Dose fractionation effects with negative pions

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Abstract
Past studies of the biological effects of negative pions have largely been limited to determining RBE and OER values in the peak and entrance (plateau) region of the pion depthdose curve (Raju and Richman, 1972; Skarsgard, 1979). The results obtained have shown a higher RBE and a lower OER value in the peak compared with the entrance region. Another possible advantage of pions in their application to radiotherapy is the expected lower degree of biological recovery after irradiation in the peak compared with the entrance region. To investigate this effect experimentally, we have irradiated Vicia faba bean roots at SIN (Swiss Institute for Nuclear Research, Villigen) in the πE3 biomedical pion beam using various fractionation schemes. The beam had an average energy of 82MeV (momentum 172 MeV/c) with a spread of 13% (8%). The dose rate, as determined with an EGG tissue-equivalent ion chamber placed at the irradiation position in the water tank (Bianchi et al., 1978), varied during the irradiation between 260 and 300 cGy (rad)/h in the entrance region and between 420 and 590 cGy (rad)/h in the peak.

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