The Rate of Recovery of Human Skin from the Effects of Hard or Soft Roentgen Rays or Gamma Rays

IT is well known that a given quantity of radiation is better tolerated by human tissues if it is administered in small doses over a long period of time, than in a single dose or in a few large ones within a short time. The greater the fractionation, the more radiation can be delivered. This is obviously because in the intervals the tissues recover to a certain extent from the damage caused by the radiation. If this recuperation were the same for all tissues, there would be no advantage in dividing the dose; the normal tissues would tolerate more, but so would the diseased tissue. However, it has been found clinically that in many types of malignant tumors it is possible by dividing the doses to produce regression of the lesion without marked permanent damage to the normal tissues, whereas this was usually impossible with a single massive dose. This means that recuperation must proceed more rapidly in the normal than in the diseased tissues in such cases. In some anatomical areas, the breaking down of the skin is one of the limiting factors to the amount of radiation which can be given. In others, the possibility of lymphedema and late necrosis of the deeper tissues will be the controlling factor in the amount and the method by which it is administered. It is important, however, to know how the skin behaves under different conditions of irradiation. Since the beginning of divided dose methods, observations have been made on the amount of radiation tolerated by the skin under different schemes of treatment. However, the results obtained by different observers show wide discrepancies. In most of the published reports, the data presented are completely inadequate to support the conclusions drawn, observations having been made on only a very few cases (four or five!). In some instances, results obtained with one set of conditions have been made the basis for conclusions regarding quite different types of radiation or schemes of fractionation. Elaborate methods of treatment have been developed on meager experimental data. A review of the literature emphasizes the statements just made. It may be well to examine the data which have been most widely accepted. Kingery (1), having observed that more radiation could be delivered if the dose was divided, realized that this must be due to recovery of some sort. He supposed that irradiation produced some hypothetical decomposition product in the tissues. The rate of loss of radiation effect would be in direct ratio to the elimination of this decomposition product. He assumed that this rate of loss would follow the same law as certain other chemical and biologic reactions, which could be represented by a logarithmic curve. Note that he assumed a logarithmic curve, and then set about proving its existence. Working with low voltage unfiltered roentgen rays, he found that a full dose could be repeated after 14 days.