Robust External-Beam Calibration of Plastic Scintillation Detectors for In-Vivo Dosimetry in HDR Brachytherapy

TL;DR

This work addresses the challenge of robust in-vivo dosimetry in HDR brachytherapy by introducing a robust external-beam calibration of a Plastic Scintillation Detector (PSD) using hyperspectral techniques to remove stem effects. The method calibrates the PSD with a 6 MV linac and validates it in an Ir-192 brachytherapy setup, delivering dose-rate measurements along transverse and depth axes and comparing to TG-43U1 references. A comprehensive uncertainty budget (including TG-43U1, afterloader, and CCU reproducibility, plus detector response) is presented, revealing that near the source the dose-gradient‑driven positional uncertainty dominates, while at larger distances the detector-signal uncertainty becomes the main contributor; this yields a practical 3–5 cm optimal calibration range. The results demonstrate the feasibility of time-resolved PSD-based IVD in HDR brachytherapy and quantify fundamental limitations inherent to IVD, guiding future improvements in PSD design and calibration strategies.

Abstract

Purpose: HDR brachytherapy is a widely adopted modality for cancer treatment. However, it is not free from error and uncertainty. In-vivo dosimetry (IVD) is the only technique that can confirm correct dose delivery. This study details and validates a calibration method for Plastic Scintillation Detector (PSD), bypassing dose gradient and positioning issues in brachytherapy calibration. Methods: The PRB-0057 PSD (Medscint, Canada) was calibrated, 1x1 mm scintillating fiber coupled to a 20 m Eska GH-4001 clear optical fiber (Mitsubishi Rayon, Japan). The fiber is connected to the Hyperscint-RP200 research platform for signal collection. Hyperspectral calibration was performed at a LINAC with a 6 MV beam, enabling removal of stem effects before brachytherapy measurements. For validation, an Iridium-192 Flexisource (Elekta Brachy, The Netherlands), Sk=29447U, was used in a motorized IBA-Blue-Phantom2 water tank (48x48x41cm3). Dose rates were measured at 10 Hz along the source z-axis at a fixed transverse distance of 1.2+/-0.05 cm in 0.2 cm steps. Relative difference (RD) between measured and TG-43U1 dose rates was assessed. A detailed uncertainty budget was associated with brachytherapy measurements. Results: Comparison shows good agreement with RD around 2.5 $\%$ at 1.2 cm, corresponding to positional uncertainties of <0.15 mm. At greater depths up to 8 cm, RDs increase to about 5 $\%$, mainly due to reduced light yield. Uncertainties depend on the source-detector distance, ranging from 3.81 to 6.39 $\%$ (k=1) over the explored range. Conclusions: Results confirm the PSD calibration effectiveness using a 6MV external beam with hyperspectral technique. Uncertainties close to the source align with positional errors and are dominated by reduced PSD sensitivity at larger distances. The study underlined the intrinsic limitation of IVD in the face of known uncertainties.

Figures (6)

• Figure 1: Schematic of the single-point plastic scintillation detector (PSD) from Medscint (QC, Canada), composed of a 1 mm diameter, 1 mm long polystyrene scintillator optically coupled to a clear PMMA optical fiber.
• Figure 2: Spectral calibration setup for the plastic scintillation detector (PSD) using a TrueBeam linear accelerator. (a) PSD positioned at isocenter with the tip aligned to the Linac kV source for scintillation spectra extraction. (b) Fluorescence spectra extraction while 6 loops of the clear fiber of around $10~cm$ of diameter is placed at the Linac kV source. (c) Acquisition of reference spectra for Cherenkov contribution I and II by irradiating the optical fiber at gantry angles $45^{\circ}$ and $315^{\circ}$, respectively, with the PSD tip placed outside the primary field at around $0.5~cm$. (d) Measurement geometry at extended source-to-detector distances ($\sim1 ~m$) for Cherenkov contributions III and IV. Both (c) and (d) setups used a 6 MV beam and a $15\times15~cm$ field size. All spectra acquisitions were performed at 1 Hz.
• Figure 3: Experimental setup used for PSD dose calibration with a 6 MV beam. The tip was positioned at $d_{\text{max}} = 1.5$ cm depth in solid water at a Source to Detector Distance (SDD) of 100 cm, in the center of a $10 \times 10$ cm$^{2}$ reference field. A 500 MU irradiation was delivered to establish the conversion between scintillation light yield and absorbed dose to water.
• Figure 4: Experimental setup at the BT unit for the MV-beam calibration validation. (a) brachytherapy measurement setup: the water tank from IBA showing the positioning unit of the PSD, catheter/source holder, and the Flexitron afterloader. (b) 3D representation of the geometry used in the experiment, showing the relative positioning of the 192Ir source and the PSD within the water phantom along with the displacement axes.
• Figure 5: Comparison between measured dose rates ($\dot{D}_M$, blue) and TG-43U1‐based dose rates ($\dot{D}_{\text{TPS}}$, red) at 10 Hz acquisition rate. (a) Measured and calculated dose rates along the z-axis of the source (depth scan). (d) Measured and calculated dose rates along the x-axis within the source transverse plane at $\theta=90^{\circ}$ (crossline scan). Error bars on $\dot{D}_M$ correspond to the standard error of the mean ($k=1$) over 31 values per dwell-position frame. (b) and (e) show the relative difference RD between $\dot{D}_M$ and $\dot{D}_{\text{TPS}}$ (Equation \ref{['eq:RD']}), with dotted lines indicating the $\pm 5~\%$ agreement range. For both b and e panel, the cyan band ($\pm~u_{Pos}$) represents the uncertainty on the RD from positional and TG43-U1 contribution only as defined in Equation \ref{['eq:uPos']}. The pink band represents the total uncertainty on the RD as propagated from Equation \ref{['eq:RD']} and as referred to in the uncertainty budget as $U_c$ from Equation\ref{['eq:combined_uncertainty']} including both $u_{\mathrm{Pos}}$ and the measurement uncertainty $u_M$. (c) and (f) panels show the positional error $\Delta r$ as calculated from Equation \ref{['eq:deltarTG43']} including measurement and positional deviations in the RD values. The cyan band shows the propagated uncertainty $\sigma_{\Delta r,\mathrm{Pos}}$ due to $u_{\mathrm{Pos}}$ only, and the pink band the total propagated uncertainty $\sigma_{\Delta r,c}$ based on $U_c$. Dashed horizontal lines indicate the $\pm 0.2~\text{cm}$ positional tolerance recommended for clinical in vivo dosimetry Freniere2018AAPM013.