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

Abstract

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 10B 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+ l0B 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 252Cf 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 10B dose rates show a systematic difference of approximately 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 10B dose rates compare very well with those measured using other methods. In the 252Cf 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.

Microdosimetric intercomparison of BNCT beams at BNL and MIT

“…These include a difference of roughly 20% in two different fast neutron beams using 23 Na activation in comparison to that using both a boron-loaded miniature TEPC and boronloaded ionization chamber, 28 a difference of 15% between gold foil activation and miniature TEPC measurements in a reactor-produced thermal neutron beam 28 and a difference of roughly 30% between gold foil activation and a boronloaded Rossi counter in an accelerator-produced epithermal beam. 18 These results are all very similar to the differences of roughly 10-20% in the MIT beam and 10-35% in the BNL beam revealed in this study. Cumulatively, these results appear to reveal a systematic difference between measurements using foil activation and boron-loaded detection devices.…”

“…The average photon energy in the epithermal beam is significantly higher than that of 60 Co or 137 Cs sources which have been used previously for the extraction of representative photon spectra from mixed beams. 18 A representative photon spectrum was therefore created directly from high gain spectra measured in these epithermal beams. The increased gain provides a greater signal-to-noise ratio, allowing an inspection of the spectrum at lower lineal energies.…”

Miniature tissue‐equivalent proportional counters for BNCT and BNCEFNT dosimetry

“…Corresponding photon and neutron absorbed doses calculated from these spectra agree with measurements made by Kota using a Rossi counter. 44 These miniature TEPC measured photon and neutron absorbed doses also show good agreement with results from dual dosimeter measurements made by Rivard. 70 These measurements utilized a tissue-equivalent ionization chamber as the neutron sensitive detector and a GM counter as the neutron insensitive detector.…”

“…This assumption is not valid, necessitating the application of correction factors to account for differences in W values between different particle types and for changes in W values as a function of charged particle energy. The W-value correction factors have been adopted from calculations by Kota et al 44 The proton edge is commonly chosen as the midpoint of the final linear segment of the recoil proton portion of the lineal energy spectrum. 65,66 This methodology of proton edge determination has been employed in the present study.…”

“…The application of TEPC microdosimetry to BNCT was followed shortly afterward by its application to the boron neutron capture enhancement of fast neutron therapy ͑BNCEFNT͒. 42 A more thorough investigation of TEPC microdosimetry for BNCT and BNCEFNT has been presented by Kota et al 43,44 The work of Wuu, Maughan, and Kota is based on the ''dual counter microdosimetric technique'' in which two detectors of different wall materials are utilized. The method of spectral subtraction using proportional counters of different wall compositions was first introduced by DeLuca et al 45 for the purpose of direct determination of elemental kerma values in neutron beams.…”