Beam shaping assembly optimization of Linac based BNCT and in-phantom depth dose distribution analysis of brain tumors for verification of a beam model

A Linac source of epithermal neutrons for BNCT is proposed in this work. A series of simulations has been carried out using the Monte Carlo code MCNPX for the determination of the final composition and configuration of beam shaping assembly (BSA) to moderate the photoneutron spectrum from the hybrid target that was designed in our previous work. The suggested configuration for BSA included 10 cm iron, 30 cm magnesium fluorine and 10 cm polytetrafluoroethylene (PTFE) in a cylindrical geometry. Ni, Pb, Bi and borated polyethylene were chosen as the collimator, reflector, gamma and neutron shield, respectively. Epithermal, thermal and fast neutron fluxes in the suggested design were 8.19E8 n/cm2 s, 2.14E5 n/cm2 s, and 4.82E7 n/cm2 s, respectively, while other IAEA parameters for BNCT facility design had been satisfied.In the next step, the calculated beam parameters were measured for a SNYDER phantom placed 10 cm from the exit of the beam shaping assembly (BSA). MCNP calculation showed that the advantage depth was 8.2 cm and the maximum therapeutic ratio was 5.05, if boron concentrations in the tumor and normal tissue were assumed to be 65 and 18 ppm, respectively. The maximum dose rate for normal tissue was 37.1 cGy/min.