Monte Carlo calculations of epithermal boron neutron capture therapy with heavy water

Much work over the past decade has centred upon the development of epithermal neutron beams for boron neutron capture therapy (BNCT) in an effort to increase thermal-neutron flux penetration and dose homogeneity throughout the brain. While heavy water has been used extensively to improve neutron penetration associated with thermal neutron beams, the effects of heavy water with epithermal neutron beams remain largely unexplored. Applying the Monte Carlo code MCNP to a heterogenous ellipsoidal skull/brain model, the effects of heavy-water replacement are studied for the JRC/ECN Petten HFR epithermal neutron beam. Thermal neutron flux and induced gamma depth dose distributions are calculated for 20% D2O replacement in comparison to standard brain and skull materials. Results are presented for both unilateral and bilateral irradiation. With bilateral irradiation, thermal-neutron flux homogeneity is substantially increased with 20% D2O replacement, thus improving the potential to give lethal doses to boron-10-loaded, disseminated cancer cells whilst avoiding local 'hot spots' to healthy tissue. Additionally, the induced gamma dose is reduced by up to 30%, substantially lowering the background dose to healthy tissue. With bilateral irradiation, 20% D2O replacement increases the therapeutic ratio from 2.25 to 2.75 for over 4 cm depth centred at the midline of the brain. These calculations use documented tumour and blood 10B concentrations for boronophenylalanine (BPA) in humans and recently documented neutron relative biological effectiveness (RBE) values.