Further Studies on the Rate of Recovery of Human Skin from the Effects of Roentgen- or Gamma-ray Irradiation

IN a previous communication, the authors have described a method for determining the amount of recovery in human skin after irradiation, and have presented data for a few specific schemes of treatment (1). This work has now been extended to cover the usual types of protracted and fractionated therapy. Data are here given which permit the computation of the approximate cumulative dose of radiation in the skin at any time during the treatment period. Studies have also been made of the effect of different fractionations of a certain dose delivered in a of radiation which, delivered in equal fractions, with any specified intensity, produced the same skin reaction as the standard threshold dose. When it was found, for instance, that two doses of 400 r each (measured in air), with a 24-hour interval between them, produced the same reaction as 525 r at one sitting, the recuperation in the 24-hour interval was calculated as follows: Of the first 400 r, enough must remain apparently effective at the end of the interval to give, when added to the second 400 r, a total effect corresponding to 525 r. That is, 125 r of the first dose specified period, and of the effect of varying the intensity of the radiation when a given total dose is delivered in a given time. It will be recalled that the experimental work consisted in determining the amount might be considered to remain effective; 275 r must have been recovered from. This implies a recovery factor for 24 hours of 275/400, or 69 per cent. It was emphasized that any such conception must be purely arbitrary. No actual amount of radiation or of ionization remains in the tissues. The effect that remains is the consequence of the total ionization produced by the original irradiation. But it is evident that some recovery has taken place, and for purposes of dosage calculation, and of obtaining some idea of the effect of the interval on the cumulative dose, it is convenient to say that the tissue acts as if a certain part of the radiation remains effective. For three or more daily treatments, similar calculations can be carried out. For details the reader is referred to the first paper. In Table I are given the available data for 200 kv. x-rays. In addition to the authors' data, those of Reisner (2) for various numbers of daily irradiations are included. Practical applications of these data will now be developed. A. The Calculation of the Cumulative Dose.—In the earlier paper it was shown that the recovery during the second day of treatment was 31 per cent, or that 69 per cent of the cumulative dose remained effective when the third treatment was to be administered, and 24 hours after the third treatment, 74 per cent. Corresponding figures obtained from Reisner's data were 65 and 74. By carrying out similar calculations for data from longer treatment periods, the values have been obtained from which the curves of Figure 1 were plotted. Points obtained from the authors' data are indicated by circles; those from Reisner's by crosses.2 A single additional point can be obtained from the data published by Low-Beer and Redisch (3) and is indicated by a triangle. The upper curve, which is well covered experimentally, shows the percentage of the cumulative dose at any day of the treatment period, when daily irradiations are given. Its use is best illustrated by an example. Example: Calculate the daily cumulative dose when 300 r are administered to a given field daily for five days.