Effects of a bolus and inhomogeneities on pion stopping distributions

In radiotherapy treatments with negative pion beams, an external bolus is often used to compensate for inhomogeneities within the body in order that the pions will have the proper stopping distribution within a tumor. However, angular beam divergence, multiple Coulomb scattering, and elastic and inelastic nuclear scattering limit the degree to which the pion stopping region can be controlled. We have used the Monte Carlo computer code PION-I to calculate pion stopping distributions for several idealized cases in order to show explicitly the effects of a number of factors on stopping distributions. Calculations have also been made for the same geometrical configurations used in measurements of pion stopping distributions in the biomedical beam at the Los Alamos Meson Physics Facility, and the calculated results are compared with experiment.