Changes in the basal cell density in pig skin after single radiation doses with different dose rates

Radiation-induced changes in the basal cell density (BCD) and the labelling index (LI) in the epidermis of pig skin were used to compare the effect of high and low dose rate irradiation. Single doses of 6, 12 and 24 Gy were applied at 1.5 and 0.02 Gy/min with two equal 137Cs sources. Punch biopsies were taken daily for 40 days. The linear BCD was calculated from histological sections. The LI was determined by an in vitro H3-thymidine labelling technique. We found a degenerative phase in the basal cell layer with a linear cell loss of 2% per day, independent of the dose and dose rate. The time and level of the minimum BCD were dose and dose rate dependent. The LI data indicated an increased proliferation rate after a 25% reduction in BCD, i.e. before the nadir was reached. From the BCD data an iso-effective dose factor of 1.8 was estimated for the two dose rates used, which was consistent with that determined from macroscopic scoring. The BCD can be used as an endpoint for comparison of different radiotherapy modalities and gives, together with the LI, further information on the time-course of the proliferation changes in the tissue.