Effect of head size on sup 10 B dose distribution

Boron neutron capture therapy (BNCT) for treatment of brain tumors is based on the utilization of large epithermal-neutron fields. Epithermal neutrons thermalize at depths of {approximately}2.5 cm inside the head and provide a maximum thermal fluence at deep-seated tumor sites with minimum damage to normal tissue. Brain tissue is a highly scattering medium for epithermal and thermal neutrons; therefore, a broad treatment field enables epithermal neutrons to enter the head over a large area. These neutrons slow down as they undergo scattering collisions and contribute to the thermal-neutron fluence at the tumor location. With the use of large neutron fields, the size of the head affects the thermal-neutron distribution and thereby the {sup 10}B absorbed dose distribution inside the head. In this paper, the authors describe measurements using a boron trifluoride (BF{sub 3})-filled proportional counter to determine the effect of head size on {sup 10}B absorbed dose distributions for a broad field accelerator epithermal-neutron source.