Calculation of dosimetry parameters for fast neutron radiotherapy

Abstract

A computer simulation of the interactions of 50 MeV d⁺ on Be and 42 MeV p⁺ on Be neutron spectra with ICRU muscle tissue and Shonka A-150 tissue equivalent plastic was performed to allow computation of the charged particle spectra that result. Nuclear data were obtained from the Evaluated Nuclear Data File (ENDF) whenever possible and from the Intranuclear Cascade and Evaporation models otherwise. The dosimetry parameters calculated are: the kerma ratio, K (subscript A-150) / K (subscript tissue); the energy required to form an ion pair, W; and the stopping power ratio, S(W over g). [See PDF for diagram] The values listed above for the kerma in air require an ionization chamber wall thickness of 0.5 cm to ensure that charged particle spectrum equilibrium will occur. Doses given at the Texas A&M Variable Energy Cyclotron (TAMVEC) using currently accepted dosimetry parameters agree within 4% at depths of 1 cm or more with doses calculated with the parameters listed above. Calculated values of S (W over g) and W for the 50 MeV d⁺ on Be and the 42 MeV p⁺ on Be neutron spectra are the same, within 0.1%. The kerma ratio for the 42 MeV p⁺ on Be neutron spectrum is about 1% greater than the kerma ratio for the 50 MeV d⁺ on Be neutron spectrum.