Low vs high dose-rate effects on the acute reaction of pig skin to cobalt-60 gamma rays

Abstract

The acute moist reaction of pig skin to regimens of low and high dose-rate irradiation with plaques containing a single, small 60Co source was studied in order to determine the application time for a continuous, low dose-rate exposure which is equivalent to a fractionated, high dose-rate course. The dose distributions for the high and low dose rate sources were identical. All irradiations were carried out without anesthesia. The visual threshold for moist desquanwtion, i.e., the margin of the initial peak reaction, served as the biological endpoint. Ten young Chester White female pigs were irradiated—eight areas on the high dose-rate (control) side, and eight corresponding areas on the low dose-rate (experimental) side. During the subsequent 60-day observation period the longitudinal and transverse diameters of the acute moist reactions were measured three times weekly (Mon.-Wed.-Fri.). For the purpose of analysis the average initial peak diameter of the moist reaction was plotted against application time for the low dose-rate (experimental) side of each pig. linear regression was employed to determine the isoeffect low dose-rate application time. It was found that a continuous, low dose rate application of 63.8 hours duration at a dose-rate of 88.6 ran per hour was equivalent to four 1100 ran high dose-rate fractions separated by 24-hour intervals. The present experimental data for 60Co gamma rays are useful in calculating the parameters of time-dose formulae desinged to equate low and high dose-rate radiotherapy.