Determination of absorbed dose to water around a clinical HDR 192Ir source using LiF:Mg,Ti TLDs demonstrates an LET dependence of detector response

摘要

Purpose: Experimental radiation dosimetry with thermoluminescent dosimeters (TLDs), calibrated in a 60Co or megavoltage (MV) photon beam, is recommended by AAPM TG-43U1for verification of Monte Carlo calculated absorbed doses around brachytherapy sources. However, it has been shown by Carlsson Tedgren Med. Phys. 38, 5539-5550 (2011) that for TLDs of LiF:Mg,Ti, detector response was 4 higher in a 137Cs beam than in a 60Co one. The aim of this work was to investigate if similar over-response exists when measuring absorbed dose to water around 192Ir sources, using LiF:Mg,Ti dosimeters calibrated in a 6 MV photon beam. Methods: LiF dosimeters were calibrated to measure absorbed dose to water in a 6 MV photon beam and used to measure absorbed dose to water at distances of 3, 5, and 7 cm from a clinical high dose rate (HDR) 192Ir source in a polymethylmethacrylate (PMMA) phantom. Measured values were compared to values of absorbed dose to water calculated using a treatment planning system (TPS) including corrections for the difference in energy absorption properties between calibration quality and the quality in the users' 192Ir beam and for the use of a PMMA phantom instead of the water phantom underlying dose calculations in the TPS. Results: Measured absorbed doses to water around the 192Ir source were overestimated by 5 compared to those calculated by the TPS. Corresponding absorbed doses to water measured in a previous work with lithium formate electron paramagnetic resonance (EPR) dosimeters by Antonovic Med. Phys. 36, 2236-2247 (2009), using the same irradiation setup and calibration procedure as in this work, were 2 lower than those calculated by the TPS. The results obtained in the measurements in this work and those obtained using the EPR lithium formate dosimeters were, within the expanded (k 2) uncertainty, in agreement with the values derived by the TPS. The discrepancy between the results using LiF:Mg,Ti TLDs and the EPR lithium formate dosimeters was, however, statistically significant and in agreement with the difference in relative detector responses found for the two detector systems by Carlsson Tedgren Med. Phys. 38, 5539-5550 (2011) and by Adolfsson Med. Phys. 37, 4946-4959 (2010). Conclusions: When calibrated in 60Co or MV photon beams, correction for the linear energy transfer (LET) dependence of LiF:Mg,Ti detector response will be needed as to measure absorbed doses to water in a 192Ir beam with highest accuracy. Such corrections will depend on the manufacturing process (MTS-N Poland or Harshaw TLD-100) and details of the annealing and read-out schemes used.