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(Jannatul Madinah Wahabi, University of Malaya, Malaysia)
Ionising radiation plays a critical role in patient care, serving as a screening tool for trauma and accidents, diagnosing illnesses and delivering cancer treatment. To ensure safety and accuracy, a radiation dosimeter is used to monitor dose exposure. However, existing dosimeters have limitations like non-tissue equivalent properties, passive readout, limited lifetime, and energy dependency. To overcome these challenges, the utilisation of a radioluminescent (RL) dosimeter is proposed. This study aimed to develop and test a prototype RL dosimetry system in clinical settings. The single-sensor system was developed using an SP101 plastic scintillator coupled with optical fibre as a light guide. When ionising radiation interacts with the plastic scintillator, light photons will be emitted and transmitted to the photodetector. The photodetector will convert the detected light into electrical pulses that are counted by a counter unit. The measured signals were recorded and converted into dose units using an in-house developed computer script. The dosimeter prototype was characterised for mammography dosimetry, which comprised calibration, energy dependency assessment, exposure time measurement, digital breast tomosynthesis (DBT) pulse evaluation, uncertainty estimation and mean glandular dose (MGD) measurement. The prototype was further characterised in radiotherapy beams to study dose linearity, reproducibility, dose rate dependency, energy dependency and percentage depth dose. Additionally, the dosimeter was utilised to measure the surface dose and dose in-target of various radiotherapy techniques, including conformal radiotherapy (CRT), intensity modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT) and stereotactic radiosurgery (SRS). A multi-sensor system was developed by adopting the time-division multiplexing (TDM) concept, with an optical switch integrated into the single-sensor design to enable input from two channels and generate a single output. The dosimeter was assessed for acquisition time for each channel, switching time and dead time. Then, the system was used to measure the surface dose during a CRT treatment. For mammography dosimetry, upon calibration, the RL dosimeter exhibited 2.87 % energy dependency within the typical tube potential used and 0.02 % discrepancy in the measurement of the exposure time. The DBT pulse rate was 3.9 pulse/s, and an uncertainty of 10.48 % was observed. Large uncertainties were attributed to non-uniform radiation fields and variations in filter materials. MGD measurement resulted in 2.5 mGy, 1.77 mGy and 3.74 mGy for rhodium, silver and aluminium filters, respectively. In radiotherapy, the dose linearity was excellent and independent of dose rate. However, it showed a 6.5 % energy dependency. The accuracy of surface dose measurement was between -1.55 % and 2.18 % for CRT and 1.05 % to 11.66 % for IMRT, VMAT and SRS. The multi-sensor dosimeter demonstrated a linear response to the delivered dose and had a dead time of 3 ms, which is equivalent to dose deficit of 0.04 mGy. Channel-2 consistently exhibited a longer acquisition time compared with Channel-1, deviating by 36.4 % from the preset acquisition time. For surface dose measurements in CRT, the accuracy of the dosimeter ranged between -6.13 % to 3.19 %. In conclusion, the RL dosimeter showed a promising solution to overcome the limitations of current dosimeters used in clinical practise. The usage encompassed low and high- energy photon beams, allowing it to be used for multimodalities setup. It showed linear dose-response, with no dose rate dependence, and could measure clinical dose and perform dose verification in various radiotherapy techniques.
Keywords: plastic scintillator dosimeter, fibre optic dosimeter, radiotherapy, mammography, in vivo dosimetry