Acute reactions in rat feet exposed to multiple fractions of X-rays per day.

Acute reactions of rat foot skin to regimens of X-irradiation involving single and multiple fractions per day were determined. The influence of an interval halfway through the treatment course was investigated for a multiple fractions per day regimen. The regimen was one in which 4 fractions of 2 Gy were given each day. Two courses of up to the tolerance dose separated by an interval of several weeks can be given without causing excessive skin damage. Repopulation during the interval was found to be responsible for the increase in tolerance. Repopulation started at about 2 weeks after the beginning of treatment. Using Barendsen's isoeffect model [4], doses which when given daily would result in the same biological effect as 4 X 2 Gy given daily, were calculated. Rat feet were irradiated with these doses. It was concluded that the isoeffect model provides good predictions of the tolerance doses in new irradiation regimens for the acute reaction of the rat foot skin.