Use of low-pressure tissue equivalent proportional counters for the dosimetry of neutron beams used in BNCT and BNCEFNT

The absorbed dose in a phantom or patient in boron neutron capture therapy (BNCT) and boron neutron capture enhanced fast neutron therapy (BNCEFNT) is deposited by gamma rays, neutrons of a range of energies and the B-10 reac tion products. These dose components are commonly measured with paired (TE/ Mg) ion chambers and foil activation technique. In the present work, we hav e investigated the use of paired tissue equivalent (TE) and TE+B-10 proport ional counters as an alternate and complementary dosimetry technique for us e in these neutron beams. We first describe various aspects of counter oper ation, uncertainties in dose measurement, and interpretation of the data. W e then present measurements made in the following radiation fields: An epit hermal beam at the University of Birmingham in the United Kingdom, a d(48.5 ) + Be fast neutron therapy beam at Harper Hospital in Detroit, and a Cf-25 2 radiation field. In the epithermal beam, our measured gamma and neutron d ose rates compare very well with the values calculated using Monte Carlo me thods. The measured B-10 dose rates show a systematic difference of similar to 35% when compared to the calculations. The measured neutron + gamma dos e rates in the fast neutron beam are in good agreement with those measured using a calibrated A-150 TEP (tissue equivalent plastic) ion chamber. The m easured B-10 dose rates compare very well with those measured using other m ethods. In the Cf-252 radiation field, the measured dose rates for all thre e components agree well with other Monte Carlo calculations and measurement s. Based on these results, we conclude that the paired low-pressure proport ional counters can be used to establish an independent technique of dose me asurement in these radiation fields. (C) 2000 American Association of Physi cists in Medicine. [S0094-2405(0)01603-5].