THE NEUTRON SENSITIVITY OF DOSIMETERS APPLIED TO BORON NEUTRON-CAPTURE THERAPY

To obtain a high accuracy in the dosimetry of an epithermal neutron be am used for boron neutron capture therapy (BNCT), the neutron sensitiv ity of dosimeters applied to determine the various dose components in- phantom has been investigated. The thermal neutron sensitivity of Mg(A r) ionization chambers, TE(TE) ionization chambers, and thermoluminesc ent dosimeters (TLD) has been experimentally determined in a ''pure'' thermal neutron beam. Values much higher than theoretically expected w ere obtained and a variation up to a factor of 2.5 was found between v alues for the thermal neutron sensitivity of different Mg(Ar) ionizati on chambers of the same type. The sensitivity of the TE(TE) ionization chamber to intermediate and fast neutrons (k(t)) has been calculated for the neutron energy spectrum in a phantom irradiated by a clinical epithermal BNCT beam, obtained using Monte Carlo simulations. The k(t) value for muscle tissue ranged from 0.87 at small depths to 0.93 at l arger depths in the phantom. The application of the thermal neutron se nsitivities to measurements in a phantom irradiated by the epithermal BNCT beam yielded up to 17% higher gamma-ray dose rate values compared with measurements using Li-6 containing caps to shield the detectors from thermal neutrons, due to a substantial perturbation of the in-pha ntom radiation field by the Li-6 cap. The application of the new k(t) values resulted in a dose from intermediate and fast neutrons about 10 % higher than the dose based on currently applied relative neutron sen sitivities of TE(TE) chambers in BNCT beams. The resulting improvement in the: accuracy of the determination of the dose from gamma rays and intermediate and fast neutrons is important in view of the required a ccuracy for dosimetry in radiotherapy. (C) 1996 American Association o f Physicists in Medicine.