Dose outside the treatment volume for irradiation with negative pions

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Abstract
Irradiation of humans with negative pions requires a knowledge of the absorbed dose and radiation quality outside the primary pion beam. In conjunction with early clinical trials at LAMPF [Los Alamos Meson Physics Facility], experimental data were obtained with microdosimetric techniques and multiwire proportional counters. Theoretical calculations were made for the neutron contribution to the dose and were consistent with these data. Measurements were made within a 40 cm .times. 51 cm .times. 76 cm water phantom for a negative pion beam. The absorbed dose outside the treatment volume was the result of: neutrons and photons from the pion interactions, treatment room background and peripheral muons, electrons and pions in the primary beam. The first 2 components were nearly isotropic and were .simeq. 0.02% of the peak dose at a distance of 24 cm from the treatment volume; the 3rd component was anisotropic and varies from 0.01-0.1% of the peak dose. Collimation of the beam increased the dose outside the treatment volume typically by 50%.