The response of murine B-16 melanoma to fractionated doses of pions.

The relative biological effectiveness (RBE) of pions has been studied in mouse B-16 melanoma transplanted into C57BL/6 mice. To determine the RBE at both high and low doses per fraction, a range of fractionation schedules was used, with 1, 4 and 10 fractions. The reference 250 kV X ray dose rate was 1.5 Gy/min which was much higher than the dose rate of pions (0.25 Gy/min). The RBE varied depending on the number of fractions and, within the same fractionation schedule, also on the dose per fraction. The RBE ranged from 1.15 for single fractions at 12.5 days of growth delay, to 1.80 for 10 fractions at 5 days of growth delay, which was determined by the time taken for the tumors to reach 5 times the average of their original volume. RBEs at the iso-effect level of 10 days growth delay were 1.20, 1.29 and 1.62 for single, 4 fractions and 10 fractions, respectively. RBE values were influenced by both the number of fractions and the dose per fraction, that is, the larger the number of fractions and the smaller the dose per fraction, the larger the value of RBE. In comparison with RBE of normal mouse skin, it was suggested that pion therapy may provide advantage over conventional photontherapy for radioresistant tumors such as this melanoma with the maximum therapeutic gain factor of 1.2. alpha/beta ratios for B-16 melanoma were also obtained from the 10 day growth delay iso-effect curve, and were 10.5 Gy and 32.6 Gy for X ray and pions, respectively.