radiation ionizing radiation

From Big Medical Encyclopedia

RADIATION by ionizing radiation — impact of ionizing radiation on biological objects. It is applied to radiation therapy and diagnosis of diseases, experimental studying biol, efficiency of ionizing radiation, and also can arise at emergencies as a result of disturbance of the accident prevention in the course of production or use of sources of ionizing radiation.

Distinguish the external and internal Lake. External O. — impact of ionizing radiation, a source to-rogo is out of an organism. Biol, external O.'s effect depends on a dose, a look and energy ionizing radiation (see), features of space and temporary distribution of a dose. It is widely used for X-ray inspection (see) and radiation therapy (see), and also in radiobiology for studying of patterns of action of ionizing radiation on biol, objects. Internal O. — impact of ionizing radiation, a source to-rogo is in an organism. It occurs continuously until the radioactive nuclide which caused it does not undergo full disintegration of a pla will not be brought out of an organism. Biol, internal O.'s effect depends on activity of the radioactive nuclide which got inside and the nature of its distribution in an organism. Depending on features of distribution of radioactive nuclides in bodies and fabrics they are divided into four groups: evenly distributed in bodies and fabrics — 24 Na, 40 K, 137 Cs, etc.; collecting preferential in a bone tissue — 8fiSr, 90Sr, 226Ra, 32P, etc.; concentrating in parenchymatous bodies — 106Ru, 210ro, etc.; 239 Rhee having the mixed type of distribution, e.g. collecting in a bone tissue and a liver. Internal O.'s dose is defined by a type of ionizing radiation (alpha, beta or the gamma-emitter), its energy, effective elimination half-life (see). Internal O. is applied for the purpose of diagnosis of various diseases (see. Radio isotope diagnosis ) and for radiation therapy (use as the open radionuclides entering a metabolism in an organism, e.g. 131 I, and closed radioactive sources, e.g. 60so, for intracavitary and interstitial introduction).

Depending on an ionizing radiation dose distinguish the suble-talny, lethal and superlethal Lake. Sublethal O. causes in an organism patol, the changes which are not leading to death. Lethal O. leads to death as a result of development of a marrowy form acute radial illness (see). At supersummer flax O. death comes during 1 — 3 days after impact of ionizing radiation owing to development of cerebral and intestinal forms of a radial illness. Depending on a type of ionizing radiation distinguish beta, gamma and x-ray O., radiation by protons, neutrons, pi-mezonamp, etc. Physical properties of ionizing radiation define its penetration, and density of ionization created by it in fabrics — it biol, efficiency.

As a result of impact of ionizing radiation uneven distribution of an absorbed dose can be observed (see. Doses of ionizing radiation ). Features of spacing of a dose are caused by a look and energy of ionizing radiation, geometry of radiation (distance from its source to the irradiated object, position of a body in relation to a source of ionizing radiation). Distribution of an absorbed dose of ionizing radiation is characterized by coefficient of irregularity (KN) showing difference of a dose from the next to a source of ionizing radiation of a body surface to the most remote from it (at unilateral O.) or to the center of a body (at multilateral O.).

Depending on O.'s geometry and volume of the irradiated fabric O. can be uniform and uneven. Uniform O. is a radiation of all organism, at Krom of distinction in distribution of an absorbed dose on a body do not exceed +10%. It meets almost exclusively in experimental conditions. Uniform multilateral O. is carried out most often from an external source of radiation, but can result also from hit in an organism of a radioactive nuclide. Uneven O. is divided into the general uneven, subto-talny, partial and local, and also by O. via trellised filters (rasters). At the general uneven O. all body is exposed to beam influence, but depending on O.'s geometry perhaps uneven distribution of a dose on a small or big axis of a body (unilateral O.) or difference from a surface to the center of a body (bilateral, quadrilateral and multilateral O.). The general uneven O. is used in radiobiology for studying of features of radiation injury and recovery processes in an organism. It was observed also in the majority of emergencies. At the general uneven impact of ionizing radiation natural decrease biol, effect in process of Ki's increase is found. Features a wedge, pictures of radiation injury as a result of uneven O. consist that at the same time the found forms of a radial illness which are not observed at uniform O. skin, oral and pulmonary forms of a radial illness develop as usual. At the KN approaching to 3 — 6 not only the hemopoietic system, but also other critical systems is surprised, at the same time the mixed or transitional forms of defeat caused by the direction of beam influence develop (hemopoietic and oral, hemopoietic and intestinal, hemopoietic and skin). At increase in KN to 7 — 10 preferential skin, intestinal and oral syndromes develop. At KN the manifestations inherent to partial influences become St. 10 leaders in a wedge, a picture of defeat.

Subtotal O. — radiation of the most part of a body at protection (shielding) of its certain areas, napr, the heads, a thorax, a stomach or extremities. In radiobiology subtotal O. is used for studying of value of the certain not struck areas of a body in development of radiation injury and the course of processes of recovery.

Partial O. is characterized by defeat only of certain areas of a body, napr, the heads, a thorax, a stomach or extremities. It is used in radiobiology for studying of value of the struck areas of a body, in uniform O.'s effects, and also for detection of features of a course of processes of radiation injury and recovery. Partial O. occurs in the course of X-ray diagnostic testing, it is used in radiation therapy, e.g., of malignant tumors. There is overseeing by people, to-rye in emergencies or in case of accidents owing to disturbance of the accident prevention were exposed to the partial Lake.

Local O. — impact on separate bodies and small body parts. It is applied in radiation therapy to treatment of various diseases, by hl. obr. malignant new growths, and also in radiobiology for assessment of value of defeat of separate bodies. During the performing radiation therapy the main objective of O. is concentration of a dose of radiation in patholologically the changed fabrics at the maximum decrease it in the surrounding not struck fabrics and in all body. For this purpose resort to multifield O., various methods of mobile O., use the means of aiming of a bunch of radiation collimating devices, filters, etc. Trellised filters (rasters) apply in radiation therapy at O. of rather big tumors without exceeding of tolerance of surface fabrics, and also in cases of repeated influence through the entrance fields which were earlier exposed to the Lake. At O. via trellised filters unirradiated intervals of fabric play a protective role that gives the chance to bring to a tumor ionizing radiation in rather high doses and at the same time not to cause essential beam damage of surrounding fabrics. Biol, effect at influence via trellised filters depends on a ratio of the areas of the closed and open sites of skin of the entrance field.

Influence of a time factor of O. is defined by dosage rate of ionizing radiation, fractionation or crushing of a dose and the general time of O. (see. Time factor of radiation ).

Depending on dosage rate distinguish: heavy-duty or super and intensive O. — influence by a single impulse with dosage rate about 10 6 — 10 12 I am glad / sec. during less than 1 sec.; intensive O. — influence with dosage rate from 1 — 5 I am glad / mines above in time of 1 sec. to 1 — 2 hour; The Island with low powers of a dose — less than 1 is glad / mines during more than 2 hours and to several weeks; The Lake with very low capacities of doses on an extent from several months to several years. The lake can happen to constant dosage rate or to the dosage rate changing in time, napr with the falling-down dosage rate.

On features of crushing of a dose distinguish single O. with various dosage rate; the fractioned O. which is carried out by parts of a total dose; ultrafraktsioni-rovanny O. — influence of a particle flux or quanta in the form of impulses lasting less than a millisecond of bucketed various duration within a second or several seconds. O. electronic or a bremsstrahlung on the betatron or the linear accelerator can be ult-rafraktsionirovanny O.'s example.

Lakes with a low power of a dose or the fractioned O. render always smaller biol, effect in comparison with influences with big dosage rate or single influence. Reduction biol, effect in these cases is caused by the fact that in the conditions of extended O. already during it, and at the fractioned influence in intervals between fractions the damaged fabrics partially manage to be recovered, as leads to decrease biol, effect. The fractioned O. is the main method in radiation therapy. Distinguish large fractionation in a single dose 600 — 1000 I am glad (6 — 10 Gr), an average — 300 — 500 I am glad (3 — 5 Gr), usual — 150 — 250 I am glad (1,5 — 2,5 Gr) and small — less than 150 I am glad (1,5 Gr). At small fractionation of O. carry usually out 2 times a day. In experimental radiobiology the fractioned influences use for studying of features of a course of recovery processes. Double O.'s method allows to estimate, e.g., quantitatively the speed of processes of recovery and to define the period of semi-recovery of radio sensitivity of an organism.

Depending on the general time of O. distinguish the instant O. which is made during less than 1 sec.; short-term O. duration of several minutes to 2 hours; extended O. — the influence happening during more than 2 hours and up to several weeks to rather small dosage rate; chronic O. — the long and more or less constant O. occurring within many months or several years. Instant O. is possible on pulse reactors and other pulse installations. Short-term O. is carried out on various irradiators with constant dosage rate or on the linear accelerator or the betatron. Extended O. is carried out with a constant or the falling-down dosage rate, and also in the conditions of the fractioned influence. Hron. The lake can come both from an external source of ionizing radiation, and at hit in an organism of radioactive materials. It is made constantly with low dosage rates or (at the fractioned O.) in small single doses. X \yugut to be exposed to such radiation radiologists and radiologists, and also other specialists working with radioactive nuclides. At violation of the rules of the accident prevention and exceeding of a marginal dose there can be disturbances of various functions of an organism or hron, a radial illness. At hron. Lake lab. animals their resilience is reduced to influence of adverse environmental factors, life expectancy is reduced, genetic changes are found, tumors develop.

The genetic effect of radiation is an important component biol, impacts of ionizing radiation. Usually mean O.'s action on posterity of the irradiated individuals by genetic effect of O. The lake causes dominant gene in formative cells mutations (see), leading to hereditary changes, mostly negative, shown already in firstgeneration; recessive genovariations, to-rye are not shown in a heterozygous state, but increase genetic Burdeness of population; chromosomal reorganizations, from to-rykh the most essential role play Reciprocal translocations (see); aneuploidies. Chromosomal reorganizations and aneuploidies increase the frequency of pre-natal death of a fruit, misbirths and inborn malformations.

Quantitative assessment of genetic effects of O. at the person is accompanied by great difficulties in connection with the low frequency of the induced mutations and insufficient volume of the studied material. Therefore the main means for quantitative assessment of genetic effect of O. the person has extrapolation of results of experiences on a lab. animals or cultures of somatic cells of the person.

Emergence of genetic changes depends on a stage of development of formative cells. Practically all population of formative cells in a female body consists of unripe oocytes, maturing to-rykh happens just before an ovulation. Therefore at assessment of genetic effects of O. for a female body proceed from radio-sensitivity of cells at a stage of unripe oocytes. Among men's formative cells such place is taken by spermatogones.

Frequency of hereditary changes at action x-ray and gamma radiations is substantially connected with conditions O. Tak, at mice the frequency of visible recessive mutations on a locus on 1 is glad (0,01 Gr) at O. spermatogoniyev in high doses makes 1,7 • 10 - 7 , and at low doses, and also hron, radiation — 0,5 • 10 - 7 . At impact of ionizing radiation with high dosage rate on mature oocytes an exit of mutations at big [400 is glad (4 Gr)] and small [50 is glad (0,5 Gr)] doses make respectively 5,5 • 10 - 7 and 1,8 • 10 - 7 , and at O. with low dosage rate and hron, influence — apprx. 0,3 • 10 - 7 .

Value of genetic effects of O. variously: e.g., the probability of congenital anomaly equal 0,001, is insignificant for the individual, but the same probability increased by number of births in population leads to emergence of considerable number of children with inborn pathology. For the individual the heaviest are genetic effects of O., to-rye cause death of a germ, an abortion or the early death of the newborn and the hidden hereditary changes (recessive mutations) are insignificant. Genetic effects are most undesirable to society, to-rye lead to increase in number of the disabled population or increase the hidden genetic Burdeness of population. The effects arising at small doses and hron, radiation have the greatest value. Expert estimates of these effects are periodically reconsidered in process of obtaining new data. At establishment of marginal ionizing radiation doses it must be kept in mind that genetic effects can be caused by radiation in as much as small doses and the threshold dose does not exist.

The marginal exposure doses of gonads established in the USSR at the expense of artificial sources of ionizing radiation (excepting their medical use) match the international recommendations and make for persons 5 rem/year (0,05 J/kg/year) which are professionally occupied with sources of ionizing radiation, for all population of 1,5 rem (0,015 J/kg) on the individual during the reproductive period (30 years), i.e. 50 mrem/year (0,0005 J/kg of i year).

Bibliography: Akoyev I. G. Problems of post-beam recovery, M., 1970; Biological effects of uneven beam influences, under the editorship of N. G. Da-renskaya, M., 1974; Bochkov N. P. Chromosomes of the person and radiation, M., 1971; Guskova A. K. and B and y with G. D au heads. Radial illness of the person, M., 1971; Relative biological efficiency of radiations, the Time factor of radiation, under the editorship of M. P. House slag, M., 1968; The Case of an acute radial illness at the person, under the editorship of N. A. Kur-shakov, M., 1962; Ionizing radiation, levels and effects, A report of the United Nations scientific commitee on the effects of atomic radiation to the general assembly, v. 1—2, N. Y., 1972.

H. G. Darenskaya; H. V. Luchnik (gen).