Evidence for a constant repair capacity over 20 fractions of X-rays

The response of mouse skin to small X-ray doses (less than or equal to 4.5 Gy) has been studied using gross skin reactions to obtain dose response curves. In order to study such small doses without giving a very prolonged series of fractions, the 'top-up' or partial tolerance design of experiment has been used. Eight or twenty priming fractions of X-rays have been 'topped up' with graded single doses of 3 MeV neutrons to bring the sub-threshold X-ray damage into the measurable range. By this means the effect of the same dose could be studied, when given either 8 or 20 times. The data were analysed to see whether each fraction was equally effective in the long or short fractionation schedules. The effectiveness remained constant, showing no significant loss of the repair capacity as the fractionation schedule proceeded.