An optical fibre dosimeter based on a terbium-doped gadolinium oxysulphide (Gd2O2S:Tb) inorganic scintillating detector (ISD) was recently proposed for external beam radiation therapy (EBRT) dosimetry applications. Although it has demonstrated many promising characteristics, an energy dependence was found during percentage depth dose (PDD) measurements. In this study, the response of a Gd2O2S:Tb based ISD to superficial x-ray energies and 6 MV EBRT photon beams has been measured and compared to absorbed dose values generated using a Monte Carlo (MC) model of a superficial x-ray treatment unit and a clinical linear accelerator treatment head. The relationship between beam energy and the response of the scintillating phosphor was investigated for depth dose and beam profile measurements. An overresponse was observed during physical measurements in the kV range that is indicative of an energy-dependent variation in scintillation efficiency of the Gd2O2S:Tb. This study indicates that this intrinsic energy dependence may result in a significant increase in signal at kV energies relative to MV irradiation, which was not accounted for in the MC model.
KEYWORDS: Sensors, Monte Carlo methods, Gadolinium, Zinc, Optical fibers, Animal model studies, Modeling, 3D modeling, Absorption, Polymethylmethacrylate
In the field of radiation therapy, optical fiber dosimeters (OFD) offer several advantages over conventional dosimeters for real-time dosimetry. Their sensing tips can be small in size affording them the potential for high spatial resolution capabilities. In previous work, a novel inorganic scintillating detector (ISD) based on Gadolinium Oxysulfide (gadox) was fabricated for in vivo optical fiber dosimetry of conformal small animal irradiators. The performance of this detector was evaluated for 40 and 80 kVp imaging beams and the 220 kVp therapy beam of the Small Animal Research Platform (SARRP). The purpose of this study was to use a validated Monte Carlo (MC) model of the SARRP to investigate (i) dose absorption in the ISD active volume and (ii) dose perturbation by the inorganic scintillating phosphor volume. A comparison was also drawn between the perturbation by the gadox phosphor and a ZnS-based phosphor. The gadox-based detector was seen to cause high levels of dose perturbation in the radiation field, leaving significant dose shadows in the irradiated media. The use of ZnS:Ag phosphor reduces the perturbations with just over half as much dose absorbed relative to the gadox phosphor. An optimized ISD design which utilizes the higher light yield of the less perturbing ZnS:Ag phosphor to allow for a smaller high-density active volume, and significantly mitigating kV dose perturbation, has been proposed.
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