Influence of beam characteristics and detector size in negative-pion dose studies.

Dose calculations were performed for a tissue phantom irradiated by uniform circular beams of negative pions with an assumed gaussian momentum distribution. The mean momentum of the pions was varied from 104.4 to 171.5 MeV/c (mean range 5-20 cm in unit-density tissue) and the momentum spread from 0 to 5% of the mean. Depth-dose curves are shown for different mean momenta and momentum spreads. The radial distribution of dose as a function of distance from the beam axis was computed at different depths for a beam with a mean momentum of 153.4 MeV/c and spread of 2%. The responses of detectors of different sizes used to measure centre-line dose for this beam were shown by calculating depth-dose curves for detectors of radii 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0 cm. Calculations were also performed for beams having radii of 1, 2 and 3 cm. Depending on particular conditions, it appears that considerable care may often be needed to infer the absorbed dose at a given location in a phantom irradiated by a negative-pion beam.