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.
Optical fibre scintillation detectors have proven to be a viable alternative in clinical radiation dosimetry. By obtaining active readouts, scintillators can be used to give real time measurements for various clinical and pre-clinical applications from radiology to radiation therapy. Gd2O2S:Pr is of interest as a scintillator due to it offering a much higher light output than other scintillators, organic scintillators in particular. Temperature dependence is exhibited by a number of organic scintillators, such as BCF-60; this however needs to be established for the inorganic scintillator Gd2O2S:Pr. This study therefore aims to characterize the temperature dependence of Gd2O2S:Pr, using the HYPERSCINT Research Platform 200. The detector was immersed in water and the temperature varied from 22.7°C to 49.0°C using a hotplate and temperature controller. Five spectra were recorded and averaged at each of the seven temperatures, in approximately 5°C increments. A decrease in total photon count with temperature was observed of 0.22%/°C between 346 and 631 nm with a decrease of 0.38 in the full width at half maximum at the photopeak of 513 nm. A method of correcting for temperature is necessary in the use of Gd2O2S:Pr as a detection material in environments where the temperature differs significantly from the calibration temperature.
FLASH-Radiotherapy (FLASH-RT) is an emerging radiotherapy technique delivering ionizing radiation beam at ultrahigh dose rates (UHDR), typically ≥40 Gy/s. Animal studies have demonstrated the safety and efficacy of the technique in killing tumor cells while significantly reducing radiation toxicity in normal tissues, compared to conventional radiotherapy (dose-rate exposure <0.03 Gy/s). A reliable real-time dosimeter system is crucial for the characterization of the so-called ‘FLASH-effect’ and an accurate beam delivery. Standard dosimeters for conventional radiotherapy saturate at this high-intensity field or cannot provide real-time measurements. In previous work, optical fiber inorganic scintillating detectors (ISDs) showed excellent linearity with shutter exposure time and tube current, indicating scintillating signals independent of the dose and dose rate, respectively. This study aims to benchmark the performance of the ISD with plastic scintillating detectors (PSDs) for an ultrahigh dose-rate x-ray beam irradiation. Relative scintillator output, signal linearity with dose and dose rate, signal-to-noise ratio (SNR), signal stability and reliability were evaluated for all detectors. In a UHDR x-ray beam irradiation, the ISDs produced a larger SNR than the PSDs. All detectors showed good linearity with tube current (R2 < 0.975) and shutter exposure (R2 >0.999). Gd2O2S:Tb showed excellent repeatability (coefficient of variation (CV) <0.1%) compared to other detectors, while the PSDs resulted in the highest reliability for a UHDR beam measurement with a CV of <0.1%. A further investigation regarding the positioning uncertainty of the ISDs during irradiation due to the detector’s angular dependency and the optimal design of the scintillator detectors for UHDR applications are required.
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.
KEYWORDS: Lithium, Ultrasonography, Monte Carlo methods, Radiotherapy, Solids, Medical imaging, Tomography, Computed tomography, Cancer, Current controlled current source
A revised version of this paper, published originally on 2016, was published on 6 January, 2017, replacing the original paper. To correct for a set-up error, the simulated treatment plan was re-run using CT images of a solid water phantom with/without the Autoscan US probe in contact with its surface. The revised paper is available at http://dx.doi.org/10.1117/12.2216653.
Changes to original text:
Section 2.2
1. A solid water phantom is employed as opposed to a Rando phantom.
2. Figure 1 is changed to illustrate new phantom.
Section 2.3
1. New treatment isocentre.
2. Figure 3 is changed to illustrate dose distribution in new phantom.
Section 3.2
1. Text of first paragraph has been revised to reflect new results.
2. Table 2 has been revised to reflect new results.
3. Figure 6 has been revised to reflect new results.
Section 4
1. Text of second paragraph has been revised to reflect new results.
KEYWORDS: Monte Carlo methods, Ultrasonography, Radiotherapy, Computer simulations, Computed tomography, Physics, Radiation dosimetry, Prostate, Data modeling, Solids, Head
The aim of this study was to quantify the dosimetric effect of the AutoscanTM ultrasound probe, which is a 3D transperineal probe used for real-time tissue tracking during the delivery of radiotherapy. CT images of a solid water phantom, with and without the probe placed in contact with its surface, were obtained (0.75 mm slice width, 140 kVp). CT datasets were used for relative dose calculation in Monte Carlo simulations of a 7-field plan delivered to the phantom. The Monte Carlo software packages BEAMnrc and DOSXYZnrc were used for this purpose. A number of simulations, which varied the distance of the radiation field edge from the probe face (0 mm to 5 mm) were performed. Perineal surface doses as a function of distance from the radiation field edge, with and without the probe in place, were compared. The presence of the probe was found to result in negligible dose differences when the radiation field is not delivered through the probe. A maximum surface dose increase of ≈1% was found when the probe face to field edge distance was 0 mm. Surface doses with and without the probe in place agreed within Monte Carlo simulation uncertainty at distances ≥ 3 mm. Using data from three patient volunteers, a typical probe face to field edge distance was calculated to be ≈20 mm. Our results therefore indicate that the presence of the probe does not adversely affect a typical patient treatment, due to the relatively large probe face to field edge distance.
This work outlines the development of a multi-pinhole SPECT system designed to produce a synthetic-collimator image of a small field of view. The focused multi-pinhole collimator was constructed using rapid-prototyping and casting techniques. The collimator projects the field of view through forty-six pinholes when the detector is adjacent to the collimator. The detector is then moved further from the collimator to increase the magnification of the system. The amount of pinhole-projection overlap increases with the system magnification. There is no rotation in the system; a single tomographic angle is used in each system configuration. The maximum-likelihood expectation-maximization (MLEM) algorithm is implemented on graphics processing units to reconstruct the object in the field of view. Iterative reconstruction algorithms, such as MLEM, require an accurate model of the system response. For each system magnification, a sparsely-sampled system response is measured by translating a point source through a grid encompassing the field of view. The pinhole projections are individually identified and associated with their respective apertures. A 2D elliptical Gaussian model is applied to the pinhole projections on the detector. These coefficients are associated with the object-space location of the point source, and a finely-sampled system matrix is interpolated. Simulations with a hot-rod phantom demonstrate the efficacy of combining low-resolution non-multiplexed data with high-resolution multiplexed data to produce high-resolution reconstructions.
We have recently developed a digital x-ray image receptor for use in mammographic procedures. The detector is based upon a photoconductor, amorphous selenium (a-Se), coupled to a polymer dispersed liquid crystal (PDLC) layer. A potential is applied across the structure to create a bias electric field in the photoconductor. When x-rays are absorbed in the photoconductor, electron-hole pairs are released. The created charges are swept to the a-Se /PDLC interface via the applied electric field, which causes potential variations across the PDLC. These variations lead to liquid crystal (LC) molecule re-orientation, which affects the propagation of readout light from an external source through the display. The readout light can be bright in this arrangement so that no secondary quantum sinks are present. Since this system is independent of light creation in contrast with a phosphor screen system, the image brightness can be adjusted independently of the number of x-rays used to make the image. The image can be digitized with a CCD camera and a frame grabber. Results will be presented on the PDLC characteristics, the system model and initial images from the detector.
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.