Evaluation of photoconductor and scintillator hybrid dosimeters for radiation therapy quality assurance

Radiation detectors have two main detection mechanisms: direct conversion, which utilises a photoconductor to directly convert radiation into an electrical signal, and indirect conversion, which utilises a scintillator to convert radiation into visible light, which is subsequently converted into an electrical signal. The photoconductor material, mercury (II) iodide (HgI2) 2 ) sensitive to visible light, has the property of gradually decreasing signal for repeated irradiation beams. Therefore, this study aimed to improve the signal magnitude and stability to repeated measurements of polycrystalline HgI2 2 by mixing 0, 4, 6, 8, 10, and 20 weight percent (wt%) of terbium-doped gadolinium oxysulfide (GOS:Tb), a scintillator material that emits visible light. The performance of these dosimeters was evaluated with a focus on dosimetric parameters such as signal quantity, short-term repeatability, dose-response linearity, and dose-rate dependence to assess their potential application in radiotherapy quality assurance (QA). The results from the dosimeters were compared with those from a metal- oxide-semiconductor field-effect transistor (MOSFET) dosimeter and an ionisation chamber. Peak was found for the dosimeter with 8 wt% GOS:Tb but decreased for the dosimeters with 10 and 20 wt% GOS:Tb. The standard deviations of short-term repeatability were 1.45%, 0.86%, 1.18%, 1.51%, 131%, and 4.48% for dosimeters with 0, 4, 6, 8, 10, and 20 wt% GOS:Tb, respectively. The dosimeters with 4 and 6 wt% GOS:Tb had better standard deviations of repeatability than the MOSFET (1.35%), but not better than the ionization chamber (0.5%). All dosimeters, the MOSFET, and the ion chamber demonstrated excellent dose-response linearity for monitor unit (MU) values ranging from 2 to 500, with R-2 values exceeding 0.9997. The sensitivity analysed using the linear slope was the highest for the dosimeter with 6 wt% GOS:Tb (0.103) and the MOSFET (0.0103) among all dosimeters. The dosimeter with 6 wt% GOS:Tb and the MOSFET exhibited deviations of 4.64% and 3.31%, respectively, at a dose rate of 100 cGy/min, thus failing to meet the evaluation standard of within 2%. The ion chamber, however, achieved compliance with a deviation of 0.15%. Therefore, it is necessary to apply correction factors to each dose rate when using hybrid dosimeters for QA. The optimal mixing ratio of GOS:Tb for manufacturing the hybrid dosimeter was determined to be 6 wt%, suggesting signal amplification through visible light and a potential improvement in operational stability through pore filling of the material.