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the sensitivity of biological objects to ionizing radiation. Irradiation gives rise to a variety of changes in cells and organisms. The manifestations of the changes are not always correlated. Therefore, in evaluating radiosensitivity it is important to take into account the criterion used to characterize it. The lethal action of radiation—cell inactivation or death and the death of multicellular organisms—usually serves as such a criterion. The lethal action of radiation can also be manifested in different forms: cells die in interkinesis after one or more divisions, and multicellular organisms die at various times after irradiation.
To evaluate radiosensitivity, biological objects are irradiated with different doses, the number of survivors is determined, and survival curves are plotted. Survival curves are usually represented on a semilogarithmic scale for cells (Figure 1) and on a linear scale for multicellular organisms (Figure 2). The survival curves are used to find LD50—that is, the lethal dose that 50 percent of the individuals survive—and DQ and D0, which reflect the “shoulder” and slope of the straight-line component of the curves (D0 is equal to the dose that reduces the survival rate by e ≃ 2.7 times on the straight-line component of the survival curve). In experiments with mammals, LD50 is usually determined for different lengths of time after irradiation—three, five, 15,30 or more days. The resulting values of LD50/5, LD50/30, and so on reflect the radiosensitivity of those systems of the organism whose primary involvement is responsible for its death during a given interval of time. Thus, the death of mice and rats three to five days after irradiation results from injury to the intestinal tract; death occurring between five and 30 days after irradiation is due to injury to the hematopoietic system. LD50 or D0 usually serves as the measure of radiosensitivity.
The radiosensitivity of cells may differ by a factor of 100, 1,000, or greater: LD50 is 200–350 rads for mammalian cells, 10,000–45,000 rads for bacteria and yeasts, and 300,000–500,000 rads for infusorians and amebas. Cell radiosensitivity is determined by the primary vulnerability of the cell structures and by their capacity for reparation and the conditions under which they develop. In general, cell radiosensitivity is heightened by an increase in the DNA content and in the number and size of the chromosomes; it decreases with an increase in the number of chromosome sets (ploidy).
The radiosensitivity of cells is also affected by their chemical composition (for example, content of endogenous thiols), their physiological state (phase of the cell cycle, phase of differentiation), the conditions during irradiation (the conditions may have a protective or sensitizing effect), and the conditions in the postradiation period (the conditions may assist or interfere with the repair and manifestation of primary injuries). Cells with an impaired repair system are highly radiosensitive. Mutations in individual genes can alter the radiosensitivity of cells immeasurably and affect various aspects of metabolism. Thus, cell radiosensitivity depends on many factors the importance of which varies from object to object.
The radiosensitivity of multicellular plants and animals also varies widely. For example, LD50 is 5,000–20,000 rads for pea and corn seeds and 100,000–250,000 rads for clover and radish seeds (LD50 is 250–700 rads for seedlings of the same plants). LD50 is 30,000–50,000 rads for adult insects and 350–700 to 1,000–1,200 rads for mammals. The radiosensitivity of plants and animals is due mainly to the radiosensitivity of their cells (in the case of mammals, the radiosensitivity of stem cells of the hematopoietic organs and gastrointestinal tract). It is also determined by factors that influence the effectiveness of regeneration of the radiation-injured organs and tissues owing to reproduction of the surviving cells. The manifestation of radiosensitivity is affected by the conditions under which the animals are kept after irradiation, since the conditions may promote or prevent recovery from radiation sickness. The radiosensitivity of cells and organisms depends not only on biological characteristics and environmental conditions but also on the physical properties of the radiation, the dose rate, and the characteristics of fractionation of irradiation.
Methods of radiosensitization, that is, artificial enhancement of the radiosensitivity of biological objects, have been devised. Study of the various aspects of radiosensitivity is important in the development of effective modes of treating radiation injuries. It is also important in the use of radiation to treat malignant tumors, to stimulate plant growth, and to induce mutations artificially.
REFERENCESOsnovy radiatsionnoi biologii. Moscow, 1964.
Timofeev-Resovskii, N. V., V. I. Ivanov, and V. I. Korogodin. Primenenie printsipa popadaniia v radiobiologii. Moscow, 1968.
Kuzin, A. M. Strukturno-metabolicheskaia gipoteza v radiobiologii. Moscow, 1970.
Akoev, I. G., G. K. Maksimov, and V. M. Malyshev. Luchevoe porazhenie mlekopitaiushchikh i statisticheskoe modelirovanie. Moscow, 1972.
Miasnik, M. N. Geneticheskii kontrol’ radiochuvstvitel’nosti bakterii. Moscow, 1974.
V. I. KOROGODIN