MIXED FIELD DOSIMETRY OF EPITHERMAL NEUTRON BEAMS FOR BORON NEUTRON-CAPTURE THERAPY AT THE MITR-II RESEARCH REACTOR

During the past several years, there has been growing interest in Boro n Neutron Capture Therapy (BNCT) using epithermal neutron beams. The d osimetry of these beams is challenging. The incident beam is comprised mostly of epithermal neutrons, but there is some contamination from p hotons and fast neutrons. Within the patient, the neutron spectrum cha nges rapidly as the incident epithermal neutrons scatter and thermaliz e, and a photon field is generated from neutron capture in hydrogen. I n this paper, a method to determine the doses from thermal and fast ne utrons, photons, and the B-10(n,alpha)Li-7 reaction is presented. The photon and fast neutron doses are measured with ionization chambers, i n realistic phantoms, using the dual chamber technique. The thermal ne utron flux is measured with gold foils using the cadmium difference te chnique; the thermal neutron and B-10 doses are determined by the kerm a factor method. Representative results are presented for a unilateral irradiation of the head. Sources of error in the method as applied to BNCT dosimetry, and the uncertainties in the calculated doses are dis cussed.