When optical fibre dosimetry systems are exposed to ionizing radiation, unwanted Cerenkov radiation and fluorescent light are produced in the fibre itself during irradiation. A number of techniques have been used to eliminate or minimize these effects. In this study time discrimination technique was used, by measuring the signal of an inorganic scintillation detector between linac pulses, after the stem effect signal has decayed to successfully eliminate the contribution of Cerenkov radiation. Dosimetric properties, including the repeatability of the ISD system response and angular dependence of the system, were tested. Percentage depth dose profiles were measured for different field sizes and compared to ion chamber measurements. The result of this study shows that the ISD system has good repeatability of the output signal when exposed to high and low radiation doses with a maximum deviation of 0.55% and 1.10%, respectively. However, the system showed a strong angular dependence in the azimuthal plane due to the detector shape. Additionally, the system overestimates the dose when measuring PDDs, this effect decreased with the decrease in field sizes.
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.
This paper presents an initial investigation into the depth dependence of an inorganic optical fibre sensor (OFS) based on physical measurements and Monte Carlo (MC) simulations, using a 6 MV flattening filter free (FFF) beam. The OFS was fabricated using an inorganic scintillating material (Gd2O2S:Tb), which was embedded in a cavity of diameter 700 μm, in a 1mm plastic optical fibre. Percentage depth dose (PDD) profiles were measured in a solid water phantom for three field sizes: 10×10 cm2 , 4×4 cm2 and 2×2 cm2 . The OFS results were then compared to an ion chamber and the W1 plastic scintillator. A MC model of an Elekta Versa HD linear accelerator (linac) was developed using the MC software packages BEAMnrc and DOSXYZnrc and then used to simulate the Gd2O2S:Tb and polystyrene scintillators. The OFS measurements over-estimated the dose when compared to the ion chamber and the W1 measurements, across the investigated field sizes, by a maximum of 30%, 20% and 15% for 10×10 cm2 , 4×4 cm2 and 2×2 cm2 , respectively. The MC simulations of the Gd2O2S:Tb and polystyrene scintillators were in good agreement with the W1 and ion chamber measurements, however, the OFS measurements were found to differ across all field sizes. Our results therefore indicate the need for further investigation into the overall contribution of the stem effect to the discrepancy between the OFS physical measurements and the ion chamber and the W1 measurements.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.