Determination of the intrinsic energy dependence of LiF:Mg,Ti thermoluminescent dosimeters for 125I and 103Pd brachytherapy sources relative to 60Co

Purpose: To determine the intrinsic energy dependence of LiF:Mg,Ti thermoluminescent dosimeters (TLD-100) for I-125 and Pd-103 brachytherapy sources relative to Co-60. Methods: LiF: Mg,Ti TLDs were irradiated with low-energy brachytherapy sources and with a Co-60 teletherapy source. The brachytherapy sources measured were the Best 2301 I-125 seed, the OncoSeed 6711 I-125 seed, and the Best 2335 Pd-103 seed. The TLD light output per measured air-kerma strength was determined for the brachytherapy source irradiations, and the TLD light output per air kerma was determined for the Co-60 irradiations. Monte Carlo (MC) simulations were used to calculate the dose-to-TLD rate per air-kerma strength for the brachytherapy source irradiations and the dose to TLD per air kerma for the Co-60 irradiations. The measured and MC-calculated results for all irradiations were used to determine the TLD intrinsic energy dependence for I-125 and Pd-103 relative to Co-60. Results: The relative TLD intrinsic energy dependences (relative to Co-60) and associated uncertainties (k = 1) were determined to be 0.883 +/- 1.3%, 0.870 +/- 1.4%, and 0.871 +/- 1.5% for the Best 2301 seed, OncoSeed 6711 seed, and Best 2335 seed, respectively. Conclusions: The intrinsic energy dependence of TLD-100 is dependent on photon energy, exhibiting changes of 13%-15% for I-125 and Pd-103 sources relative to Co-60. TLD measurements of absolute dose around I-125 and Pd-103 brachytherapy sources should explicitly account for the relative TLD intrinsic energy dependence in order to improve dosimetric accuracy. (C) 2014 American Association of Physicists in Medicine.