The response of mouse epidermis to fractionated doses of pi mesons.

An extensive series of preclinical experiments are described in which the relative biological effectiveness (RBE) of pions from TRIUMF has been determined for mouse skin. To measure the RBE at both high and low doses per fraction, a range of fractionation schedules was used, with 1, 2, 4, 10, and 20 fractions. Because the pion dose rate is much lower than the dose rate of X or gamma-rays used in radiotherapy, two sets of reference X ray data were obtained: one at the same dose rate as the pions (15 cGy min-1), and another at the more conventional 150 cGy min-1. This allowed a "biophysical" RBE for equivalent dose rates and a "practical" RBE for preclinical evaluation to be calculated. The pion RBE was significantly higher for 20 small fractions than for fewer large doses. The absolute value of RBE depends upon the dose rate of the reference X ray treatments. The "practical" RBE increased from 1.05 at high doses to 1.45 at the smallest dose per fraction tested. The "biophysical" RBE for equivalent dose rates ranged from 1.20 at high doses to 1.5 for 20 fractions.