Influence of the variation of energy spectra with depth in the dosimetry of ¹⁹²Ir using LiF TLD

Absolute dose measurements around low activity seeds used in brachytherapy are difficult to perform with ionisation chambers. The physical size of the chamber does not allow good resolution close to the seeds and the ionisation current generated is very low. Small thermoluminescent dosimeters (TLDs) overcome these problems but the energy dependence of their response should be considered. In this work, dose in polystyrene was measured at several distances from the high activity ¹⁹²Ir source (370 GBq) of a remote afterloading device using an ionisation chamber and LiF TLD chips. These data show that over a range of 1-10 cm from the source the sensitivity of LiF varies by up to 8.5%. This is attributed to the higher response of LiF to the lower photon energies, and to the shift of the photon spectrum to lower energies with increasing depth. The sensitivity of LiF to ¹⁹²Ir was also calculated by weighting the energy-dependent response of LiF by the Monte Carlo calculated photon spectra. The calculations give a similar change in sensitivity with distance from the source.