Radiation biology is a scientific discipline which deals with the study of the action of ionizing radiation on healthy and diseased tissue. It makes us to understand the sequence of events (damage or repair) that occurs after the absorption of radiation energy. There are many variables that determine the biological response to radiation exposure. These include the dose, dose rate, fractionation, nature of exposure (whole body/partial body), cell cycle, presence of radioprotectors /sensitizers and age of the exposed individual etc.
CELL
Cell is the basic unit of life. Living organisms are made up of either a single cell or many cells. Human being is a multi-cellular organism built up of 10
14 cells. Cell consists of a nucleus, which is surrounded by a viscous liquid known as cytoplasm. Both the cell and cytoplasm are enveloped by a membrane, which is known as cell membrane or plasma membrane.
The constituents of cytoplasm control the functions of the cell. The nucleus contains tiny thread like structures known as chromosomes, which are made up of deoxyribonucleic acid (DNA) and protein. The DNA molecules contain all the information required for the cellular function in coded form and thus control the nature and growth of the individual.
Sections of chromosomes, which contain information for specific functions, are called genes. Cells of similar nature constitute a tissue and different tissues form an organ. Many organs constitute a system (like respiratory system, digestive system, hemopoietic system and nervous system).
Cells are mainly classified into two categories: (i) somatic cells, and (ii) reproductive (or germ) cells. Somatic cells constitute various tissues such as skin, liver, brain etc. Germ cells are those, which participate in the reproductive process. They are sperm in males and ovum in females. All somatic cells in human body contain 46 chromosomes as 22 pairs and two sex determining chromosomes. Reproductive cells contains only 23 chromosomes.
INTERACTION OF RADIATION WITH TISSUE
Radiation deposits energy in tissues randomly and rapidly (<10-10Sec) via excitation, ionization and thermal heating, in turn produces moving
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15 electrons. These electrons interact with atoms and molecules leading to chemical and molecular changes. These changes may appear as biological effects such as chromosome breakage, cell death, oncogenic transformation and acute radiation sickness. These effects appear after a period of time (latent period), which may vary from minutes to years.
Major portion of the radiation energy appear as heat, with little biologic significance.
Radiation interactions that produce biologic effects are classified as direct and indirect action. In direct interaction, the radiation ionize or excite the molecules such as DNA, RNA and protein directly. It involves rupture of cell membrane and break of chromosome structure, resulting in DNA strands break. The fragments of chromosomes produced in a direct interaction, can join together to form chromosomes with abnormal structures. This is known as chromosomal aberration. The frequency of chromosomal aberrations increases with the radiation dose and hence the magnitude of aberrations is a biological indicator of radiation dose absorbed in human body. Chromosomal aberration analysis (CAA) is useful in determining the radiation dose received by a person who is accidentally exposed to high radiation dose (>100 mGy).
In indirect mode, the radiation interacts with the medium and produce radicals which in turn interact with the target molecule. For example, radiation interacts with oxygen and water molecules present in the cell.
These interactions produce a large number of free radicals, which are uncharged atoms or molecules with an unpaired electron and hence are highly reactive. Human body tissue is composed of 70-85% water, and the major interaction (three-fourth) is indirect action. The X and γ rays effects in macromolecules of living system are mainly due to indirect interactions.
The absorption of radiation by a water molecule (radiolysis) results in ion pairs (H2O+, H2O–),which are unstable and forms free radicals H*
(hydrogen) and OH* (hydroxyl) as follows:
H2O → H2O+ + H2O– (ion pairs) H2O+ → H+ + OH*
H2O– → H*+ OH–
OH* + OH* = H2O2 (hydrogen peroxide) H* + O2 = HO2* (Hydroperoxyl radical)
Free radicals are extremely reactive chemical species and perform variety of reactions. They act as strong oxidizing or reducing agents by combining with macromolecules. For example, free radicals interfere with cell functions and may inactivate cellular mechanism or break DNA bonds. The damaging effects of free radicals is enhanced with presence of oxygen. In the case of low LET radiation, free radicals are the primary agents that cause the
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biologic effects. Approximately two-third of all radiation induced damage is considered to be caused by the hydroxyl free radical.
Repair mechanisms exists within cells which repair the cells and return to pre irradiated state. For example DNA single strand break, base damage can be repaired, by specific endonucleases and exonucleases that are present. The Fig. 2.1 presents the physical and biologic responses to ionizing radiation.
Fig. 2.1: Interaction of radiation in cell: Physical and biological response of ionizing radiation
Cells are more radiosensitive during the phase of cell division known as mitosis, when the chromatin material is being distributed to daughter cells.
Hence rapidly dividing cells like intestinal epithelium, bone marrow cells, reproductive cells are more radiosensitive. Highly differentiated tissues like muscle and brain are least radiosensitive.
Direct and indirect interactions of radiation with cell ultimately result in (i) cell modifications like gene mutation, cell transformation and chromosome aberrations and (ii) cell death.
LINEAR ENERGY TRANSFER
The linear energy transfer (LET) is a parameter that describes the average energy deposition per unit path length of the incident radiation and it is expressed in keV/μm. All radiations are capable of producing same type of biologic effects, but the magnitude of the effect per unit dose differs. In other words different radiations of equal dose do not produce the same level of biologic response. To evaluate the effectiveness of different radiations the term Relative biological effectiveness (RBE) was introduced and it is defined as follows:
RBE Dose of 250 kVp X-rays required to produce certain eff
= eect
Dose of test radiation required to produce the same efffect
17 These effects or endpoint includes chromosomal mutation, cataract formation and acute lethality etc. The relationship between RBE and LET is shown in Fig. 2.2. The RBE is proportional to LET at the beginning, which is relevant for low LET radiations (X, γ, e). Later it increases with LET, suggesting deposition of higher energy in the tissue, which is applicable to high LET radiations(α). Beyond 100 keV/μm, the RBE decreases with increasing LET, due to overkill. This means that radiation deposits excess energy than that necessary to kill the cell. RBE also depends on the total dose and dose rate of the radiation. The LET of the X and γ rays and tissue penetration are listed in Table 2.1 below.
Fig. 2.2: The relation between LET and RBE
Table 2.1: The LET of X and γ rays and tissue penetration in HVL X and γ LET(keV/μm) HVL(mm) in Tissue Source rays Energy (keV)
80 1.0 38 Diagnostic X-rays
120 1.4 43 Co-57
140 1.5 46 Tc-99m
364 2.8 65 I-131
511 3.5 70 Positron emitters
1000 5.2 100 Co-60
BIOLOGIC EFFECTS
The harmful effects of radiation in human body are classified as (i) somatic effects and (ii) Genetic effects. The radiation effects, arises due to the damage of the somatic cell are called somatic effects. It is produced in an exposed
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individual during his life time. The magnitude of the somatic effects vary with nature of exposure (whole body or partial exposure). The hereditary effects are due to damage to reproductive cells and manifest in the progeny of the exposed person.
Early Somatic Effects (Whole Body Irradiation)
Somatic effects may appear immediately after exposure, within a few hours to weeks or much later (after years - decades). The early effects are due to an acute exposure (large doses over a short period of time) and attributed to depletion of cell population due to cell death. Table 2.2 shows the early somatic effects due to the acute whole body exposure to low LET radiation.
The amount of radiation damage depends on the rate at which the radiation is delivered. High dose delivered in a short time may result in severe damages to tissues. The same dose delivered over several months allows the repair mechanisms to function fully. Hence, somatic effects listed in the table may not manifest, if the person receives the dose over a prolonged period.
Table 2.2: Early somatic effects due to acute whole body exposure to low LET radiation*
Dose range Effect
Less than 0.1 Gy No detectable damage
Above 0.1 Gy Chromosome aberrations detectable(1-2 dicentrics in 500 cells)
Above 0.5 Gy Above effect + transient reduction in WBC count: emporary sterility in males
Above 1.0 Gy Above + radiation sickness like loss of appetite, nausea, vomiting , diarrhea (NVD) : complete recovery possible at low doses
Above 3.0 Gy Severity of above effects increases + damage to blood forming organs (bone marrow, spleen, lymph node): death in 4-8 weeks possible (> 10%)
3.0 to 5.0 Gy All the above with increased severity + Anemia, infection, high fever (bone marrow syndrome), about 50% of the exposed persons may die within 60 days (LD 50/60).
8.0 to 15.0 Gy Severity of above effects increases + cells in the gastrointestinal system get severely damaged leading to gastrointestinal syndrome (GIS) like diarrhea, weight loss and fever. Death may occur in 1-2 weeks (100 %).
Above 10.0 Gy All the above + manifestation of skin damage
> 25 Gy All the above + severe depression, fatigue, delirium, coma etc. (Central nervous system syndrome[CNS]). Death occurs in a few hours to days.
*Source: AERB lecture notes
19 Early Somatic Effects (Partial Body Irradiation)
Partial body exposure to the above dose ranges produces only local effects.
A whole body exposure to a dose of 4 Gy can be lethal. But exposure of a part of the body to the dose will not be life threatening. However, it can produce certain serious local effects (Table 2.3). The seriousness of the local effects too depends on the dose rate and the period of exposure etc. All the early somatic effects do have a threshold dose, below which they do not occur. Beyond the threshold dose, severity of the effect increases with the dose. Since the threshold doses are much higher than the normal occupational dose, these effects may not occur from normal occupational exposure to radiation.
Table 2.3: Early somatic effects due to acute partial body exposure to low LET radiation
Dose Region Effect
0.15 Gy Testes Temporary sterility
3.5 – 6.0 Gy Testes Permanent sterility 1.5 – 2.0 Gy Ovaries Temporary sterility 2.5 – 6.0 Gy Ovaries Permanent sterility 3 Gy Hair follicles Epilation (fall of hair)
5 Gy Eye Cataract (after 2-3 years)
6 Gy Skin Reddening of skin (erythema)
Permanent epilation
10-20 Gy Skin Burns, blisters, wounds, death of
tissues Late Somatic Effects
Exposure to low levels of radiation over a prolonged period, may lead to late effects. Persons who recover from early somatic effects too may develop late somatic effects in life. Late effects are characterized by latent period, which can be as long as 30 years. The important late effects are cataract and cancer. Dose needed to produce cataract may be greater than 8 Gy of fractionated irradiation (not acute) with low LET radiation and the latent period may be 5-10 years. The biological process required to transform the damaged cells to a cancer cell is very complex and the latent period may vary from 2-5 years for leukemia (blood cancer) and 5-30 years or more in the case of cancers of the lung, bone etc.
Hereditary Effects
Hereditary effects occur in the progeny of exposed individuals when reproductive cells carrying radiation-induced damages (mutations) participate in the process of fertilization. Only that amount of radiation dose to reproductive organs, which occurs up to the time of conception, can affect the general characteristics of the offspring. However, the present
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knowledge on hereditary effects is limited to laboratory animals. No evidence for increase in hereditary effects was observed in human population exposed to both high and low dose of radiation. Hereditary and both early and late somatic effects of radiation can be classified into two categories, namely (i) deterministic effects and (ii) stochastic effects.
Deterministic Effect
A deterministic effect (non stochastic) is one “which increases in severity with increasing absorbed dose in affected individuals”. It results in cell killing due to degenerative changes in the exposed tissues. It may appear at higher doses (> 0.5 Gy) and soon after the dose is received. It have threshold dose, below which the effect is not seen. All somatic effects except cancer, as mentioned above are deterministic effects of radiation. All these effects will definitely appear in the exposed individual, if the radiation dose received is above the respective threshold doses. Examples are skin erythema, epilation, organ atrophy, fibrosis, cataract, blood changes, and reduction in sperm count. The radiation dose required to produce such effects are very large and are likely to occur only as the result of radiation accidents and patients irradiated in radiotherapy. It is unlikely in diagnostic Radiology, both for patients and occupational workers. As such deterministic effects can be completely avoided by limiting the dose levels well below the threshold doses.
Stochastic Effect
A stochastic effect is one in which “the probability of occurrence increases with increasing absorbed dose rather than its severity”. It is very important at very low levels (< 0.5 Gy). Any dose, however small, is effective for a certain level of risk for induction of stochastic effects. The risk increases as the dose increases. It have no threshold dose and the chance of occurrence increases with dose and independent of sex and age. Stochastic effects are the principle health risk from low level radiation, which is likely in diagnostic radiology and Nuclear medicine. Radiation induced cancer and genetic effects are examples for stochastic effects. Hence the risk of stochastic effects cannot be completely avoided. However, it can be minimized to an acceptable level.
Radiation Induced Cancer
Radiation induced cancers may appear only some years after the radiation was received. This is known as latent period, which may extend from 10 to 30 years. The risk of inducing a particular type of cancer is measured by comparing the number of cancers produced in the irradiated population sample in excess of those expected in the same size of un irradiated population sample. This is known as Risk factor for the particular cancer.
Leukemia has the highest risk factor among all cancers, since tissue cells
21 are rapidly dividing. There is no safe dose limit and all doses of radiation carry some form of risk. Hence, radiation induced cancers can not be prevented, only be reduced by minimizing the radiation dose.
Genetic Effects
The radiation effects produced in the successive generation of the exposed individual are called genetic effects.
Genetic effects are caused by radiation induced damage to the genes or chromosomes in the ova or spermatozoa. The above damage is due to ionization and subsequent faulty recombination of the molecules which make up the chromosomes. As a result the biological code contained by the genes on the chromosomes gets altered and produce structural abnormality in the chromosome. It will be passed on to the future generations if reproduction takes place.
Severe genetic effects are not observed due to short life span of the individual and inability to reproduce. Only less severe effects are seen in the human population as a result of reproduction. Epidemiological studies carried out on 30,000 children born to the survivors of atom bomb (500 mSv) do not show an increase in the incidence of genetic disorders. There is no direct evidence of either elevated cancer risks or genetic disorders among human population exposed to low level radiation.
To protect the future generations, minimize the use of artificial radiations and adopt protection devices such as gonad shield in children, during radiography. Fig. 2.3, summarizes both early and late biologic effects due to radiation exposure from 0.001 mSv to 20 Sv.
Fig. 2.3: The early and late biologic effects of radiation with dose ranging from 0.001 mSv to 20 SV
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RADIATION EFFECTS ON DNA
The ionizing radiation deposits energy in the DNA molecule and produce chemical changes, which leads to structural changes. This includes (i) hydrogen bond breakage, (ii)molecular dehydration, and (iii) intermolecular and intramolecular cross linking. The rupture of hydrogen bond, which link the DNA base pairs, may result in irreversible structural changes in the molecule. DNA molecular breakage may occur as single strand breaks, double strand breaks, base loss or base changes occurring in DNA. Single strand breakage between the sugar and the phosphate can rejoin. Oxygen can cause the broken strand become peroxidized, and prevent it from rejoining. Thus presence of oxygen potentiates the radiation damage.
A double strand break are genotoxic lesions that can result in chromosome aberrations. This may lead to carcinogenesis through activation of oncogenes, inactivation of tumor suppressor genes, or loss of herterozygosity. Single strand breaks are easily repairable than double strand breaks as far as the low LET radiation is concerned (Fig. 2.4).
Fig. 2.4: Radiation action on DNA molecule:
Single and double strand breaks
Molecular cross linking is another common structural change, as result of reactive sites at the point of chain breakage. Cross linking may be between two DNA molecules, DNA and a protein, two base pairs within DNA. The loss of change of base is considered as mutation.
23 Chromosome breaks produced by radiation do occur and can be observed microscopically. Chromosomal damage that occurs before DNA replication is called chromosome aberrations, whereas that occur after DNA synthesis is called chromatid aberrations. In the later, only daughter of the one of the damaged chromatids pair is affected. Repair of chromosomal aberrations depends on stage of the cell cycle, and type and location of the lesion. There is a strong force of cohesion between broken ends, resulting in rings and dicentrics formation.
Chromosomal aberrations in human lymphocytes can be scored and used as a biological dosimeter to estimate radiation dose. For this lymphocytes are cultured from human blood sample. It is stimulated to divide, allowing karyotype to be performed. The cells are arrested at metaphase, and the frequency of rings and dicentrics are scored. Total body doses > 0.25 Gy can be estimated in the above method.
RADIATION EFFECTS IN UTERO
Developing embryo is extremely sensitive to ionizing radiation, as it is characterized by cell proliferation, migration and differentiation. As per the Bergonie and Tribondeau’s laws of radiosensitivity, highly differentiated cells are more radiosensitive. The response after radiation depends upon the total dose, dose rate, radiation quality and gestation period of the pregnant women. The type of damage includes prenatal death, congenital abnormalities, growth impairment, reduced intelligence, genetic aberrations and an increased cancer risk.
Gestational periods is divided into three stages namely (i) Preim-plantation, (ii)major organogenesis, and (iii) fetal stage. These stages vary with radiosensitivity and result in different radiation response at each stage.
Preimplantation begins with the union of the sperm and egg and continue until the zygote is embedded in the uterine wall. During this period (9-14 days) the conceptus is more sensitive, any radiation damage may result in prenatal death. The congenital abnormalities are low, though it is not completely absent. The most sensitive time of exposure in humans is 12 hr after conception (when two pronuclei fuse together to one cell) and again at 30 and 60 hr, when the first two divisions occurs. Animal study revealed an increase in the spontaneous abortion rate after a dose of about 50-100 mGy during the preimplantation period. Later a radiation dose of >250 mGy is required induce prenatal death and the spontaneous abortion rate is reported as 30-50%.
Major organogenesis is the period between 2-8 wk (15-50 days) after
Major organogenesis is the period between 2-8 wk (15-50 days) after