Development and evaluation of a thallium (I) bromide dosimeter for intracavitary radiotherapy quality assurance

In intracavitary radiotherapy, incorrect source locations can result in excessive doses to normal tissues. Therefore, it is essential to accurately evaluate the source location. In this study, we investigated a digital line dosimeter based on thallium (I) bromide (TlBr) to improve the existing analogue verification method. Therefore, a polycrystalline TlBr unit cell dosimeter was manufactured, and the measurement performance of iridium-192 (Ir-192) sources was evaluated. We found that the dosimeter's reproducibility satisfied the evaluation criteria of 1.5% with a relative standard deviation of 1.44%. Moreover, the linearity showed excellent results (linear coefficient, R 2 = 0.9999). The distance dependence showed a difference of 0.03 cm at 50% intensity when compared to the inverse square value, whereas the angular dependence showed a large difference when compared to a diode. As the angle increased, the intensity gradually decreased, resulting in a difference of up to 41.9%. These results demonstrate the excellent performance of the TlBr dosimeter. However, because of the significant influence of angular dependence, a measurement distance that minimises this error should be applied when manufacturing line dosimeters in the future.