Monte Carlo calculations to characterize the source for neutron therapy facilities

Modern radiation treatment planning for photons includes full 3D modeling o f the adsorbed dose distribution, accurate inclusion of the patient anatomy , and consideration of significant changes in material density and composit ion. Such efforts are founded in an accurate description of the radiation s ource and the beam delivery system. Modern fast neutron therapy facilities employ highly penetrating beams and isocentric beam delivery. Treatment pla nning is largely based on analytic models adapted from photon codes and int eraction cross sections normalized to macroscopic attenuation. However, the recent PEREGRINE initiative at Lawrence Livermore Laboratory offers the po ssibility of fully stochastic modeling if the neutron source can be adequat ely described. In this article we report neutron source modeling of three h igh energy facilities. Neutron production is based on the intra-nuclear cas cade model of the LAHET code while neutron transport through the beam deliv ery system is managed by MCNP using cross section libraries extended to 100 MeV neutron energy. PEREGRINE is then used to transport the neutron beam t hrough typical phantoms. The resulting neutron sources are in excellent agr eement with the limited experimental information and the measured phantom d ata are well described by the PEREGRINE transport using the LAHET/MCNP dete rmined neutron sources. (C) 1999 American Association of Physicists in Medi cine. [S0094-2405(99)02705-4].