Robust external-beam calibration of plastic scintillation detectors and uncertainty analysis forin-vivodosimetry in...

Objective.High dose rate brachytherapy (BT) is a widely adopted modality for cancer treatment. However, it is not free from error and uncertainty.In-vivodosimetry (IVD) is the only technique that confirms correct dose delivery. This study details and validates a calibration method for plastic scintillation detector (PSD), bypassing dose gradient and positioning issues in BT calibration.Approach.The PRB-0057 PSD (Medscint, Québec, Canada), a1×1mm scintillating fiber coupled to a 20 m Eska GH-4001 clear optical fiber (Mitsubishi Rayon, Japan) of the same diameter, was calibrated and connected to the Hyperscint-RP200 research platform for optical signal collection. Hyperspectral calibration was performed at a LINAC with a kV and a 6 MV beam, enabling the independent stem effect removal. For validation, BT measurements with aSk=29447U iridium-192 Flexisource (Elekta Brachy, Veenendaal, The Netherlands) were performed in a motorized IBA-Blue-Phantom²water tank. Dose rates were measured at 10 Hz along the source's vertical/transverse axis at a fixed transverse distance of1.2±0.05cm in 0.2 cm steps. Calibration accuracy was evaluated using relative differences (RDs) between measured and TG-43U1 dose rates, converted to inverse-square equivalent positional errors. A detailed uncertainty budget was established to the measurement setup.Main results.Comparison agreed with RDs around 2.5% at 1.2 cm, corresponding to inverse-square positional uncertainties of<0.15mm. At greater distances, up to 8 cm, RDs increase to about 5%, corresponding to inverse-square positional uncertainties up to 3 mm, mainly due to reduced light-yield. Uncertainties found to depend on the source-detector distance, ranging from 3.92% to6.42% (k=1)over the range of explored distances.Significance.Results confirm the effectiveness of a 6 MV external beam PSD calibration to be used in time-resolved IVD. Uncertainties close to the source are consistent with the Afterloader and IBA motorized unit reproducibility and are mainly dominated by reduced detector response at larger distances. Our study further underlined the intrinsic limitation of IVD in the face of known uncertainties.