POST-RADIATION RECOVERY

From Big Medical Encyclopedia

POST-RADIATION RECOVERY — ability of the organism struck with ionizing radiation to clinical recovery, recovery of structure and functions. The item of century passes at all levels and includes the various processes which are carried out by various mekhanizkhma (e.g., recovery of conformation of molecules and membrane structures, enzymatic DNA repair, recovery of cells or removal them together with the damaged fabrics, a reparative angenesis, reactions of compensation, etc.).

Distinguish clinical and biological P. century. Clinical, or observable, recovery — reduction of signs of observed defeat of cells, fabrics and systems of an organism, the loudspeaker to-rogo is characterized by existence of the eclipse period, rise periods of changes and the subsequent permission. At the same time the maximum of manifestations of damage is always divided by a big or small time term from the moment of the end of beam influence, and, therefore, recovery generally begins after a maximum of manifestations beam damages (see). Biological recovery — gradual reduction of total quantity of primary and subsequent damages of structures of cells to all systems of an organism. Total quantity of damages changes at once after the end of beam influence only in one direction — towards reduction.

Rather fully it is possible to estimate P. century in an organism on change of its resistance to repeated influence ionizing radiation (see). To predict resistance to reirradiation and to quantitatively estimate full value biol, recovery of an organism, it is possible to characterize irradiation efficiency only at the accounting of all sum of damages. It is connected with the fact that the beam damages which are shown observed changes and a wedge, symptomatology make a small part of the general damage of an organism. The prevailing part of damages is hidden and it is necessary to apply special diagnostic methods to their identification.

For quantitative assessment of observed changes use an effective dose — an estimated value of the dose corresponding to observed defeat. The term «effective dose» is usually applied to the characteristic of efficiency of radiations with their various duration and at repeated influences. The effective dose is less than absorbed dose because of existence of recovery processes in an organism during beam influence. At hron, radiations in small doses the speed of recovery processes can be commensurable with a speed of formation of damage, as a result radial illness (see) does not develop. For the characteristic of a condition of an organism in various intervals of time after the end of beam influence it is necessary to consider residual radiation injury. After a wedge, recovery resistance of an organism to the subsequent beam influence is reduced because of existence of the residual (hidden) radiation injury. At reirradiations the effective dose consists of doses of reirradiation and the dose corresponding to residual radiation injury.

The item of century is carried out with a certain speed, edges is calculated in size of the defeat which developed to a certain term of observation and is expressed percentage of the general defeat taken for 100% (relative speed) or in terms of a dose (absolute speed) by which the general defeat of an organism in unit of time decreases. Time necessary for reduction of the general defeat half, is called the period of semi-recovery.

There are different theoretical ideas of the dominating value of these or those systems of an organism (e.g., systems of a hemopoiesis, immune system) in P.'s processes century. Attempts of more sweeping generalization based on division of all variety of damages into several groups, napr, early and late, specific and nonspecific or reversible, partially reversible and irreversible changes are made. However without mathematical description it was not represented possible to create the general theory of P. of century, edges would allow to estimate and predict development of defeat and recovery on a strict quantitative basis. This lack was met by the formalized P.'s theories of century based on the mathematical description of processes of defeat and recovery. Among them Blair's hypothesis (N. of A. Blair, 1958) developed for the solution of practical tasks by Deyvidson has the leading value (N. O. of Davidson, 1960). Its initial position is the assumption that radiation injury develops in proportion to a dose of chronic or repeated exposure, and processes of recovery go with the absolute speed proportional to the size of this defeat. At the same time an irreversible part of defeat is proportional to the saved-up dose. Reversible and irreversible parts of defeat are summed up; death of an organism occurs when their sum exceeds the limit transferred by an organism. On the basis of this hypothesis the formal model P. of century is created, in a cut it is connected with reduction of size of a reversible part of defeat. The size of an irreversible part makes a constant share of the general defeat, smaller at gamma and x-ray radiation and big at influence of other types of ionizing radiation.

Deyvidson modified the equation for definition of residual radiation injury (t)) in terms of a dose:

D (t) = D [f + (l-f) of *e - λt ],

where D — an exposure dose; f — a share of irreversible beam damages; (1 — f) — a share of reversible beam damages; e — the basis of natural logarithms; λ — a constant of recovery; t — time which passed after radiation. E.g., at the general gamma irradiation in a dose 300 I am glad (3 Gr) for the purpose of suppression of immunity at renal transplantation residual defeat in 56 days will make 75 is glad (0,75 Gr), from to-rykh 30 is glad (0,3 Gr) will make so-called irreversible damages (10%). In case of need reirradiation with the same efficiency the dose of repeated influence shall be on 75 is glad (0,75 Gr) less, i.e. 225 is glad (2,25 Gr).

Original positions of a hypothesis of Blair — Deyvidsona found confirmation in numerous subsequent researches which, however, showed need of entering into it of basic amendments. The research of cellular bases of P. of century revealed that idea of exponential reduction of defeat is very rough approach over time to reality since cellular kinetics in such radio sensitive systems as went. - kish. the path and a hemopoiesis, makes essential changes to P.'s course of century. The relative recovery rate (in shares or percent from the size of defeat) was very significantly dependent on the size of a dose, dosage rate, frequency of radiation, etc. The period of semi-recovery which can change depending on a number of conditions is not a constant. Idea of constancy in time of an irreversible part of defeat also was inexact. It is established that rate of post-radiation recovery processes is proportional to quantity of an absorbed energy and speed of its accumulation. With increase exposure doses (see) the relative speed of P. of century of an organism increases if the increasing volume of damages owing to the arising difficulties for functioning of recovery mechanisms (shortage of plastic and power material, restriction of speed of transport of necessary metabolites, disturbances of systems of regulation, etc.) will not begin to interfere with manifestation of this pattern. For this reason at further increase in a dose the speed of post-radiation recovery begins to decrease. The termination of damage and reduction of its share reduce over time a relative recovery rate. The share of a so-called irreversible damage also decreases over time. At the same time it the greatest at molecular level and the smallest, often equal to zero, at organismic level.

Perhaps full and resistant a wedge, recovery of an organism at a radial illness, despite the known inferiority of recovery of its separate systems. After a wedge, recovery reduction of a residual share of radiation injury continues. Release of an organism from a considerable part of the latent injury facilitates development of compensatory and adaptive reactions. Therefore in later terms resistance to reirradiation and probability of development of the secondary and remote pathology depend on insufficiency of these reactions, on a state fiziol, systems of an organism to a large extent.



Bibliography: Akoyev I. G. Problems of post-beam recovery, M., 1970; Akoyev I. G., Maximov G. K. and Tyazhelova V. G. Quantitative patterns of a radiation syndrome, M., 1981; Yarmonenko S. P. Radiobiology of the person and animals, M., 1977.


I. G. Akoyev.

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