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Changes in the rate of repopulation during multifraction irradiation of mouse skin

Medical Physics Department, Royal Postgraduate Medical School, DuCane Road, London W.12, and The Gray Laboratory, Cancer Research Campaign, Mount Vernon Hospital, Northwood, Middlesex HA6 2RN

The skin reaction on the feet of albino mice has been used to investigate the relative effects of repopulation and repair of sublethal injury at various stages during a course of "daily" 300 rad fractions. Previous two-dose experiments have shown that with increasing time after a priming dose of 1,000 rads, additional dose is needed to produce a given skin reaction. 500 rads were required after the first 24 hours, whereas only 30 rads per day were needed to counteract the subsequent repopulation.

In the present series the average dose increment that was required between 4 or 9 daily fractions was almost 200 rads per interval, and was not attributable to repopulation. Between the 9th and 14th fractions this average dose increment increased because of a contribution from an increased rate of repopulation.

After 4 fractions there was no measurable repopulation for two weeks. After 9 fractions approximately 50 rads per day were needed to counteract repopulation. After 14 fractions approximately 130 rads per day were needed for the first week, falling to about 20 rads per day in the second week. 130 rads per day corresponds to a doubling time of about 18 hours for the basal cells of the epidermis, which could then repopulate the damaged tissue in approximately one week.

Received for publication August 1, 1972. Revision received February 1, 1973.

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