Effect of different pion stopping volumes on the radiation response of mouse small intestine.

This radiobiological investigation was based on measurements of crypt cell survival in mouse small intestine when the animals were exposed to 5 cm3 (spot), 350 cm3 and 3010 cm3 (the latter two were spot scans) pion stopping volumes generated by the multi-channel Piotron of the Swiss Institute for Nuclear Research. The experimental data obtained indicate that there was a decrease in biological effectiveness when the pion treatment volume was enlarged, irrespective of whether the pion dose was delivered in a single exposure or in four fractions. The r.b.e. values for pion beams relative to 200 kVp X-rays for the different experimental conditions used in this study are presented. The phenomenon of decreasing biological effectiveness with increasing pion stopping volume may be attributed to the following two factors: (1) when the pion stopping volume is increased there is a corresponding dilution of the high l.e.t. component of the beam; (2) the biological test system used may be sensitive to radiation dose rate which varied by a ratio of about 20 for the pion volumes used in this study.