Evaluation of polycrystalline thallium (I) bromide dosimeter for radiotherapy quality assurance

In radiotherapy, dose assessment is performed as part of quality assurance (QA) to verify the dose accuracy of the treatment administered to patients. Various dosimeters are used depending on the part of the body and the desired purpose, and tools such as the thermoluminescent dosimeter (TLD) and semiconductor dosimeter are used as in vivo dosimeters. However, TLDs have demonstrated measurement errors of approximately 11.8%, and the diodes used in semiconductor dosimeters fluctuate in performance owing to radiation damage. Consequently, various photoconductor materials are being researched to replace diodes. In particular, Thallium (I) bromide (TlBr) is being used as a substitute material for semiconductors because of its low cost and high performance. Therefore, this study determined the optimised performance of TlBr, which is a potential photoconductor material to replace other materials commercially, using the particle-in-binder deposition method. Upon evaluating the reproducibility and linearity, all sensors demonstrated remarkable performance at 6 MV and 15 MV as the results demonstrated a relative standard deviation (RSD) less than 1.5% and R-2 value above 0.9998, which were the evaluation criteria. A 3:1 material ratio was chosen, as the performance demonstrated in this case was superior to that of other material ratios. The results of the monitor unit rate dependence and percent depth dose (PDD) were compared with those of silicon diode and thimble chambers, which are widely used for dose measurement. According to the monitor unit rate dependence results, the RSD was 0.66% at 6 MV and 0.27% at 15 MV with the standard set at 100 MU, indicating that the diode was outperformed by less than 1%. Similar results were obtained for the PDD with the ion chamber, and the surface dose required for in vivo dose measurement indicated an error within 8.67%. Therefore, this study provides basic data for polycrystalline TlBr dosimeters, which can replace the existing semiconductor dosimeters for QA.