Variations of pion RBE in a volume with an asymmetric beam configuration.

The relative biological effectiveness for the radiation response of mouse jejunum for pions was determined in a clinically relevant asymmetric treatment geometry. A cylindrical volume of 570 ccm, with mouse holders in proximal and distal positions, was irradiated by spot scan technique with a sector of 31 concentric pion beams. The physical dose distribution within the treatment field was homogeneous and the average dose rate was 8.5 cGy/min. The aim was to determine RBE differences between the proximal and distal edge of the field and to compare it with the predictions. The RBE, relative to 200 kVp X-rays, was 1.3 for proximal position and 1.4 for the distal one. The results are in agreement with the calculations of the therapy planning computer program and with the microdosimetric characteristics at the two positions. The variation of the RBE is smaller than for the same field irradiated by a single pion beam.