Calculation of the TLD700:LiF energy response from Ir-192 using novel Monte Carlo and empirical methods

Lithium fluoride thermoluminescent dosimeters (TLDs) exhibit a dependence on the energy of the radiation beam of interest so need to be carefully calibrated for different energy spectra if used for clinical radiation oncology beam dosimetry and quality assurance. TLD energy response was investigated for a specific set of TLD700:LiF(Mg,Ti) chips for a high dose rate (192)Ir brachytherapy source. A novel method of energy response calculation for (192)Ir was developed where dose was determined through Monte Carlo modelling in Geant4. The TLD response was then measured experimentally. Results showed that TLD700 has a depth dependent response in water ranging from 1.170 ± 0.125 at 20 mm to 0.976 ± 0.043 at 50 mm (normalised to a nominal 6 MV beam response). The method of calibration and Monte Carlo data developed through this study could be easily applied by other Medical Physics departments seeking to use TLDs for (192)Ir patient dosimetry or treatment planning system experimental verification.