Radiobiology of pions at TRIUMF.

Radiobiological studies at TRIUMF of the effects of pion beams have been carried out using cultured cells, mice and pigs. CHO cells in gel/medium were used for RBE determinations throughout the dose distributions. The RBE was shown to increase with depth in the stopping region, while the average RBE value decreased with increasing width of the stopping peak and also increased with increasing field size. The results suggest that the effects of peak width and field size will approximately cancel out when treatment volumes are changed, provided that all three dimensions are changed more or less proportionally. The peak center RBE values ranged from 1.2-1.3 for single doses. The oxygen enhancement ratio (OER) for pions was 2.2 compared to 2.8 for X rays. Split dose recovery was found to be somewhat reduced in the peak region. In vivo studies with mouse and pig skin have been done for 1 to 20 fractions. For mouse skin, pions showed an RBE of 1.5-1.6 for fraction numbers greater than 10. The RBE for pig skin was 1.4 for early reactions, but it may be higher for medium term reactions.