Tissue equivalence in clinical neutron dosimetry: comparison of dose distributions in nine tissue substitutes for a d(14)Be neutron beam.
Depth dose distributions for a d(14)Be neutron beam, measured separately for total and gamma absorbed dose, are presented for nine tissue substitutes and for two field sizes. These data are used to examine methods to transform depth dose curves from one material to another. Best results are found when the local depths are transformed by constant empirical factors which are independent of depth and field size. As a physical interpretation of the empirical factors, kerma-weighted mean-free-path lengths are calculated for the interaction of the Essen neutron beam with the materials. The ratios of these free path lengths agree with the empirical factors within +/- 10%. However, for clinical dosimetry, a direct comparability of spatial absorbed dose distributions measured in two different phantom materials is only given if their corresponding transformation factor is near unity.