MONTE-CARLO CALCULATIONS OF EPITHERMAL BORON NEUTRON-CAPTURE THERAPY WITH HEAVY-WATER

Much work over the past decade has centred upon the development of epi thermal neutron beams for boron neutron capture therapy (BNCT) in an e ffort to increase thermal-neutron flux penetration and dose homogeneit y throughout the brain. While heavy water has been used extensively to improve neutron penetration associated with thermal neutron beams, th e effects of heavy water with epithermal neutron beams remain largely unexplored. Applying the Monte Carlo code MCNP to a heterogenous ellip soidal skull/brain model, the effects of heavy-water replacement are s tudied for the JRC/ECN Petten HFR epithermal neutron beam. Thermal neu tron flux and induced gamma depth dose distributions are calculated fo r 20% D2O replacement in comparison to standard brain and skull materi als. Results are presented for both unilateral and bilateral irradiati on. With bilateral irradiation, thermal-neutron flux homogeneity is su bstantially increased with 20% D2O replacement, thus improving the pot ential to give lethal doses to boron-10-loaded, disseminated cancer ce lls whilst avoiding local 'hot spots' to healthy tissue. Additionally, the induced gamma dose is reduced by up to 30%, substantially lowerin g the background dose to healthy tissue. With bilateral irradiation, 2 0% D2O replacement increases the therapeutic ratio from 2.25 to 2.75 f or over 4 cm depth centred at the midline of the brain. These calculat ions use documented tumour and blood B-10 concentrations for boronophe nylalanine (BPA) in humans and recently documented neutron relative bi ological effectiveness (RBE) values.