Sensitivity studies of beam directionality, beam size, and neutron spectrum for a fission converter-based epithermal neutron beam for boron neutron capture therapy

Sensitivity studies of epithermal neutron beam performance in boron neutron capture therapy are presented for realistic neutron beams with varying fil ter/moderator and collimator/delimiter designs to examine the relative impo rtance of neutron beam spectrum, directionality, and size. Figures of merit for in-air and in-phantom beam performance are calculated via the Monte Ca rlo technique for different well-optimized designs of a fission converter-b ased epithermal neutron beam with head phantoms as the irradiation target. It is shown that increasing J/phi, a measure of beam directionality, does n ot always lead to corresponding monotonic improvements in beam performance. Due to the relatively low significance, for most configurations, of its ef fect on in-phantom performance and the large intensity losses required to p roduce beams with very high J/phi, bearn directionality should not be consi dered an important figure of merit in epithermal neutron beam design except in terms of its consequences on patient positioning and collateral dose. H ardening the epithermal beam spectrum, while maintaining the specific fast neutron dose well below the inherent hydrogen capture dose, improves beam p enetration and advantage depth and, as a design,le by-product, significantl y increases beam intensity. Beam figures of merit are shown to. be strongly dependent on beam size relative to target size. Beam designs with J/phi ap proximate to 0.65-0.7, specific fast neutron doses of 2-2.6x10(-13) Gy cm(2 )/n and beam sizes equal to or larger than the size of the head target prod uced the deepest useful penetration, highest therapeutic ratios, and highes t intensities. (C) 1999 American Association of Physicists in Medicine. [S0 094-2405(99)00409-5].