European Nuclear Medicine Guide
The biological effects of ionising radiation are based on physical and chemical reactions that follow the so-called radiobiological chain of action. The physical phase takes place in fractions of a second (10-15-10-12 s) and is characterised by the absorption of ionising energy. The resulting radicals react with other radicals/atoms/molecules during the chemical phase (10-12-10-5 s). These chemical events can cause biological damage at various levels with the biological phase persisting until the end of life. At the molecular level, DNA damage is predominantly implicated in determining the cellular outcome. The latter can be cell death, senescence, or carcinogenesis, for example. At the tissue level, deterministic effects resulting from cell destruction give rise to acute and chronic adverse reactions. Finally, at the systemic level, the occurrence of bystander and abscopal effects and the manifestation of adaptive response mechanisms can be observed after ionising radiation. Modifying factors for these biological radiation effects are for example the oxygen level, the dose rate and fraction, and the radiation type and energy. [1]
When ionizing radiation crosses living matter, it deposits energy along its path. If the energy deposition is higher than the binding energy of an orbital electron, the latter leaves the atom which leads to the formation of ion pairs. This process is called ionization. At lower energies, the orbital electrons can gain enough energy to elevate them to higher energy shells, and the atom in the cell is raised from the ground state to a higher energy level. This process is called excitation. Both processes may convert atoms and molecules into free radicals with very reactive unpaired electrons. Radicals can also react with neighbouring molecules after which chain reactions may occur. It is widely accepted that DNA within the cell is the critical target for radiation damage that can occur after direct or indirect action. In direct action, the atoms of the DNA itself may be ionized or excited leading to a chain of physical and chemical events which may eventually produce biological damage. Alternatively, the energy can also be deposited in cellular water after which a complex series of chemical changes occurs. This process is called radiolysis and must be seen as the indirect action of ionizing radiation in living cells. During radiolysis, free hydroxyl and other highly reactive radicals are produced. Despite their short existence (10-11-10-13 s), they are capable of diffusing a few micrometres to reach and damage cellular DNA. Moreover, oxygen can modify the reaction by enabling creation of other free radical species with greater stability and longer lifetimes. [2,3]
Radiation exposure may lead to a wide range of lesions in DNA and proteins. These lesions comprise single-strand breaks (SSBs), double-strand breaks (DSBs), base damage, protein-–protein cross-links, and protein-–DNA cross-links. Low LET radiation causes approximately the same amount of base damage as SSBs and only a few DSBs. Nevertheless, DSBs must be seen as the most extensive expressions of radiation damage, since it was shown that both at high and low doses, unrepaired and misrepaired DSBs correlate with radiosensitivity and survival. There is also experimental evidence for a causal link between the generation of DSBs and the induction of chromosomal translocations with carcinogenic potential.[4]
The repair of DNA lesions is essentially carried out by enzymatic reactions. A diverse array of DNA repair mechanisms has been identified. There exist a variety of DNA repair mechanisms, but most of them depend on the presence of double-stranded DNA with two complementary DNA molecules/copies of the genetic information, one on each strand of the double helix. If the damage is limited to base damage and SSBs, those mechanisms include base excision repair, nucleotide excision repair, and mismatch repair. The repair of DSBs is more complex and involves non-homologous end joining (NHEJ) and homologous recombination. If DNA repair fails, cellular injury may manifest as mutation, chromosome aberrations, transformation of cell morphology, reproductive failure, or cell death. [5]
It is observed that radiosensitivity is a function of the metabolic state of the tissue being irradiated (law of Bergonié and Tribondeau, 1906): radiation sensitivity is (1) indirectly proportional to the degree of cell differentiation and (2) directly proportional to the reproductive activity. Low degree of differentiation, high proliferation rate for cells and high growth rate for tissues result in increased radiosensitivity. This is why a fetus is considered to be more sensitive to radiation exposure than a child or a mature adult.
When a tissue is irradiated, the response is determined principally by the amount of energy deposited per unit of mass. The International Commission on Radiological Protection (ICRP) defines the absorbed dose (D) as the mean energy absorbed per unit of mass of tissue or organ (e.g. 1 Gy = 1 J absorbed by 1 kg body mass). However, the density at which energy is deposited as a charged particle travels through matter by a particular type of radiation is an important element in determining the biological outcome. This density is denoted as the Linear Energy Transfer (LET) (keV/µm) and must be regarded as the derivative dE/dx, where dE is the average energy locally imparted to the medium by a charged particle of specified energy in traversing a distance of dx. X-, γ- and β-rays (electrons) have LET values between 0.2 and 10 keV/µm and are considered as low-LET radiation. Protons, neutrons, α-particles and Auger electrons have higher LET-values between 10 and 100 keV/µm and are therefore considered as high-LET radiation. [6] It is evident that high-LET radiation primarily leads to direct DNA ionisation and thus causes biological effects. Low-LET radiation, on the other hand, mainly causes indirect DNA damage through radiolysis. [7] The direct action of ionizing radiation is predominantly for high-LET radiation, whereas indirect action is the predominant reaction after exposure of cells with low-LET radiation.
Their effect of low-LET radiation, such as photons and electrons, is characterized by cell survival curves with a pronounced shoulder and a relatively shallow slope in the subsequent linear portion. Radiation with a higher LET shows a much higher ionization density. Hits occur in close succession. Their spacing is so small that several or many are located within the same cell. The cell survival curves show a narrow or no shoulder. The linear part declines steeply.
Figure x: Representative survival curves for low-LET particles and high-LET radiation. The vertical axis (surviving fraction) is in a logarithmic scale whereas the Dose is displayed linear
Cellular radiosensitivity in response to low-LET radiation strongly depends on the oxygen concentration (pO₂). Oxygen stabilizes the radicals produced as a result of radiation exposure. This prolongs their lifetime and increases the cell damage they cause. Oxic cells exhibit a shallower shoulder and a steeper decline in the linear portion of the cell survival curves.
Figure x: Effect of oxygenation on cell survival
This effect on cell survival can be quantified. For a defined survival rate, 0.01 (= 1%) in Figure x, dose values Doxic and Danoxic are determined from the respective intersection points with the survival curves, and the oxygen enhancement ratio (OER) is calculated as follows:
OER=Danoxic / Doxic
In mammalian cells, the OER value ranges between 2.5 and 3.5. Anoxic cells exhibit reduced, while oxic cells exhibit increased radiosensitivity.
In radiobiology the LET is limited in its use, because the LET value changes as the particle loses energy in its passage through tissue. For this reason, the term Relative Biological Effectiveness (RBE) is used as an indicator of the differing biological efficacies of various radiation types. The RBE is defined as the ratio of an absorbed dose of a reference low-LET radiation to an absorbed dose of the test radiation that gives an identical biological endpoint. Furthermore, the RBE of a specific radiation type is influenced by the studied biological endpoint, the dose, the temporal dose distribution (dose rate, fractionation), environmental factors (H20, O2), and relative radiation sensitivity (individual, cell type, cell cycle phase).. Low-LET radiation will have an RBE of less than 1; high-LET radiation will have an RBE greater than 1. A LET of about 100 keV/μm is considered optimal in terms of producing a biologic effect, because at this density of ionization, the mean separation in ionizing events is equal to the diameter of the DNA double helix. This phenomenon is of particular relevance as it ensures the highest probability of causing DSBs. [6] For example, targeted α-therapy (TAT) has a LET of 100 keV/µm, while Auger electron therapy (AET) has a LET of 4-26 keV/µm [8], which results in the highest probability of causing DSBs (cfr targeted α-therapy, Auger therapy). At even higher LET values, more energy will be deposited than is necessary to cause damage. This phenomenon is called ‘overkill effect’. [9] Because the RBE is too specific to be used in radiation protection, the ICRP has selected radiation weighting factors WR for different radiation types to be used in the framework of stochastic effects at low doses. To calculate the equivalent dose (unit Sievert, Sv), the radiation weighting factors are multiplied with the mean absorbed radiation dose. [6]
Cellular responses to ionizing radiation involve complex mechanisms to maintain genomic stability and ensure cell survival. These include the delay of cell proliferation and DNA damage repair. In case of exceeding DNA damage thresholds, various forms of cell death depending on the dose and cellular context can be triggered, e.g. apoptosis, necrosis, mitotic catastrophe, autophagy-dependent death, or senescence.
Adaptive response describes the phenomenon where exposure to a low priming dose (1-100 mGy) induces cellular changes that reduce the detrimental effects of a subsequent higher radiation dose. The mechanisms behind include DNA damage repair, antioxidant, and stress response pathways. [6]
The conventional paradigm states that radiation effects occur only in cells in which energy was deposited (Grotthus-Draper law 1817, 1842). There is now more and more experimental evidence that biological responses (i.e. oxidative stress, free radicals, epigenetic changes) are also seen in cells that have not received a direct energy deposition from radiation but are influenced by signals transmitted from neighbouring cells. This is called the bystander effect. [6]
It has been observed that local radiation therapy to the primary tumour site can cause the shrinkage of untreated metastases located distant from the irradiated site. This phenomenon is believed to be mediated by the immune system. Thereby, radiation is able to induce immunogenic cell death, which is triggered by the release of damage-associated molecular patterns (DAMPs) by the irradiated tumour cells. These DAMPs have the capacity to prime circulating cytotoxic T cells via antigen-presenting cells, inducing tumour cell death at distant sites. It is imperative to acknowledge the frequent expression of surface molecules by tumour cells, which serve to impede the function of cytotoxic T cells. [11]
Biological effects of ionizing radiation can be classified into deterministic effects (tissue reactions) and stochastic effects. [6, 12]
Deterministic effects are effects for which both the incidence and the severity increase above a threshold dose with increasing dose. The time at which tissue reactions can be detected after the radiation exposure varies. Early (days-weeks) tissue reactions may be of the inflammatory type resulting from the release of cellular factors, or they may be reactions resulting from cell loss. Late (month-years) tissue reactions can be of the generic type, if they arise as a direct result of damage to that tissue, or they can arise as a result of early cellular damage. Both early and late deterministic effects can appear after whole body exposure (acute radiation syndrome) and/or partial body (i.e. skin erythema, epilation, cataract) exposure above a specific threshold dose. The ICRP published thresholds doses (corresponding to doses that result in about 1% incidence) for various organs and tissues and judged that for doses <100 mGy (both for low- and high-LET radiation) no tissues express clinically relevant functional impairment. [12]
Cell killing is crucial to the development of deterministic effects. Cell survival curves are commonly used to study the survival of tissue target cells. The cell survival fraction ‘S’ as a function of the radiation dose ‘D’ is predominantly described using the linear-quadratic model.
S=exp{-(αD+βD2)}
In this model, αD describes the linear component as a single-track non-repairable event that is proportional to the dose on a semi-log plot of cell survival (log) versus irradiation dose (linear). βD2 describes the quadratic component that reflects the accumulation of sublethal damage (multiple, potential repairable single-tracks) leading to increasing sensitivity of cells at higher radiation doses. The ratio α/β is the dose at which the linear and quadratic components of cell killing are equal and this ratio has been useful to compare the early and late responses of tissues (see figure x). Early or acute tissue reactions have α/β ratios ~10 whereas for late effects values between 2 and 5 are found. LET, dose rate, dose fractionation, and the presence of oxygen can affect the survival curve, elements which are important in radionuclide therapy. Moreover, because of the combination of radionuclide decay and biological clearance of the radiopharmaceutical, the dose rate fluctuates in time. For this reason the ‘biological effective dose’ (BED) was introduced to compare different treatment types. [13,14]
Figure x: These cell survival curves illustrate typical differences in the dose-response curves of early- (purple: α/β-ratio of ≈3 Gy) and late- (blue: α/β-ratio of ≈12 Gy) responding tissues. Note that the α/β-dose is the dose where the contributions to the cell kill from the αD-term is the same as for the βD2-term, as indicated with brackets for the late-responding tissues from [15]
Stochastic effects are defined as those effects for which the probability of occurrence increases with the dose. The two major stochastic effects are cancer and genetic effects.
It is generally assumed that DNA damage response processes in single cells are of critical importance to the development of cancer after radiation exposure. Cancer risks are estimated on the basis of probability and are derived mainly from epidemiological data from the Life Span Study (LSS) of the atomic bomb survivors of Hiroshima and Nagasaki. Although there is limited epidemiological evidence of stochastic effects at effective doses <100 mSv, the linear-non-threshold (LNT) hypothesis was introduced by ICRP as the best practical approach to manage cancer risk from radiation exposures to low dose rates. According to ICRP Publication 103, the detriment-adjusted nominal risk coefficient for cancer for the whole population after exposure to radiation at low dose rate is 5.5% per Sv effective dose. This figure must be seen as a mean value for the entire population, because there is strong evidence that cancer risk also depends on the age at exposure, the gender, and the specific organ exposed. Exposure at an early age results in higher nominal risk factors, whereas females are slightly more susceptible than males.
To date, there have been no documented cases of radiation-induced genetic effects in humans. Nevertheless, the ICRP set the nominal risk coefficient for genetic effects for the whole population at 0.2% per Sv effective dose because of compelling evidence that radiation causes heritable effects in experimental animals. [6]
The biological effects after radiation in utero can be both deterministic and stochastic.
The deterministic effects of radiation on the fetus depend on two factors: the irradiation dose and the developmental stage of the unborn development at the time of exposure. The principal effects encompass neonatal death, malformations, growth retardation, IQ reduction, and congenital defects. The risk is most significant during organogenesis and in the early fetal period, less in the second trimester, and least in the third trimester. It is, however, assumed that these effects have a dose-threshold of 100 mGy.
For stochastic effects, the life-time cancer risk after in utero exposure is considered to be similar to that following irradiation in young children. [6]
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12. ICRP. P118 ICRP Statement on Tissue Reactions and Early and Late Effects of Radiation in Normal Tissues and Organs, 2011. https://journals.sagepub.com/doi/pdf/10.1177/ANIB_41_1-2
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