SU-E-T-263: Luminescent Dosimetry to Measure the Out-Of-Field Low and High LET Dose Components in High Energy Photon and Proton Therapy Beams.

Abstract

Luminescent dosimetry using thermoluminescent detectors (TLDs) and optically stimulated luminescent detectors (OSLDs) were used in mixed radiation fields containing both low LET (photons and protons) and high LET (neutrons)components to obtain their out-of-field absorbed dose, dose equivalent and quality factor. LiF Thermoluminescent Detectors (TLDs) 600 and 700 chips with dimensions 0.31×0.31×0.038 cm3 were used in a 25.4 cm diameter Bonner sphere centered 42 cm from the isocenter of a 15×x15 cm2 field to measure the secondary doses for 10, 15 and 18 MV photons and a 200 MeV proton therapy beam. From the sensitivity difference to LET radiation between the210 and 280 C peaks in the glow curve, the areas under the peaks were used to obtain the absorbed dose, dose equivalent and QF of the secondary radiation. The OSLD detector measured the low LET dose component to compare with the TLD dose measurement. The neutron calibration of the TLDs was obtained from an Am-Be source at the Argonne National Laboratory. The photon and proton TLD and OSLD calibrations were obtained in 6 MV and 200 MeV beams, respectively. From the two-peak analysis of the TLDs in the Bonner sphere the ratios of the neutron dose to photon dose were 0.001, 0.014 and 0.17 for 10, 15 and 18 MV, respectively. The low LET OSLD measurements agreed within 10% of the TLD results. From the dose equivalent measurements the QFs (+/-14%) obtained were 4.5, 3.9 and 4.0 for these beam energies. For the 200 MeV proton beam the ratio of neutron to proton dose was 0.28 with a measured QF of 13. Luminescent detectors in a Bonner Sphere provide measurements of the secondary photon, proton and neutron doses and provide an estimate of the neutron QF.