Multiple-fraction experiments have been carried out to determine the response to repeated small doses of 240 kV X rays down to 45 rad per fraction, using the mouse skin reaction system. A method of irradiating without anesthetic was developed so that up to 64 fractions could be given within 8 days; over this time, proliferation was negligible.
It was found that the total dose required to produce a given reaction continued to rise with the number of fractions above 30 fractions, in contradiction to the recent conclusions of Dutreix and colleagues. The plot of reciprocal total dose against size of each fraction was shown to be linear. This finding led to an analysis in terms of a function Fe, which is proportional to the slope of the chord of the appropriate cell survival curve from the origin to the dose per fraction used.
The cell survival curve derived here was well fitted by an equation of the form