Heterogeneous head phantom for validating treatment planning system in boron neutron capture therapy

In clinical boron neutron capture therapy (BNCT), the distribution of dose to a heterogeneous medium that is predicted by a treatment planning system (TPS) should be experimentally validated. A head phantom specifically developed for this purpose is described and demonstrated herein. The cylindrical phantom exhibits distinct regions made from four materials (polymethyl methacrylate, calcium phosphate, air, and boric acid) to approximate a head structure with explicitly defined skin, skull, and brain tissue with a cavity and tumor within. Multiple gold activation wires were consecutively embedded in the phantom along the neutron beam central axis to provide direct measurements of ¹⁹⁷Au(n,γ)¹⁹⁸Au reaction rates at various depths and interfaces, enabling comparison with numerical predictions. The heterogeneous head phantom was irradiated and tested at the Tsing Hua Open-pool Reactor BNCT facility, and the measured reaction rates were compared with the results of two calculation methods: Monte Carlo simulations based on an analytical geometry model and TPS predictions based on voxel-based geometry created from computed tomography images. The specifications of the phantom and reported results can serve as a reference suite of test problems for BNCT dosimetry.