From Big Medical Encyclopedia

TIME FACTOR of RADIATION — dependence of character and degree of manifestation of beam reaction of biological objects on duration of radiation.

The FACTOR of CHANGE of the DOSE 179

On duration is distinguished by four main types of radiation: short-term, extended (about-tragirovannoye), fractional (fractioned), chronic. Understand single short beam influence at big dosage rate as an acute exposure. Extended radiation happens during rather long time, continuously and at the low power of a dose. At fractional radiation impact of ionizing radiation is distributed in time by separate portions (fractions); intervals between them can be different (at therapeutic radiation they are estimated preferential in days or hours). The chronic exposure is a long radiation by small doses, a cut goes continuously or with nek-ry intervals for many months or years. This type of radiation is allocated especially because of qualitative differences of beam reaction of an organism.

To one of indicators of temporary distribution of radiation can serve dosage rate (see Dosage rate of radiation). On the majority of the registered indicators biol. actions deceleration of power of an ionizing radiation dose is followed by easing of effect. Dependence of effect of radiation on dosage rate or fractionation is connected with the recovery processes happening in an organism in the field of local radiation. Therefore the phenomenon of easing of effect can be expressed much more weakly' or be absent at use of radiation with high linear power loss (see. Linear power transmission) or at impact of any kinds of ionizing radiation on the fabrics having low level of proliferative activity and fiziol. reparations. Nervous tissue, bones, liver, muscles, etc.

F are among such. century of the lake is widely used in radio biological researches during the studying of the main patterns biol. actions of ionizing radiation, and also in development of practical recommendations about use of radiation as means of change of life activity of vegetable and animal organisms. In a wedge, radiobiologies F. century of the lake finds application during the development of the most effective techniques of radiation therapy of malignant tumors with use of methods of dynamic fractionation of a dose (see. Radiation therapy), establishments of the equivalent modes of radiation, etc.

The ratio between dosage rate, a total dose and medical effect at interstitial radiation of tumors is studied at cancer therapy of skin most in detail. On the basis a wedge, experience optimum on an equivalent of action the dose 6000 is accepted I am glad (60 Gr) for 168 hours at dosage rate 35,7 is glad (0,357 Gr) in an hour. As evaluation criterion any manifestation of action of radiation is accepted (e.g., percent of treatment of tumors, emergence of an erythema, wet Epi-dermita, a necrosis, etc.). Between a total dose, lasting a course of radiation and the nature of fractionation there is a difficult interrelation, the analysis the cut was carried out by F. Ellis who offered the concept of the so-called nominal standard dose (NSD). Within this concept the possibility of quantitative definition of a sufficient dose at radiation of tumors on the basis of achievement of a certain level of a tolerant (transferable) dose was for the first time shown. The ratio between the size of a total dose on a course (in labor) by criterion of achievement of tolerant reaction of connecting fabric is normal (D), the number of fractions (N) and duration of a course of treatment in days (T) is described by Ellis's formula:

D = NSD go-and,

where NSD — a nominal standard dose in ret (a therapeutic equivalent I am glad).

The number of fractions and size of a dose for fraction have bigger value in definition of irradiation efficiency, than the general duration of a course. It means that the size of a total dose of fractional irradiation more depends on change of size of a dose for fraction, than on change of duration of a course. Dependence of efficiency of a total dose on duration of a course is more essential at a large number of rather small fractions and is rather less significant at trace amount of big fractions. It means that lengthening of a course and consequently, and intervals between radiations renders more expressed biol. effect. Further the concept of NSD gained development, the factor of VDF (time, a dose, fractionation), size was entered into a formula of calculation to-rogo it is proportional to partial tolerance, i.e. reflects relative level biol. radiation effects at the corresponding combination of the general time of a course of treatment, size of a dose and the nature of fractionation for this specific mode of radiation. Variable exponents for N are offered and specific ways of calculation of NSD in the form of tables, nomograms and schedules are developed.

See also Doses of ionizing radiation, Ionizing radiation, Beam damages, Radiation.

Bibliography: Biological bases of radiation therapy of tumors, under the editorship of S. P. Yar-monenko, M., 1976; D arensky N. G, etc. Relative biological efficiency of radiation, Time factor of radiation, M., 1968; R at d e r-

m and N A. I., Weinberg M. Sh. and Ruo of l K. I shako. Remote gamma therapy of malignant tumors, M., 1977; Tyubiana M., etc. Physical bases of radiation therapy and radiobiology, M., 1969; X about l and V. V N.

Radio biological bases of radiation therapy of malignant tumors, L., 1979; it, Development and current state of the concept of NSD, Medical radio-gramophones., t. 27, No. 7, page 79, 1982; Yarmonenko S. P. Radiobiology of the person and animals, M., 1984; Duncan W. Nias A. N of Clinical radiobiology, Edinburgh and. lake, 1977; Ellis F. Dose, time and fractionation, clinical hypothesis, Clin. Radiol, v. 20, p. 1, 1969; Fletcher G. h! Textbook of radiotherapy, Philadelphia, 1980; Hall E. J. Radiobiology for the radiologist, Hagerstown, 1978.

V. V. Holin.