oxygen effect in radiobiology

From Big Medical Encyclopedia

OXYGEN EFFECT in radiobiology — dependence of biological effect of ionizing radiation on the size of partial pressure of oxygen (pO 2 ) in the irradiated fabric or in the environment surrounding the irradiated biological object. To. aa. it is shown in decrease in extent of damages (an inactivation of enzymes, phages and viruses, emergence of aberation chromosomes, death of cells and organisms, etc.) at radiation biol, objects in the conditions of the lowered pO 2 (see. Hypoxia ), and also in strengthening of damages upon transition from anaerobic conditions to aerobic and at further increase in pO 2 to certain limits.

With increase in pO 2 in the environment surrounding biol, objects, from 0 to 150 — 160 mm of mercury., i.e. at change of concentration of oxygen from 0 to 21%, biol, action ionizing radiation (see) amplifies, and it is the sharpest with increase in concentration of oxygen from 0 to about 10%. To the specified concentration of oxygen in the environment there correspond their smaller values in body tissues or directly around cells in the intercellular environment; To. aa. is implemented in them at lower sizes pO 2 . E.g., in tissues of mammals To. aa. it is shown at pO 2 apprx. 20 mm of mercury. in the intercellular environment.

Size K. aa. it is characterized by coefficient of oxygen strengthening (the oxygen relation). The coefficient of oxygen strengthening represents the relation of the ionizing radiation doses causing identical damage to anaerobic conditions D anaerobe and on air or in the atmosphere of pure oxygen D O 2  :

- (D anaerobe / D O 2 )

This size is not identical to different biol, objects.

Size K. aa. the hl depends. obr. from a type of ionizing radiation and conditions of radiation. To. aa. it is accurately shown at influence x-ray, gamma radiations (see), beta radiations (see) and other radiations which are characterized by small density ionization (see). At influence of particles with the big linear power transmission (LPT) of radiation (see. linear power transmission ) To. aa. decreases also since alpha radiations (see) almost it is not shown. Therefore To. aa. to different types of radiations it is not identical: the coefficient of oxygen strengthening makes 2 — 3 for x-ray and gamma radiations, 1,5 — 1,7 for bystry neutrons of different energy and 1,1 — 1,3 for alpha particles with energy of 2 — 4 MEV. Radiations with the LPE close values (x-ray and the gamma radiation, the accelerated electrons, protons with energy of 100 — 600 MEV) are characterized also by neighbors To. aa. For comparison of dependence biol, actions of different types of ionizing radiation from the oxygen mode use the size of a ratio of their coefficients of oxygen strengthening — a so-called factor of advantage. It characterizes advantage of action of some one type of ionizing radiation on biol, the objects which are in a condition of a hypoxia.

So, for bystry neutrons the factor of advantage in relation to x-ray or to gamma radiation is equal 1,4 — 1,7 that characterizes their big efficiency at influence on biol, objects in the conditions of a hypoxia, including at radiation therapy of tumors as in them pO 2 it is lowered.

Physical. - the chemical mechanism K. aa. at an indirect radiation effect it is connected with participation of oxygen in formation of active products of a radiolysis of water and organic radicals. To. aa. can take place and at direct action of ionizing radiation on organic molecules. Its mechanism in these cases is connected with formation of organic peroxides. At decrease in pO 2 formation of active products of a radiolysis of water and the changed organic molecules decreases that reduces efficiency biol, radiation effects. Decrease To. aa. at a direct radiation effect has a talk with high LPE the maximum extent of radiation damages in this case, and at indirect action of such radiations — formation of big concentration of primary radicals IT that provides high performance them biol, actions even in anaerobic conditions. However change of radiation resistance (see. Radiochuvstvitelnost ) at radiation of an animal organism in the conditions of a hypoxia it cannot be explained only with these mechanisms. At a hypoxia the difficult complex of neuroendocrinal reactions develops (narrowing of blood vessels in intestines, a spleen and other radio sensitive bodies, the increased release of adrenaline, change of tissue respiration etc. therefore sensitivity of an organism to ionizing radiation changes.

Understanding of essence To. aa. at beam influences it is necessary for studying of primary mechanisms of action of ionizing radiation, identification their relative biological efficiency (see), and also for search and development of methods of use of radio protective agents (see. Antiactinic protection ).

To. aa. finds practical application and at radiation therapy (see). Raising the oxygen content in a tumor, napr, by hyperbaric oxygenation, and creating hypoxemic conditions in surrounding fabrics, it is possible to strengthen beam impact on tumor cells, at the same time reducing damage of healthy fabrics.

Bibliography: Tank 3. and Alexander P. Fundamentals of radiobiology, the lane with English, page 270, M., 1963; Sverdlov A. G. Biological effect of neutrons and chemical protection, page 143, L., 1974; Eydus L. X. Physical and chemical bases of radio biological processes and protection against radiations, page 76, M., 1972.

A. G. Sverdlov.