The Problem of Recovery from Radiation Effects

Abstract

The evaluation of the recovery of tissues from the effects of radiation offers one of the most difficult problems of radiation therapy. This paper considers the subject from three aspects: (1) biologic, (2) photochemical, and (3) clinical. other stimulus, is capable of producing three types of effects: Facts concerning the recovery of single cells, tissues, and organs gathered from experimental and clinical observations will be presented and an attempt made to evaluate them. 1. Recovery Considered from the Point of View of General Biology: From the point of view of general biology, the problem of recovery from effects of radiation is one of cellular physiology. Radiation, just as any (a) Reversible effects, i.e., transient changes, ending in complete morphologic and functional restoration of the original integrity of the tissues. (b) Conditioned reversible effects, i.e., transient changes, ending in incomplete restoration. These changes may be followed by complete morphologic restoration but with modified function, or the morphological picture and the cell function may both be fully restored, but a second dose of radiation of the same amount, or even a smaller, will not lead to restoration of the original tissue state. (c) Irreversible changes, i.e., changes that do not permit ultimate restoration at all. Which of the three above-mentioned effects occurs depends on the amount of radiation and the so-called “specific susceptibility” of cells (e.g., phase of mitosis, metabolism, etc.).2 Only reversible effects can be considered as leading to recovery. 2. Recovery Considered from the Point of View of Photochemistry: According to our present concepts, irradiation produces new substances in the cells of the human and animal body which have been held to be toxic compounds. On the basis of this photochemical theory of the effects of radiation, Holthusen (32) has explained recovery as follows: (a) The substances formed photochemically are recombined into their original compounds by a reversible chemical reaction. In photochemistry there are many examples of such reactions. (b) The photochemical products are removed from the tissue by mechanical means, e.g., diffusion (“washing away effect”). This theory is supported by experiments of Ellinger (11), using collodion as a model of a cell membrane. 3. Recovery Considered from the Clinical Point of View: From a clinical point of view, recovery means restoration of organ structure and function. This restoration may result from replacement of damaged cells by proliferation of cells which have escaped radiation injuries, or from the recovery by the injured cells of their original function. The latter is designated as “true recovery” and the former as “pseudorecovery.”