Neutron dose distributions in tissue irradiated by negative pion beams

Abstract

Monte Carlo computations are performed for four idealized uniform, circular negative pion beams. The individual dose contributions are shown from direct ionization by pions, from secondary protons, from neutrons, and from the heavier particles produced by nuclear absorption of pions. The resulting neutron dose distributions, which are important in therapeutic applications, are analyzed. Isodose contours for the total doses are presented together with isodose contours for neutron doses. The results show quantitative trends in the neutron dose distributions for different beam conditions and different treatment-volume sizes. They also show the sharpness of the boundaries of the high-dose regions that can be obtained with pion beams. The calculations indicate that only under special circumstances would the neutron dose outside the tumor volume limit the treatment dose.