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 reaction products. These dose components are commonly measured with paired (TE/Mg) ion chambers and foil activation technique. In the present work, we have investigated the use of paired tissue equivalent (TE) and TE+B-10 proportional counters as an alternate and complementary dosimetry technique for use in these neutron beams. We first describe various aspects of counter operation, uncertainties in dose measurement, and interpretation of the data. We then present measurements made in the following radiation fields: An epithermal 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-252 radiation field. In the epithermal beam, our measured gamma and neutron dose rates compare very well with the values calculated using Monte Carlo methods. The measured B-10 dose rates show a systematic difference of similar to 35% when compared to the calculations. The measured neutron + gamma dose 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 measured B-10 dose rates compare very well with those measured using other methods. In the Cf-252 radiation field, the measured dose rates for all three components agree well with other Monte Carlo calculations and measurements. Based on these results, we conclude that the paired low-pressure proportional counters can be used to establish an independent technique of dose measurement in these radiation fields. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(0)01603-5].