is in the range of 2.5–3.5, meaning that a 2.5–3.5 times higher dose is needed to achieve the same killing effect in hypoxic cells compared to normoxic cells. Indirect effects, relying on reactive oxygen species (ROS) production, are the dominant process associated with low LET radiation and explain the importance of oxygen for low LET radiation. Hypoxic regions within a tumor may therefore show radioresistance to low LET radiation. The OER has an intermediate value for neutrons. Based on this concept, a massive work on oxygen-based radiosensitization is being done and is discussed in Chap. 5.


The damage caused by ionizing radiation in the body can become clinically apparent as a number of different health effects. The type and severity of the effect are strongly dependent on dose and exposure conditions, but also on the health status of the exposed individual. For radiation protection purposes, and to ensure the safe use of radiation in society, the health effects of ionizing radiation exposure are classified into two types [113]:


Deterministic effects, which are also called tissue reactions, are those for which there is a defined threshold below which the effect is not expected to occur. In addition, the severity of the effect increases with dose. The acute radiation syndromes are examples of early effects following high doses. However, deterministic effects are not a synonym for acute effects, as some, e.g., fibrosis, can occur much later.


Stochastic effects have no threshold, and the occurrence of the effect is probabilistic, such that any exposure to ionizing radiation increases the risk of these effects. The severity of the effect is not related to the dose. Stochastic effects tend to manifest many years postexposure and include cancer and heritable effects.


High-dose penetrating radiation causes damage both to functional tissues and to stem cell compartments. In general, maintenance of health depends on a balance between loss and replacement of cells in many, but not all, organs and tissues of the body, reflecting physiological “wear and tear.”


Cellular damage is known to occur after exposing tissues to ionizing radiation. If the number of cells damaged is small relative to the total number of stem cells in the tissue, then the remaining stem cells can repopulate adequate numbers of functional cells. Consequently, there will be no obvious loss of tissue function. Conversely, if the stem cell population is reduced below a critical size, the tissue will cease to function efficiently, either transiently or permanently.


Organs and tissues differ in their sensitivity to radiation (Chap. 7), and the damage from radiation particularly affects the more radiosensitive cells, for example the lymphocytes in the lymphatic tissue, red bone marrow precursor cells, and crypt cells in the mucosal lining of the gastrointestinal tract.


Whether or not recovery will be possible will strongly depend upon the rate at which viable stem cells (that is, those cells undamaged or repaired) can repopulate the depleted stem cell population by self-renewal. The whole process of recovery is dependent upon feedback mechanisms stimulated by the body’s recognition of depleted functional cell numbers. Following exposure of a large proportion of or all of the body, the normal steady state of cellular regeneration for tissues throughout the body is interrupted: cells and tissues break down and cannot be replaced. This is the basis for the observed threshold for such deterministic effects or tissue reactions.


It is, however, very important to note that there is a variation in sensitivity among individuals in an exposed population with any particular dose and exposure scenario. This variation reflects differences in the ability of individuals to cope with radiation-induced cellular damage, which is influenced by the age and state of health of the individual at the time of irradiation [85].


When individuals are exposed to sufficiently high doses of acute, penetrating ionizing radiation, the acute radiation syndrome begins with the prodromal phase [114, 115]. Following this, there will be a latent period, which represents the time period between initial exposure and manifestation of full acute radiation syndrome (ARS) due to a lack of cell renewal, as described above. The severity of the initial prodromal effects, the time for their development, the timing and any symptoms experienced during the latent period, and the type and severity of the full manifestation of ARS are all dependent on the dose and exposure scenario. This is described in more detail in Fig. 2.24 (Box 2.23).

A graph of radiation exposure versus time after radiation exposure plots the zones of initial prodromal phase, latent phase, and acute manifestation phase.

Fig. 2.24
Radiation syndrome phases (Created with BioRender)


A graph of radiation exposure versus time after radiation exposure plots the zones of initial prodromal phase, latent phase, and acute manifestation phase.


The clinical signs and symptoms of high-dose radiation exposure are observed up to ~6 days after exposure (with a high degree of uncertainty). These come as soon as a few minutes after a very high dose.

The symptoms of deterministic effects are dependent on dose (deterministic), with increased symptoms associated with higher doses.

In general, individuals exhibit flu-like symptoms, vomiting, diarrhea, and headache. For doses in the region of:
1–2 Gy, these are classified as “mild,” and we would expect 10–50% people vomiting, and others experiencing fatigue and weakness.

2–4 Gy, these are classified as “moderate,” following which 70–90% people would be constantly vomiting, 2–6 h after exposure; 50% people would have a headache; 10–80% people would have a slight increase in body temperature.

4–8 Gy, these are “severe,” following which ~100% of people would be vomiting <1 h after exposure; 50–80% people would have a headache; most others would have a constant fever <1 h after exposure; some people might lose consciousness or feel confused; 10% of individuals would have diarrhea 1–8 h after exposure.

8 Gy, these are “very severe/lethal” (depending on the medical resources available); most people lose consciousness fairly quickly; temperature peak at about 41 °C is usually observed, and many patients would present with skin burns at these doses.


The clinical signs and symptoms of high-dose radiation exposure are observed up to ~6 days after exposure (with a high degree of uncertainty). These come as soon as a few minutes after a very high dose.


The symptoms of deterministic effects are dependent on dose (deterministic), with increased symptoms associated with higher doses.


In general, individuals exhibit flu-like symptoms, vomiting, diarrhea, and headache. For doses in the region of:
1–2 Gy, these are classified as “mild,” and we would expect 10–50% people vomiting, and others experiencing fatigue and weakness.

2–4 Gy, these are classified as “moderate,” following which 70–90% people would be constantly vomiting, 2–6 h after exposure; 50% people would have a headache; 10–80% people would have a slight increase in body temperature.

4–8 Gy, these are “severe,” following which ~100% of people would be vomiting <1 h after exposure; 50–80% people would have a headache; most others would have a constant fever <1 h after exposure; some people might lose consciousness or feel confused; 10% of individuals would have diarrhea 1–8 h after exposure.

8 Gy, these are “very severe/lethal” (depending on the medical resources available); most people lose consciousness fairly quickly; temperature peak at about 41 °C is usually observed, and many patients would present with skin burns at these doses.


1–2 Gy, these are classified as “mild,” and we would expect 10–50% people vomiting, and others experiencing fatigue and weakness.


2–4 Gy, these are classified as “moderate,” following which 70–90% people would be constantly vomiting, 2–6 h after exposure; 50% people would have a headache; 10–80% people would have a slight increase in body temperature.


4–8 Gy, these are “severe,” following which ~100% of people would be vomiting <1 h after exposure; 50–80% people would have a headache; most others would have a constant fever <1 h after exposure; some people might lose consciousness or feel confused; 10% of individuals would have diarrhea 1–8 h after exposure.


8 Gy, these are “very severe/lethal” (depending on the medical resources available); most people lose consciousness fairly quickly; temperature peak at about 41 °C is usually observed, and many patients would present with skin burns at these doses.


Following these initial signs and symptoms, for doses less than approximately 6 Gy, the latency period is generally fairly asymptomatic, and individuals usually start to feel a little better. Then, unless radiation has been identified as the cause of the observed prodromal symptoms, often nothing is done, because the symptoms can be mistaken for those of many other non-radiation-related illnesses. However, if ionizing radiation has been identified as a potential cause, differential white blood cell counts should be taken as a marker of the potential severity of the effects. A summary of the different types of ARS is given in Fig. 2.25.

A line graph of dose versus time plots a baseline for therapy. A declining curve with phases of cerebral death, gastrointestinal death, and hematopoietic death.

Fig. 2.25
The dominant syndromes leading to death vary with dose and time postexposure. Therapy is possible for doses lower than approximately 8–10 Gy (depending on medical resources) (Created with BioRender)


A line graph of dose versus time plots a baseline for therapy. A declining curve with phases of cerebral death, gastrointestinal death, and hematopoietic death.


Following exposures greater than around 2 Gy, and with this syndrome dominating up to around 10 Gy, the fall in blood cell counts may result in death from septicemia or hemorrhage, due to bone marrow failure, unless the symptoms can be treated. When the bone marrow is acutely exposed to radiation, this causes hypoplasia, aplasia, and/or hemolysis of cells. This leads to a sudden and dose-dependent reduction in the stem cell population, and ultimately atrophy of the lymph nodes and spleen. Differentiating and maturing cells may initially be only marginally affected. Depletion of cellular components of blood leads to infection and hemorrhage.


The stem cell population may attempt to recover and, if successful, increasing numbers of granulocytes will appear in the blood about 3 weeks after exposure. Loss and recovery of blood platelet cell numbers follow a similar dose- and time-related pattern.


The severity of the radiation effect can be estimated based on differential white blood cell counts (neutrophils and lymphocytes). If neutrophil and lymphocyte levels are measured repeatedly following initial exposure (the half-life of circulating neutrophils is only about 6–8 h), this can give an indication of the likely severity of the ARS or other tissue effects: A large initial peak of neutrophils and a rapid drop-off could indicate a dose ~>5 Gy.


The mucosal crypt stem cells provide the protective mucosal cell lining of the intestinal tract wall. Due to the high turnover of these cells, particularly in the small intestine, damage to these cells results in a denudation of the gut surface as the epithelial cells are not replenished, within 5–10 days after exposure of the gastrointestinal tract to doses of radiation >1 Gy. Leakage of blood from damaged blood vessels into the gut then occurs, and blood appears in the feces. Simultaneously, translocation of normally harmless intestinal bacteria from the gut through the damaged blood vessels occurs, leading to infection. Once in the blood, these bacteria become pathogenic. Symptoms include severe bloody diarrhea, anemia, severe electrolyte disturbances, malnutrition, and sepsis.


This gastrointestinal syndrome is seen in individuals who have received acute doses to the gastrointestinal tract in excess of about 8–10 Gy.


With the traditional paradigm of the dependence of severity of response on cell turnover, it was thought for a long time that the effects in the brain, beyond direct cell killing, were minimal. However, we now know that ionizing radiation can otherwise affect the way the brain functions, e.g., through changes in mediation of substance release.


For doses to the brain >~15 Gy, swelling (edema) of the brain, cerebral death (breakdown of the nerve impulse pathways), and generalized shock lead to coma and death. At such high doses, this happens very quickly, with loss of consciousness followed by death within a few hours or days at most, before the wider systemic prodromal reaction can start.


At lower doses, the regulatory functions of the central nervous system (CNS) within the body are affected—either through vascular injury or through changes in how various neurotransmitters are released or by affecting the functioning of the brain itself. After whole-body exposure, the prodromal symptoms in the case of brain effects can also be detected as abnormalities on an electroencephalogram (EEG). This “neurovascular syndrome” tends to manifest around 10 Gy, and the vascular changes lead to hypertension, dizziness, confusion, impaired cognitive function, and neurological deficit later on. For cerebrovascular (and cardiovascular) effects, the assumed threshold is approximately 0.5 Gy.


It should be noted that multiple-organ dysfunction syndrome (MODS) can also occur—this is a clinical syndrome with the development of progressive and potentially reversible physiological dysfunction in two or more organs or organ systems induced by a variety of acute insults, like ionizing radiation.


Cell proliferation is generally slower in the lung than in the hematopoietic or gastrointestinal systems; however, in the weeks and months following initial exposure, pulmonary effects may lead to death due to massive respiratory failure. Damage to the cells lining the alveoli may result in acute inflammation of the lungs (pneumonitis) at doses in the range of 5–15 Gy. This leads to pulmonary edema, which can result in adult respiratory distress syndrome and secondary bacterial and viral pneumonia. Pulmonary failure then occurs due to fibrosis as a direct result of the radiation itself or as a result of infection, between around 6 months and 2 years or more postexposure.


Local radiation injury (LRI) may be defined as a setting of signs and symptoms following local overexposure to ionizing radiation of the skin. Although sometimes called cutaneous radiation syndrome, this term applies better to skin manifestations in the context of ARS.


Skin injuries caused by the high initial dose occur initially as burning, itching, and acute pain coupled with very painful primary erythema (reddening of the skin). This is usually followed by edema, accumulation of fluid in the skin as a result of tissue damage. Cutaneous syndrome is usually characterized by a fairly short latency phase, but if edema occurs within a few hours, this will usually result in very severe ARS. After a few days, hair loss occurs and the skin starts to break down leading to ulceration and necrosis—tissue death occurs. Bacteria may use this as an entrance to the body ultimately followed by sepsis. Skin transplantation or amputation may be needed. As a late effect, telangiectasia and secondary erythema (and associated pain) can be very long lasting.


Evidence of the deterministic effects of radiation on the embryo and fetus is derived almost entirely from animal experiments. Extrapolation of the results of these studies can be used to predict the consequences of radiation exposure in humans.


The effects on the embryo depend on the time of exposure relative to its development. When the number of cells in the embryo is small (i.e., in the first 6 days of pregnancy) and the cells are not yet specialized, damage is frequently seen in animals as failure of the embryo to implant in the wall of the uterus. In humans, the only manifestation of this would be a late or missed menstrual period. However, evidence from in vitro human embryo research has shown that the survival of even one cell in the early embryo before implantation can allow normal development, since all the necessary genetic components are present in each cell of the embryo at this stage of development. The consequences of any of these cells carrying a point mutation are unknown, but the possibility of stochastic (genetic, heritable) effects occurring cannot be excluded.


Because of the lack of direct human evidence, it is useful to look in brief at the animal data. The data taken from animal experiments suggest that threshold doses in humans for radiological protection purposes are in the order of 0.05 Gy for reabsorption of preimplantation embryos; 0.05 Gy for minor skeletal abnormalities; 0.20 Gy for impaired fertility in the female; 0.2 Gy for functional disorders of the central nervous system; and between 0.20 and 0.50 Gy for serious skeletal abnormalities and growth retardation. Such information provides a basis for guidelines to ensure that pregnant women are adequately protected.


Brain development has been particularly well studied in animals. It is when neurons (the information-conducting cells in the brain) are developing and when they are migrating to their predetermined sites in the cerebral cortex that irradiation is most damaging. In humans, this corresponds to between 8 and 25 weeks postconception. Only a very small amount of human data exists. For example, data were published in 1984 from a relatively small study on intellectual disability in children exposed in utero following the atomic bombs dropped on Hiroshima and Nagasaki in 1945.


Intellectual disability is associated particularly with irradiation between the 8th and 15th weeks following conception. From these data, it has been estimated that the excess probability is about 40% per Gy; that is, at a dose of 1 Gy, 40 out of every 100 children exposed would be expected to experience severe intellectual disability. This compares with a background frequency of 0.8%. It is less marked between the 16th and 25th weeks, and no effect has been seen at other times of pregnancy.


The uncertainties at each measured dose point are extremely wide, because of the small numbers. Thus, the presence or absence of a threshold for developmental effects remains highly uncertain. However, school performance and IQ scores have been measured for children irradiated in utero, with a decrease of approximately 30 points at 1 Gy for children irradiated in the 8th to 15th week of pregnancy (but not before or after) [116].


A variety of additional effects can occur, but of particular note, ionizing radiation can also cause nephropathy, which is reduced renal function, leading to progressive scarring kidneys and ultimately failure months to years following exposure.


Other tissue effects may be seen many years postradiation exposure, for example cataract, which has an assumed threshold of approximately 0.5 Gy but which for low dose likely has a very long latency period. This topic is further considered in Chap. 8.


The probability of detecting tissue reactions, characterized by loss of tissue function, in healthy individuals following exposure to radiation is non-existing in some tissues at doses of up to a few hundred mGy. In other tissues, the threshold of detection is above a few thousands of mGy. Above the threshold, the probability of a tissue reaction increases steeply in a sigmoid manner, with the severity of effect increasing linearly with dose. It is important to note that protracting the dose will result in a lower frequency of effects and less severe symptoms at a given dose compared with acute exposure [113, 117].


The range of doses associated with death from these syndromes after acute exposure to low linear energy transfer (LET) radiations is given in Table 2.8.Table 2.8
Range of doses associated with death after exposure to low LET radiations

Whole-body absorbed dose

Principal effect contributing to illness or death

Time of death after exposure

1–6 Gy

Damage to bone marrowa

30–60 days

5–15 Gy

Damage to gastrointestinal tract and lungsb

10–20 days

>15 Gy

Damage to nervous system and shock to cardiovascular system

1–5 days

aDose range considered to result in 50% of an exposed population dying (LD50) without medical treatment is LD50 = 3–4 Gy
bDamage to vasculature and cell membranes, especially at high doses, is an important factor in causing death


In an exposed population, there is a chance of death of approximately 5% of the population (5 persons dying in a population of 100) exposed to about 2 Gy or of about 50% without medical treatment (lethal dose, LD50) within the dose range of 3–4 Gy. Most individuals would be expected to die at doses between about 6 Gy and 10 Gy, unless they receive treatment to prevent infection and bleeding. Above about 10 Gy, death is very likely, even after attempts to stimulate the bone marrow or bone marrow transfusion from a suitable donor. The risk of death thus also depends on the number of exposed individuals, and the available expertise and facilities for appropriate treatment, as discussed further in Chap. 7.


High exposures do not always prove fatal, especially if the irradiation is nonuniform so that sufficient vital bone marrow stem cells are spared. Recent advances in immunology and in the administration of growth factors or cytokines to accidentally irradiated persons may rescue the bone marrow so that the hematopoietic syndrome might no longer be the limiting lethal condition. Matched stem cell transplantation is an alternative, provided that such stem cells are available at short notice. Death would then depend on whether damage to the lungs or intestine was sufficient to cause fatal pneumonitis or breakdown of the gut wall.


Table 2.9 shows proposed values of the LD50 and/or ED50 and 1% thresholds for a selection of the most important conditions of ARS (Table 2.10).Table 2.9
Parameters for acute mortality (various sources including ICRP, 2007)

Threshold (Gy)

LD50 (Gy)

1%

Bone marrow syndrome

First aid only

3.0

1.5

Supportive treatment

4.5

2.2

Pneumonitis

10.0

5.5

Gut syndrome

15.0

10.0
Table 2.10
Parameters for acute morbidity (various sources including ICRP, 2007)

Threshold (Gy)

ED50 (Gy)

1%

Prodromal

Vomiting

2

0.5

Diarrhea

3

0.5

Lung fibrosis

5

2.7

Skin burns

20

8.6

Hypothyroidism

60

2.3

Cataract

3

1.3

Temporary sterility

Males

0.7

0.5

Females

3.5

0.8


Cancer develops in tissues through the accumulation of various mutations over several conceptual stages [118]. Initiation of the process can occur following exposure to various environmental agents including radiation, but further changes in neoplastic development require a complex interaction between various factors in the host and environment. For this reason, it is not possible to attribute causal relationships between a particular environmental agent (in this case, radiation exposure) and cancer in individuals [119]. Instead, attribution is made for increased cancer incidence in an exposed population over a known baseline rate either pre-exposure or in a nonexposed population. This attribution is expressed through risk estimates.


Present risk estimates for cancer following radiation exposure are based on a number of epidemiological studies, most notably the Life Span Study (LSS) of the Japanese atomic bomb survivors. The study is a gold standard against which the results of other studies on long-term radiation effect on humans are evaluated. In the latest analysis of mortality patterns between 1950 and 2003 [120] of the 50,234 deceased cohort members with dosimetric measurement data, there were 10,929 deaths from solid cancers and 695 deaths from hematological malignancies. Of these, 527 (4.8%) solid cancer deaths can be attributed to radiation exposure from the bomb in 1945. A dose-dependent increase in the rate of solid cancer deaths can be observed (Table 2.11).Table 2.11
Observed and excess death from solid cancer and non-cancer diseases (adapted from Ozasa et al. 2012)

Colon dose (Gy)

Number of subjects

Number of deaths

Number of excess cases

Attributable fraction (%)

<0.005

38,509

4621

2

0

0.005−

29,961

3653

49

1.3

0.1−

5974

789

46

5.8

0.2−

6536

870

109

12.5

0.5−

3424

519

128

24.7

1−

1763

353

123

34.8

2+

624

124

70

56.5

Total

86,611

10,929

527

4.8


In the analysis of solid cancer incidence among the LSS population between 1958 and 2009 [53], the latest follow-up data of a cohort of 105,444 people who were alive without known history of cancer was presented. For a person exposed at age 30, the excess relative risk (ERR) for any cancer by the age of 70 was estimated to be 0.50 per Gy without adjusting for smoking. The dose-response was linear with an estimated ERR of 0.64 per Gy for females, but for males, a linear quadratic fit was observed instead, with ERR of 0.20 per Gy at 1 Gy and 0.010 per Gy at 0.1 Gy (Fig. 2.26).

2 line graphs. a plots excess relative risk versus attained age for smoking E R R. 4 lines, 2 follow an increasing trend, and 2 remain stable along the increasing lines. b plots cases per 10,000 person-year versus attained age for the solid cancer rate. 6 lines follow an increasing trend.

Fig. 2.26
Smoking effects on solid cancer baseline rates. (a) Smoking ERR as a function of attained age for males (black curves) and females (gray curves). The solid curves represent lifelong smokers, while the dashed curves represent past smokers from the age at which they quit (shown are male past smokers quitting at age 50 years and female past smokers quitting at age 55 years). (b) Total smoking risk for current smokers, past smokers, and those who never smoked (thin solid curves) for males and females. The curves represent typical smoking histories. Male smokers started at age 20 years and smoked 20 cigarettes per day, while female smokers started at 30 years and smoked 10 cigarettes per day (reproduced with permission from Grant et al. © 2017 Radiation Research Society) [53]


2 line graphs. a plots excess relative risk versus attained age for smoking E R R. 4 lines, 2 follow an increasing trend, and 2 remain stable along the increasing lines. b plots cases per 10,000 person-year versus attained age for the solid cancer rate. 6 lines follow an increasing trend.


At the moment, 0.1 Gy is the lowest dose for which the overall cancer risk from radiation exposure can be reliably estimated. Uncertainties from various factors such as limited statistical power, dosimetric uncertainties, and confounders begin to grow increasingly large and mask any possible effects in lower dose ranges or site-specific risk estimation. Unless properly addressed, these uncertainties distort the results and lead to erroneous estimation of risk [119].


Together with radiation-induced cancers, the hereditary effects of radiation are stochastic effects. By comparison with cancer, induced hereditary diseases are considered to be a minor component of the total stochastic disease risk due to radiation exposure of an individual or of the population generally.


There is little direct human evidence of hereditary effects; however, it is clear that ionizing radiation can cause mutations of the types seen in hereditary effects.


Multifactorial diseases are an additional class of effect, which combine heritable aspects in addition to influence from environmental factors. These include congenital abnormalities present at birth or chronic conditions, which appear later in life (Box 2.24).


There are three classes of Mendelian-type gene mutations, where genes are inherited from each parent:(a)
Dominant conditions, where even in the heterozygote (a person inheriting one mutant and one normal gene), the abnormality is seen in the individual. Their effects in the homozygote (double dose of the mutant gene) are usually more severe, if not lethal. An example of a dominant gene condition is Huntington’s chorea (HC), which is characterized by nerve cell damage and changes in physical, emotional, and mental state. HC is caused by a faulty gene on chromosome 4.
 (b)
Recessive conditions, which have an effect only when present in the homozygote (two genes with the same, disease-linked, mutation). Recessive disorders are usually rare, as the mutation would need to be inherited from both parents. However, some recessive genes even when present in a single dose, i.e., heterozygote accompanied by a dominant normal gene, do still confer slight deleterious effects. An example of a recessive gene disorder is cystic fibrosis, which is caused by mutations on a gene located on chromosome 7.
 (c)
Sex-linked conditions, which involve genes located on the X chromosome. A large proportion of mutations that are inherited are related to the X chromosome. Since there is only one X chromosome in males, mutant genes here act as dominant genes in males who suffer, whereas they are masked in the female with two X chromosomes who act as carriers. Mutations in these genes will exert their effect in females only when present in homozygotes and therefore appear as a recessive condition. Half the male offspring of a carrier mother will suffer and half her female offspring will be carriers. Examples of sex-linked conditions are color-blindness and hemophilia.


Dominant conditions, where even in the heterozygote (a person inheriting one mutant and one normal gene), the abnormality is seen in the individual. Their effects in the homozygote (double dose of the mutant gene) are usually more severe, if not lethal. An example of a dominant gene condition is Huntington’s chorea (HC), which is characterized by nerve cell damage and changes in physical, emotional, and mental state. HC is caused by a faulty gene on chromosome 4.


Recessive conditions, which have an effect only when present in the homozygote (two genes with the same, disease-linked, mutation). Recessive disorders are usually rare, as the mutation would need to be inherited from both parents. However, some recessive genes even when present in a single dose, i.e., heterozygote accompanied by a dominant normal gene, do still confer slight deleterious effects. An example of a recessive gene disorder is cystic fibrosis, which is caused by mutations on a gene located on chromosome 7.


Sex-linked conditions, which involve genes located on the X chromosome. A large proportion of mutations that are inherited are related to the X chromosome. Since there is only one X chromosome in males, mutant genes here act as dominant genes in males who suffer, whereas they are masked in the female with two X chromosomes who act as carriers. Mutations in these genes will exert their effect in females only when present in homozygotes and therefore appear as a recessive condition. Half the male offspring of a carrier mother will suffer and half her female offspring will be carriers. Examples of sex-linked conditions are color-blindness and hemophilia.


A low dose of irradiation can be defined as acute and chronic. An acute low dose is defined as less than 0.1 Gy (100 mGy), while a chronic low dose is defined as less than 6 mGy/h (or Sv equivalent). In this low-dose range, there are a variety of phenomena that dominate the dose-response relationship and lead to nonlinear and unpredictable outcomes.


A key finding in low-dose radiobiology is that the effects seen are not directly proportional to the dose received. Rather, there are a number of factors such as genetic background, age, gender, and lifestyle, which can modify the outcome. After higher doses, DNA strand breaks are the predominant cause of radiation effects, and these are more directly related to dose deposited in the tissue or cells. Figure 2.27 depicts the usual dose-response relationship with the low-dose region shown as of uncertain outcome. The expanded section shows the variety of factors and outcomes which can be expected.

A graph of stochastic effect versus effective dose plots low dose extrapolations and deterministic effects. A magnified graph of the low dose extrapolations plots adverse effects versus dose for uncertainty, transitional, and hormetic zones.

Fig. 2.27
Usual dose-response relationship with the low-dose region shown as of uncertain outcome. (Figure from Kugathasan and Mothersill, 2022 [121])


A graph of stochastic effect versus effective dose plots low dose extrapolations and deterministic effects. A magnified graph of the low dose extrapolations plots adverse effects versus dose for uncertainty, transitional, and hormetic zones.


Mechanisms involved in non-targeted effects are described in Sect. 2.8.2, and low-dose hypersensitivity, hormesis, and adaptive response mechanisms are described in Chap. 3. Global mechanisms underlying LDR are mentioned here and include production of oxidative stress, mitochondrial and membrane channel changes, signaling to neighboring cells, release of exosomes carrying modified cargos, and changes in the proteome. It is important to recognize that these changes may be proactive damage responses and not harmful per se. Change does not necessarily equate with harm.


It is quite common that while a high dose/amount/rate of some medication or procedure is detrimental, a low dose is beneficial. Classical well-known examples include physical exercise (as opposed to forced labor), immunization (as opposed to virulent infection), and—directly related to biologically active radiation—controlled sun tanning (as opposed to sunburns and skin cancer caused by overexposure). Therefore, low-dose radiation effects may well be different from the effects of high doses. Actually, people have been using ionizing radiation for centuries: already, Herodotus and Hippocrates described healing properties of what we now know as radon springs. Radon treatment is considered to be a legitimate tool by mainstream medicine in Europe, especially for treating arthritis and other inflammatory diseases [122]. During the past four decades approximately, there has been a growing body of biological evidence regarding low-dose radiation effects. This evidence is concurrent with the shift in radiobiology from a DNA-centric view on radiation damage to a more systemic view that incorporates multi-level protection and nonlinear systems—adaptive response [123].


There is emerging evidence that low doses induce cellular and intercellular changes, which can lead to adaptive metabolic alterations. Adaptive responses against accumulation of damage—also of non-radiogenic origin—were also discovered [124]. Many studies demonstrated that radiation effects are far from linear [125]. Moreover, experimental, epidemiological, and ecological studies have shown that low doses of ionizing radiation can be beneficial to health [126, 127].


Beneficial low-dose effects of an agent that is harmful in high doses are called hormesis. Back in 1884, Hugo Schulz observed that low doses of many toxic agents, mercury and formaldehyde for example, enhanced the vitality of yeast cells. The term “hormesis” was introduced by John Ehrlich (also in the context of chemical toxicity) in 1942 [128]. The term “hormesis” is applied now to any kind of biphasic dose-response, i.e., when low doses of some agent are beneficial while higher doses are detrimental [128]. Physical exercise (as opposed to hard labor) is a typical example of hormetic response. According to the present knowledge, “hormetins”—agents inducing hormesis—include but are not limited to heat and oxidative stress, various food components, micronutrients, intermittent fasting, calorie restriction, etc. [129]. Radiation hormesis is the most thoroughly investigated among all hormesis-like phenomena.


Speaking about radiation hormesis, we should point out two somewhat different uses of the terms “hormesis” and “low dose.” Since radiation carcinogenesis is often considered as the single most important health hazard of ionizing radiation, radiation hormesis is usually understood in the narrow sense that low radiation doses may suppress cancer. In this narrow sense, curing arthritis or pneumonia is not viewed as a hormetic effect. Accordingly, there are two quite different meanings of the term “low dose.” In the context of radiation protection and many fields of radiobiology, “low dose” is understood to be 100 mGy or less as defined above. However, in the field of radiation therapy, the daily dose fraction is typically 2000 mGy and 6 weeks of therapy amounts to a total dose of 60,000 mGy—hence a single 1000 mGy dose to treat pneumonia may be regarded as a low dose [130].


Low-dose hyper-radiosensitivity (HRS) and induced radioresistance (IRR) describe a type of survival curve which has a dose range usually below 500 mGy acute dose, where the dose-response is significantly more radiosensitive than the overall fit to the higher dose points would suggest (see Fig. 2.28). The phenomenon is seen in a large variety of both tumor and normal cell lines and has been detected in human skin from patients [131]. It is seen following acute and fractionated irradiation meaning that it is likely to be relevant for radiotherapy and diagnostic radiology/medical imaging. It was first described by Lambin et al. (1993) and Marples and Joiner (1993) [132, 133]. The HRS phenomenon results in a significant reduction of clonogenic cell survival, increase in chromosome breaks, micronuclei, unrepaired DSB, or gene mutations after a single low dose in the range of 100–800 mGy. The maximal HRS effect is generally obtained at 200 mGy and corresponds to a biological effect equivalent to a dose 5–10 times higher. The mechanism of HRS/IRR is discussed in Chap. 3 (Box 2.25).

A line graph plots survival versus dose. The H R S line begins (0, 1), reaches a peak at (0.4, 0.82), follows a decreasing trend, and ends at (2.5, 0.3). The L Q line begins at (0, 1) and ends at (2.5, 0.3). Values are estimated.

Fig. 2.28
Low-dose hyper-radiosensitivity (HRS) can be observed in a typical survival curve. The dashed line represents the linear-quadratic (LQ) model, while the solid line shows the induced repair (IR) model


A line graph plots survival versus dose. The H R S line begins (0, 1), reaches a peak at (0.4, 0.82), follows a decreasing trend, and ends at (2.5, 0.3). The L Q line begins at (0, 1) and ends at (2.5, 0.3). Values are estimated.


Low-dose hypersensitivity: Increased sensitivity to low-dose radiation which is not apparent at doses above 0.5 Gy.


Adaptive response: The ability of a low first dose of radiation to “protect” against the effects of a subsequent high dose.


Hormesis: Beneficial effects seen after low-dose exposure compared to unirradiated controls.


Bystander effects: One of the non-targeted effects defined as radiation-like effects seen in cells which did not get any energy deposition but which received signals from irradiated cells.


Genomic instability: Detection of non-clonal chromosomal damage or other DNA changes in distant progeny of cells which are genetically normal in the first postirradiation mitosis.


Lethal mutations: A form of genomic instability, detected as a permanently reduced plating efficiency of progeny cells which survived irradiation.


Non-DNA-targeted effects (NTE) refer to effects in cells, tissues, organs, or individuals which have not themselves received any radiation energy deposition but are in receipt of signals from irradiated entities. They include bystander effects, abscopal effects, clastogenic effects, genomic instability, and lethal mutations. Sometimes, adaptive responses and low-dose hypersensitivity are included as NTE, but although they can be induced by signaling in bystander cells, they are not strictly speaking NTE as they occur in directly exposed cells. Box 2.25 defines the terms. Box 2.25 shows the different effects observed in bystander cells and progeny cells compared to those seen in directly irradiated cells. The lists are the same showing that signaling can induce in bystanders most of the effects associated with low-dose direct radiation exposure. An NTE dose-response saturates in the low-dose region (Fig. 2.28). In general, increasing the dose beyond 0.5 Gy produces no additional NTE.


The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) defines bystander effect as a radiobiological effect that is transmitted from irradiated cells to neighboring unirradiated cells, generating biological alterations in the receiver cells that can influence the radiation-associated cancer risk [134]. As a communicative effect, bystander effects occur mainly at the primary site over a few millimeters or cellular diameters. This effect is mediated by the secretion of soluble factors or by signaling through gap junctions as well as through networks involving inflammatory cells of the microenvironment [135].


The term radiation-induced bystander effect (RIBE) is described as the ability of irradiated cells to transport manifestations of damage to other cells which were not directly targeted by irradiation. An irradiated cell sends out signals and induces response in nonirradiated neighboring cells. The intensity of the bystander response in nonirradiated cells is not necessarily proportional to the dose delivered to the irradiated cells and can occur even at low doses. The RIBE is highly dependent on the cell tissues concerned and the irradiation sources (such as radiation doses, LET, dose rates) and can influence the nature of the bystander factors secreted by irradiated cells, the intensity of the bystander response in nonirradiated cells, and the timing of the events in the bystander signaling [136]. This amplification can cause similar radiation-induced effects in cells not directly exposed to radiation and exhibit the heritable changes that include cellular damage, DNA damage, mutations, chromosomal aberrations, chromosomal instability, senescence, apoptosis, genomic instability, micronucleation, oncogenic transformations, etc. [137–139].


Some RIBEs can have deleterious effects, which involve the type of cell inducing the bystander signal after irradiation and the type of cells receiving these signals. Such effects can be determined by intercellular communication and level of amplification of original consequences of the event. Knowledge of the mechanism(s) by which non-targeted bystander effects are activated is still in its infancy and not well understood; however, it is believed that multiple pathways are involved in this phenomenon and also different cell types respond differently to bystander signaling.


RIBE is believed to be an incredibly complex phenomenon considering the involvement of sheer number of proteins, inorganic molecules, and cofactors. This effect encompasses a number of distinct signal-mediated effects (Figs. 2.29 and 2.30). Lately, communication of bystander signals between adjacent cells connected by gap junctions has been studied extensively. Signaling molecules are propagated through direct intercellular communication via gap junctions or through diffusible secretion in the surrounding environment of irradiated and bystander cells. Exosomes and signaling mRNAs also play a potential role in mediating bystander effect [140]. Exosomes can be released by bystander cells exposed to radiation-induced UV biophoton signals [141, 142], while miRNAs have a pivotal role in intercellular signaling between irradiated and bystander cells [143]. ROS and secondary messengers (such as nitric oxide), protein kinase, as well as cytokines (such as TGF-β and TNF-𝛼) are also considered to be involved in RIBE. Additionally, irradiated dying cells (predominantly from apoptotic rather than necrotic cells) release cell-free chromatin (cfCh) particles, which can integrate into genomes of surrounding healthy cells to induce extensive genomic instability (DNA damage) and inflammation [144]. In the absence of macrophages, cfCh shows direct involvement in the activation of H2AX by bystander cells. The bystander effect can be observed in different cell types with different endpoints.

A diagram of gap junctions, calcium flux, and binding of exosomes, bystander factor molecules, and ultraviolet photons to receptors activating p 53, R O S, and nitric oxide and leading to their propagation.

Fig. 2.29
Probable players driving the non-targeted effects of radiation

An illustration describes the examples and roles of the immune system, free radicals, modified gene expression, and epigenetic modulators in radiation-induced bystander effects.

Fig. 2.30
Factors involved in RIBE (created with BioRender)


A diagram of gap junctions, calcium flux, and binding of exosomes, bystander factor molecules, and ultraviolet photons to receptors activating p 53, R O S, and nitric oxide and leading to their propagation.


An illustration describes the examples and roles of the immune system, free radicals, modified gene expression, and epigenetic modulators in radiation-induced bystander effects.


The term abscopal or out-of-field effect is an in vivo phenomenon in normal tissue that describes the occurrence of radiation-like damage in organs that have never been irritated. In other words, abscopal effects are bystander effects in vivo. Abscopal effects are known to occur after exposure to high or low doses of ionizing radiation in vivo and are often observed after high doses of targeted partial-body radiotherapy [145, 146]. The mediation of the effect is attributed to systemic factors such as the blood or the endocrine system [136, 147–149]. The immune system is also thought to play an important role. Experiments show that high levels of macrophage activation and neutrophil infiltration in mice are a consequence of radiation-triggered recognition and elimination of apoptotic cells [150]. The abscopal effect on normal tissue differs conceptually from the abscopal effect on tumors, which is often described in radiation oncology. The abscopal effect on tumors refers exclusively to systemic antitumor immune responses induced by radiotherapy alone or in combination with immunotherapy to only part of the tumor load. These antitumor immune responses are capable of completely eliminating primary tumors and unirradiated metastases in patients. For more information about the abscopal effect on tumors, see Chap. 5.


Clastogenic factors (CFs), potential biomarkers of a prooxidant state, are composed of endogenous lipid peroxidation products, cytokines such as necrosis factor alpha, unusual nucleotides, and other oxidants with chromosome-damaging properties. They are frequently noticed in the plasma of patients exposed to radiation [151]. Subsequently, it has been shown that CFs are not specific for irradiated subjects (Table 2.12), but are found in a variety of pathological conditions accompanied by oxidative stress. In both conditions, they can be considered as biomarkers of oxidative stress [152] as well as risk factors for carcinogenesis.Table 2.12
Clastogenic factors (irradiation)

Clastogenic factors (irradiation)

Therapeutic and accidental exposure

(Goh and Summer, 1968; Hollowell and Littlefield, 1968)

Exposure at Chernobyl

(Emerit et al., 1994; Emerit et al., 1997)

A-bomb survivors

(Pant and Kamada, 1977)

PUVA treatment for psoriasis

(Alaoui-Youssefi et al., 1994; Emerit et al., 2011)


The non-targeted effect is a dynamic complex response of epigenetic dysfunctions, DNA damage, and cell death in nonirradiated tissues as consequences of secretion of clastogenic factors—“chromosome breakage factors” from irradiated cells. The formation of these breakage factors (CF) with their chromosome-damaging actions is mediated by the superoxide anion radicals, which are regularly inhibited by exogenous superoxide dismutase (SOD). These free radicals are an initiator of a series of events leading to formation of clastogenic materials. In vitro experiments provide strong evidence for the role of O2 in those cells exposed to superoxide-generating systems, such as the xanthine–xanthine oxidase reaction, a phorbol 12-myristate-13 acetate (PMA)-stimulated photodynamic reaction. The supernatant of these cells contains CF, while cell-free systems do not lead to CF formation. Studies of CFs originating from observations on the plasma from irradiated persons were shown to induce chromosomal aberrations when co-cultured with cells from unexposed persons (Fig. 2.31). However, this phenomenon is common in a large number of health defects as well [153].

2 flow diagrams depict the assays to screen for clastogenic factors, components of clastogenic factors, and possible mechanisms of action of clastogenic factors.

Fig. 2.31
Clastogenic factors (created with BioRender)


2 flow diagrams depict the assays to screen for clastogenic factors, components of clastogenic factors, and possible mechanisms of action of clastogenic factors.


TNF-𝛼 and inosine triphosphate (ITP) stimulate the production of superoxide by monocytes and neutrophils. The lipid peroxidation product, 4-hydroxynonenal, inhibits superoxide production; however, it has the capacity to decrease the activity of DNA polymerases by inactivating their sulfhydryl groups leading to genotoxic effects. Formation of CF often damages/changes the chromatid structure; which indicates that they are not immediate and occur late in the S phase or in the G2 phase of cell cycle where they have duplicated their chromatids. These chromosome-damaging effects can be detected by classical cytogenetic techniques.


Ionizing irradiation is known to have mutagenic and carcinogenic potential for the exposed host as it induces chromosomal aberrations in directly exposed cells.


Genomic instability (GI) is a hallmark of cancer cells, which includes variations of increased frequencies of base pair mutation, microsatellite instability (MSI), and chromosome instability (CIN) [154]. GI is a complex multiple-gene event marked during the development of some but not all cancers and also induced effectively by ionizing radiation. Radiation can provoke cellular communications eliciting a cascade of cellular events, which results in the destabilized genome in irradiated as well as unirradiated (bystander) cells. Radiation-induced genomic instability (RIGI) is observed in the progeny of irradiated cells as a delayed and elevated stochastic appearance of de novo chromosomal aberrations, gene mutations, and reproductive cell death [137, 155]. The effects of instability occur at a stable rate and are persistent in the postirradiation survivors for many generations.


Radiation-induced bystander effects are also involved in RIGI [156] due to contribution of indirect (by stimulating the reactive intermediates over many generations) and delayed effects (delayed DNA breakage, delayed reactivation of p53, delayed induction of various phenotypes) to cellular outcomes after radiation exposure. More detailed molecular studies on RIGI can provide deep insights into radiation-induced carcinogenesis (Box 2.26).


Genomic instability (GI), a characteristic of most cancers, is a complex multigene event and is often expressed by the appearance of chromosome aberrations many generations later.

Microsatellite instability or chromosomal instability due to mutations in DNA repair genes or mitotic checkpoint genes is the underlying basis for GI in hereditary cancers.

In sporadic (non-hereditary) cancers, GI occurs at least at the early stages of cancer development.


Genomic instability (GI), a characteristic of most cancers, is a complex multigene event and is often expressed by the appearance of chromosome aberrations many generations later.


Microsatellite instability or chromosomal instability due to mutations in DNA repair genes or mitotic checkpoint genes is the underlying basis for GI in hereditary cancers.


In sporadic (non-hereditary) cancers, GI occurs at least at the early stages of cancer development.


Biological effects of IR-induced GI are transmitted over several generations after irradiation via the progeny of surviving cells with delayed phenotypic expression, but not uniformly. Delayed manifestations of induced GI include delayed cell death, chromosomal instability, and mutagenesis.


The incidence of GI is significantly higher than that of conventional gene mutation, which eventually induces delayed reproductive death or delayed lethal mutations and increases the frequency of giant cells, micronuclei, senescence-like growth arrest, apoptosis, or necrosis in the progeny of surviving cells [157], suggesting that one of the potential initiators of RIGI is delayed cell death.


Exposure to sparse LET or dense LET radiation produces non-clonal chromosome aberrations (NCCAs), a highly significant feature for delayed chromosomal instability, genome heterogeneity, and complexity, in clonal descendants or stem cells that result in transmission of chromosome-type and chromatid-type aberrations to their progeny after irradiation [158].


Radiation may induce a type of GI in cells which results in an increased rate of spontaneous mutation that persists for many generations of cells. Clonal populations of cells surviving radiation exposure indicate such instability in a fraction of irradiated cells, which can persist longer over generations. Subpopulation of genetically unstable cells may arise from irradiated cells with a high frequency of even featureless minisatellite mutations [159], signifying the delayed appearance of certain mutational events in the progeny of irradiated cells.


The mechanism of perpetuation in progeny populations is thought not only to be epigenetic but also to involve an excess generation of ROS over the course of time, cell-to-cell gap junction communication, dead and dying cells in the unstable population, and/or secreted factors from unstable cells (Fig. 2.32).

An illustration describes several factors and their corresponding effects during the initiation, perpetuation, and induction of delayed effects in RIGI.

Fig. 2.32
Mechanisms involved in radiation-induced genomic instability (Created with BioRender)


An illustration describes several factors and their corresponding effects during the initiation, perpetuation, and induction of delayed effects in RIGI.


DNA-damaging agents (such as X-rays, IR, restriction endonuclease Hinfl), radiomimetic drugs (bleomycin and neocarzinostatin), DNA DSBs, and DNA damage at the site of their decay are considered as effective initiators of RIGI. In some cases, sufficiently small or powerful environmental cues can directly exert their impact upon a cell’s DNA, which is a critical target for RIGI. DNA strand breaks, the most lethal lesions induced by IR, activate a number of cellular DDR signaling cascades such as the activation of DNA damage-sensing and early transduction pathways, cell cycle arrest, and DNA repair. To a certain degree, it could convert the initial sites of DNA DSBs to unforeseen structures and results in reorganized chromatin domains and a disrupted genome structure, evident as a mutation induction. Generation of gross chromosomal rearrangements, or multiple intrachromosomal aberrations, or DNA damage signatures is accountable for the initiation of GI.


RIGI is transmitted through many generations after irradiation, suggesting that the memory of unrepaired DNA damage can be perpetuated over time by a number of processes involving ROS, communication through cell-to-cell gap junction, unstable dying cell population, and/or secreted factors from unbalanced cells. RIGI appears to be independent of the p53 status of the irradiated cells, but a number of genetic factors influence the expression of the unstable phenotype.


Radiation-induced DNA DSBs could cause nonlethal, “potentially unstable chromosome regions (PUCR)” and altered chromatin architecture within the nucleus through DNA repair, which are transmissible through the progeny of surviving cells for many generations after irradiation [160]. Indeed, though PUCRs are potentially unstable, they are capable of persisting for prolonged periods through bridge-breakage-fusion (BBF) cycle [161] and thus could be the regions susceptible for causing delayed DNA breakages [162], inducing telomere instability and delayed cell death.


PUCRs can possibly be reactivated by large deletions or abnormal positioning of telomeres, loss of nuclear matrix-attachment regions (MARs), translocations of the chromosomes, distorted nucleosome, and altered nuclear architecture, leading to upregulating or silencing gene transcription, delayed p53 reactivation, and delayed manifestation of GI in the progeny of surviving cells (Table 2.13).Table 2.13
PUCR effects

PUCRs near the telomeres

PUCRs in the interstitial regions

Could cause telomere instability, chromosomal aberrations involving telomeric sequences

Interstitial telomeric sequences are potentially more unstable than non-telomeric sequences

Less detrimental to the cell, as it would result in loss of less genetic material

More destructive as it may lose chromosome fragments or large deletions

Lead to genomic instability across many generations

Lead to different consequences in the long-term progeny


IR-induced DSB repair defects predominantly persuade various delayed phenotypes, indicating that delayed DNA damage is associated with delayed phenotypes. It is expected that delayed DNA damage arising in the progeny of surviving cells activates the uniquely sensitive tumor suppressor p53 protein, a multifunctional, highly regulated, and promoter-specific transcription factor. It is known to depend on the kinase ATM, which acts via the downstream kinases Chk2/hCds1 and mediates phosphorylation of various nuclear proteins, including p53. Stabilized and activated p53 protein transactivates a variety of downstream gene products, which direct either a prolonged cell cycle arrest in G1, senescence-like growth arrest or an apoptotic pathway.


RIGI enhances the accumulation of genomic alterations, resultant of delayed unscheduled DNA breakage, which triggers deferred activation of p53 in the progeny of irradiated cells; however, RIGI can be induced in all cell types regardless of the presence and status of a p53 function. Reactivated PUCRs and delayed DNA breakage are directly or indirectly involved in the delayed expression of instability phenotypes (Box 2.27).


Radiation-induced genomic instability (RIGI) is characterized by an elevated and persistent rate in the accumulation of de novo genetic alterations in the progeny of irradiated cells after the initial insult.

Delayed manifestations, e.g., chromosomal instability, mutational events, and cell death, are the potential initiators of RIGI for multiple generations following irradiation or exposure to DNA-damaging agents.

Unirradiated progeny cells display phenotypic changes due to RIGI at delayed times after radiation of the parental cells.

Along with changes in DNA, epigenetic aberrations may be involved in RIGI, suggesting that epigenetics may also be the link to understand the initiation and perpetuation of GI.


Radiation-induced genomic instability (RIGI) is characterized by an elevated and persistent rate in the accumulation of de novo genetic alterations in the progeny of irradiated cells after the initial insult.


Delayed manifestations, e.g., chromosomal instability, mutational events, and cell death, are the potential initiators of RIGI for multiple generations following irradiation or exposure to DNA-damaging agents.


Unirradiated progeny cells display phenotypic changes due to RIGI at delayed times after radiation of the parental cells.


Along with changes in DNA, epigenetic aberrations may be involved in RIGI, suggesting that epigenetics may also be the link to understand the initiation and perpetuation of GI.