This work considers the feasibility of using a novel optical fiber based sensor, employing a Terbium-doped Gadolinium Oxysulfide (Gd2O2S:Tb) inorganic scintillator, as a real-time in-vivo dosimetry solution for applications in low dose rate (LDR) prostate brachytherapy (BT). This study specifically considers the influence of scintillator geometry (hemi-sphere tip vs cylindrical cavity), PMMA fiber core diameter (0.5 mm vs 1.0 mm), and sensor housing material (stainless steel vs plastic) on the measured scintillation signal. Characterization measurements were performed using a Silicon Photon-Multiplier (SiPM) detector and a commercial water phantom system, integrated with custom 3D printed components to allow for precise positioning of the LDR BT radiation source with respect to the optical fiber sensor. Significant differences in the rate of fall-off in the scintillation signal, with distance from the source, were observed between the different scintillator geometries considered. The hemi-sphere tip geometry was shown to be the most accurate, tracking with the expected fall-off in dose-rate, within measurement uncertainty. Reducing the fiber core diameter from 1.0 mm to 0.5 mm resulted in a 6-fold reduction in the detected scintillation signal. A further 57 % reduction was observed when housing the 0.5 mm fiber within a stainless steel LDR BT needle applicator. Initial results demonstrate the feasibility of employing an optical fiber sensor, for applications in LDR BT, given the excellent agreement of measurements with theoretical expectations. Furthermore a calibration process has been described for converting the detected scintillation signal into absorbed dose / dose-rate, using our water phantom based experimental set-up.

An optical fibre sensor based on radioluminescence, using the scintillation material terbium doped gadolinium oxysulphide (Gd₂O₂S:Tb) is evaluated, using a 3D printed anthropomorphic phantom for applications in low dose-rate (LDR) prostate brachytherapy. The scintillation material is embedded in a 700 µm diameter cavity within a 1 mm plastic optical fibre that is fixed within a brachytherapy needle. The high spatial resolution dosimeter is used to measure the dose contribution from Iodine-125 (I-125) seeds. Initially, the effects of sterilisation on the sensors (1) repeatability, (2) response as a function of angle, and (3) response as a function of distance, are evaluated in a custom polymethyl methacrylate phantom. Results obtained in this study demonstrate that the output response of the sensor, pre- and post-sterilisation are within the acceptable measurement uncertainty ranging from a maximum standard deviation of 4.7% pre and 5.5% post respectively, indicating that the low temperature sterilisation process does not damage the sensor or reduce performance. Subsequently, an LDR brachytherapy plan reconstructed using the VariSeed treatment planning system, in an anthropomorphic 3D printed training phantom, was used to assess the suitability of the sensor for applications in LDR brachytherapy. This phantom was printed based on patient anatomy, with the volume and dimensions of the prostate designed to represent that of the patient. I-125 brachytherapy seeds, with an average activity of 0.410 mCi, were implanted into the prostate phantom under trans-rectal ultrasound guidance; following the same techniques as employed in clinical practice by an experienced radiation oncologist. This work has demonstrated that this sensor is capable of accurately identifying when radioactive I-125 sources are introduced into the prostate via a brachytherapy needle.

Source localisation and real-time dose verification are at the forefront of medical research in brachytherapy, an oncological radiotherapy procedure based on radioactive sources implanted in the patient body. The ORIGIN project aims to respond to this medical community’s need by targeting the development of a multi-point dose mapping system based on fibre sensors integrating a small volume of scintillating material into the tip and interfaced with silicon photomultipliers operated in counting mode. In this paper, a novel method for the selection of the optimal silicon photomultipliers to be used is presented, as well as a laboratory characterisation based on dosimetric figures of merit. More specifically, a technique exploiting the optical cross-talk to maintain the detector linearity in high-rate conditions is demonstrated. Lastly, it is shown that the ORIGIN system complies with the TG43-U1 protocol in high and low dose rate pre-clinical trials with actual brachytherapy sources, an essential requirement for assessing the proposed system as a dosimeter and comparing the performance of the system prototype against the ORIGIN project specifications.

The aim of this study was to investigate the over-response of an inorganic optical fibre sensor (OFS) when measuring percentage depth dose curves (PDDs) with respect to an ion chamber by means of physical measurements and Monte Carlo (MC) simulations. The sensor was constructed by filling a cavity (700 μm diameter and 7 mm deep), which was made in a PMMA (polymethyl methacrylate) plastic optical fibre, with an inorganic scintillating material: terbium doped gadolinium oxysulphide (Gd2O2S:Tb). The MC software packages BEAMnrc and DOSXYZnrc were used to develop a MC model of an Elekta Versa HD linear accelerator (linac), which was then used to simulate the Gd2O2S:Tb scintillator. The results of the PDD measurements showed a depth dependence of the OFS, however the percentage differences between the ion chamber and the OFS measurements showed that as the radiation field size decreases, the difference between the two measurements decreases from 16.5% to 5.1% for 10x10 cm2 and 2x2 cm2, respectively. The MC simulation of the sensor showed a good agreement compared to physical measurements at shallow depth in the phantom; however, discrepancies were observed at depth, which was less pronounced for 4x4 cm2 than for 10x10 cm2. The results of this study indicate that including Cerenkov radiation measurements is essential to accurately quantify the overresponse and the higher discrepancy between the measured and simulated PDD profiles of the OFS.