Zhiyan Xiao

Performance of KCl:Eu2+ storage phosphor dosimeters for low-dose measurements

Recent research has demonstrated that europium doped potassium chloride (KCl:Eu(2+)) storage phosphor material has the potential to become the physical foundation of a novel and reusable dosimetry system using either film-like devices or devices similar to thermoluminescent dosimeter chips. The purposes of this work are to quantify the performance of KCl:Eu(2+) prototype dosimeters for low-dose measurements and to demonstrate how it can be incorporated into clinical application for in vivo peripheral dose measurements. Pellet-style KCl:Eu(2+) dosimeters, 6 mm in diameter, and 1 mm thick, were fabricated in-house for this study. The dosimeters were read using a laboratory photostimulated luminescence detection system. KCl:Eu(2+) prototype storage phosphor dosimeter was capable of measuring a dose-to-water as low as 0.01 cGy from a 6 MV photon beam with a signal-to-noise ratio greater than 6. A pre-readout thermal annealing procedure enabled the dosimeter to be read within an hour post-irradiation. After receiving large accumulated doses (~10 kGy), the dosimeters retained linear response in the low-dose region with only a 20% loss of sensitivity comparing to a fresh sample (zero Gy history). The energy dependence encountered during low-dose peripheral measurements could be accounted for via a single point outside-field calibration per each beam quality. With further development the KCl:Eu(2+-)-based dosimeter could become a versatile and durable dosimetry tool with large dynamic range (sub-cGy to 100 Gy).

The role of activator concentration and precipitate formation on optical and dosimetric properties of KCl: Eu2+ storage phosphor detectors

PURPOSE The activator ion (Eu(2+) in KCl:Eu(2+)) plays an important role in the photostimulated luminescence (PSL) mechanism of storage phosphor radiation detectors. In order to design an accurate, effective, and robust detector, it is important to understand how the activator ion concentration affects the structure and, consequently, radiation detection properties of KCl:Eu(2+). METHODS Potassium chloride pellets were fabricated with various amounts of europium dopant (0.01-5.0 mol.% Eu(2+)). Clinical radiation doses were given with a 6 MV linear accelerator. Radiation doses larger than 100 Gy were given with a (137)Cs irradiator. Dose response curves, radiation hardness, and temporal signal stability were measured using a laboratory PSL readout system. The crystal structure of the material was studied using x ray diffraction and luminescence spectroscopy. RESULTS The most intense PSL signal was from samples with 1.0 mol.% Eu. However, samples with concentrations higher than 0.05 mol.% Eu exhibited significant degradation in PSL intensity for cumulated doses larger than 3000 Gy. Structural and luminescence spectroscopy showed clear evidence of precipitate phases within the KCl lattice, especially for high activator concentrations. Analysis of PL emission spectra showed that interactions between Eu-Vc dipoles and Eu-Vc trimers could explain trends in PSL sensitivity and radiation hardness observations. CONCLUSIONS The concentration of the activator ion (Eu(2+)) significantly affects radiation detection properties of the storage phosphor KCl:Eu(2+). An activator concentration between 0.01 and 0.05 mol.% Eu in KCl:Eu(2+) storage phosphor detectors is recommended for linear dose response, good PSL sensitivity, predictable temporal stability, and high reusability for megavoltage radiation detection.

Temporal signal stability of KCl:Eu2+ storage phosphor dosimeters

PURPOSE Current KCl:Eu(2+) prototype dosimeters require a wait time of 12 h between irradiation and dosimetric readout. Although irradiating the dosimeters in the evening and reading on the following day works well in the clinical schedule, reducing the wait time to few hours is desirable. The purposes of this work are to determine the origin of the unstable charge-storage centers and to determine if these centers respond to optical or thermal excitation prior to dosimetric readout. METHODS Pellet-style KCl:Eu(2+) dosimeters were fabricated in-house for this study. A 6 MV photon beam was used to irradiate the dosimeters. After x ray irradiation, dosimeters were subjected to external excitation with near-infrared (NIR) light, ultraviolet (UV) light, or thermal treatment. Photostimulated luminescence (PSL) signals temporal stability was subsequently measured at room temperature over a few hours using a laboratory PSL readout system. The dosimeters were also placed in a cryostat to measure the temperature dependence of the temporal stability down to 10 K. RESULTS Strong F-band was present in the PSL stimulation spectrum, indicating that F-centers were the electron-storage centers in KCl:Eu(2+) where an electron was stored at a chlorine anion vacancy. Due to deep energy-depth (2.2 eV), F-centers were probably not responsible for the fast fading in the first a few hours post x ray irradiation. In addition, weak NIR bands were present. However, there was no change in PSL stabilization rate with intense NIR excitation, suggesting that the NIR bands played no role in the PSL fading. At temperatures lower than 77 K there was almost no signal fading with time. Noticeable PSL was observed for undoped KCl samples at room temperature, suggesting that Cl(2) (-) V(k) centers served as hole-storage centers for both undoped and doped KCl where a hole was trapped by a chlorine molecular ion. V(k) centers were stable at low temperature and became mobile at room temperature, probably causing the observed PSL fading with time. On the other hand, V(k) center could be stabilized by Eu(2+) activator or oxygen in the lattice, leading to the stable component in the PSL. A thermal process at elevated temperatures (60 °C or higher) was able to significantly accelerate the migration process resulting in a fast stabilization of PSL. However, this could not be accomplished using intense UV excitation. CONCLUSIONS Thermal treatment enables KCl:Eu(2+) prototypes to be ready for readout in 1 h without the need of applying a large time-dependent correction factor. However, this cannot be achieved using optical preexcitation.

TU-C-BRE-04: 3D Gel Dosimetry Using ViewRay On-Board MR Scanner: A Feasibility Study

PURPOSE MR based 3D gel has been proposed for radiation therapy dosimetry. However, access to MR scanner has been one of the limiting factors for its wide acceptance. Recent commercialization of an on-board MR-IGRT device (ViewRay) may render the availability issue less of a concern. This work reports our attempts to simulate MR based dose measurement accuracy on ViewRay using three different gels. METHODS A spherical BANG gel dosimeter was purchased from MGS Research. Cylindrical MAGIC gel and Fricke gel were fabricated in-house according to published recipes. After irradiation, BANG and MAGIC were imaged using a dual-echo spin echo sequence for T2 measurement on a Philips 1.5T MR scanner, while Fricke gel was imaged using multiple spin echo sequences. Difference between MR measured and TPS calculated dose was defined as noise. The noise power spectrum was calculated and then simulated for the 0.35 T magnetic field associated with ViewRay. The estimated noise was then added to TG-119 test cases to simulate measured dose distributions. Simulated measurements were evaluated against TPS calculated doses using gamma analysis. RESULTS Given same gel, sequence and coil setup, with a FOV of 180×90×90 mm3, resolution of 3×3×3 mm3, and scanning time of 30 minutes, the simulated measured dose distribution using BANG would have a gamma passing rate greater than 90% (3%/3mm and absolute). With a FOV 180×90×90 mm3, resolution of 4×4×5 mm3, and scanning time of 45 minutes, the simulated measuremened dose distribution would have a gamma passing rate greater than 97%. MAGIC exhibited similar performance while Fricke gel was inferior due to much higher noise. CONCLUSIONS The simulation results demonstrated that it may be feasible to use MAGIC and BANG gels for 3D dose verification using ViewRay low-field on-board MRI scanner.

TH‐C‐19A‐12: Two‐Dimensional High Spatial‐Resolution Dosimeter Using Europium Doped Potassium Chloride

PURPOSE Recent research has shown that KCl:Eu2+ has great potential for use in megavoltage radiation therapy dosimetry because this material exhibits excellent storage performance and is reusable due to strong radiation hardness. This work reports our attempts to fabricate 2D KCl:Eu2+ storage phosphor films (SPFs) using both a physical vapor deposition (PVD) method and a tape casting method. METHODS A thin layer of KCl:Eu2+ was deposited on a substrate of borosilicate glass (e.g., laboratory slides) with a PVD system. For tape casting, a homogenous suspension containing storage phosphor particles, liquid vehicle and polymer binder was formed and subsequently cast by doctor-blade onto a polyethylene terephthalate substrate to form a 150 μm thick SPF. RESULTS X ray diffraction analysis showed that a 10 μm thick PVD sample was composed of highly crystalline KCl. No additional phases were observed, suggesting that the europium activator had completed been incorporated into the KCl matrix. Photostimulated luminescence and photoluminescence spectra suggested that F (Cl-) centers were the electron storage centers post x ray irradiation and that Eu2+ cations acted as luminescence centers in the photostimulation process. The 150 μm thick casted KCl:Eu2+ SPF showed sub-millimeter spatial resolution. Monte Carlo simulations further demonstrated that the admixture of 20% KCl:Eu2+ and 80% low Z polymer binder exhibited almost no energy dependence in a 6 MV beam. KCl:Eu2+ pellet samples showed a large dynamic range from 0.01 cGy to 60 Gy dose-to-water, and saturated at approximately 500 Gy as a Result of its intrinsic high radiation hardness. CONCLUSIONS This discovery research provides strong evidence that KCl:Eu2+ based SPF with associated readout apparatus could Result in a novel electronic film system that has all the desirable features associated with classic radiographic film and, importantly, water equivalence and the capability of permanent identification of each detector. This work was supported in part by NIH Grant No. R01CA148853. The authors would like to thank Paul Leblans (AGFA Healthcare, Belgium) for many helpful discussions on this topic.

TU‐C‐108‐06: Performance of KCl:Eu2+ Storage Phosphor Dosimeters for Low Dose Measurements

Purpose: Recent research has demonstrated that europium doped potassium chloride (KCl:Eu2+) storage phosphor material has the potential to become the physical foundation of a novel and reusable dosimetry system using either film‐like devices or devices similar to thermoluminescent dosimeter chips. In this work, we will quantify the low dose detection resolution of the KCl:Eu2+ prototype dosimeter and demonstrate how it can be incorporated into clinical application for in vivo peripheral dose measurements. Methods: The prototype KCl:Eu2+ dosimeters used in this study were manufactured according to standard materials processes, and had a physical makeup similar to thermoluminescent dosimeter chips. The lowest dose‐to‐water of 0.01 cGy in this work was achieved underneath a fully closed multileaf collimator at a source‐to‐surface distance of ∼200 cm. For doses greater than 100 Gy, prototype dosimeters were irradiated using the XRLM4 beamline at the Center for Advanced Mircrostructure and Devices synchrotron facility at Louisiana State University in Baton Rouge, LA. To compare sensitivity, two commercial storage phosphor detectors were also irradiated under the same conditions. The dosimeters were read using a laboratory photostimulated luminescence detection system. Results: KCl:Eu2+ prototype storage phosphor dosimeter is capable of measuring a dose‐to‐water as low as 0.01 cGy from a 6MV photon beam with a signal‐to‐noise ratio greater than 6. Agfa MD10, Fuji STIII and KCl:Eu2+ prototypes were within 10% in sensitivity. A pre‐readout thermal annealing procedure enables the dosimeter to be read within an hour post irradiation. The dosimeter retains a constant response after receiving large accumulated doses (∼10 kGy). The energy‐dependence encountered during low dose peripheral measurements can be accounted for via a single point outside‐field calibration per each beam quality. Conclusions: With further development the KCl:Eu2+− based dosimeter could become a versatile and durable dosimetry tool with large dynamic range (sub‐cGy to 100 Gy). This work was supported in part by NIH Grant No. R01CA148853.

SU-E-T-46: Optimization of Activator Concentration in KCl:Eu2+ Storage Phosphors for Megavoltage Dosimetry

PURPOSE Recent work has shown that the storage phosphor, KCl:Eu2+, has great potential to become the physical foundation of a reusable, highresolution planar dosimeter. The activator ion (Eu2+) plays an important role in storage phosphor performance. The effects of activator concentration on dosimetric properties must be understood to design a robust dosimeter with a linear dose response, high sensitivity, and is easily integrated into clinical application. In this work, we investigate the effects of adjusting the activator concentration in KCl:Eu2+ on dose response, sensitivity, and temporal stability. METHODS Six mm diameter, 1mm thick KCl:Eu2+ pellets were fabricated using a hydraulic press. Europium concentration was varied between 0.01-5.0 mole percent. Pellets were irradiated with a 6MV photon beam. Signal stability and dose response linearity were studied using a laboratory photostimulated luminescence (PSL) readout system. PSL emission and stimulation measurements were obtained at room temperature to determine if concentration-induced effects occurred in the electronic structure. RESULTS The highest PSL intensity was achieved for samples with approximately 1 mole % europium. Significant degradation of PSL intensity was observed for samples with europium concentration greater than 1 mole %. PSL stimulation data showed that no additional storage centers were created with increased europium concentrations. Fading curve measurements showed a thermal treatment at 70 °C for 15 minutes was able to eliminate the fast fading component, regardless of concentration. CONCLUSIONS Concentration quenching of CsBr:Eu2+ (commercially available computed radiography storage phosphor panel) occurs at much lower activator concentrations (∼0.05 mole % Eu2+), compared to KCl:Eu2+. The higher quenching concentration of KCl:Eu2+ may indicate that precipitate formation has an insignificant effect on PSL properties. Further, KCl:Eu2+ dosimeters can be manufactured with an activator concentration as low as 0.01 mole percent while maintaining a linear dose response and reasonable PSL sensitivity. This work was supported in part by NIH Grant No. R01CA148853.

TU-A-BRB-02: Temporal Signal Stability of KCl:Eu2+ Storage Phosphor Dosimeters

Purpose: It is known that the PSL (photo‐stimulated luminescence) signal fades with time after x‐ray irradiation and eventually reaches a plateau. The purpose of this work is to determine whether x‐ray generatedelectrons stored in non‐dosimetric shallow traps can be liberated prior to dose readout. If so, we would achieve a rapid signal stabilization so the wait time could be shortened to, for example, one hour or less. Methods: Pellet‐style KCl: Eu2+ dosimeters were fabricated in‐house for this study. PSL signal stability after x‐ray irradiation was studied by interfering with a thermal annealing and optical annealing with near infra‐red light, respectively prior to dosimetric readout. Results: Without optical annealing, PSL faded with time at a rate of 13.6%/hr for the first 2 hrs and 6%/hr for the next 3 hrs after irradiation and eventually reached a fading rate of about 0.5%/hr after 12 hrs. PSL intensity decreased with increasing NIR optical annealing time; however, there was only marginal improvement in the PSL stabilization rate, indicating that the shallow electron traps that are responsible for the fast‐fading for the first few hours were not efficiently emptied. 15‐minute thermal annealing treatments at either 60 or 70°C stabilized the PSL to a slow decay rate of approximately 0.4%/hr over a time period of 24 hrs post irradiation. Moreover, 90% of PSL sensitivity can be retained. More stable PSL signal, 0.18%/hr and 0.08%/hr decay rate for 90 and 120°C thermal annealing, respectively, can be achieved at the expense of PSL sensitivity. Conclusions: Thermal annealing immediately after x‐ray irradiation is an effective way to remove electrons stored in the shallow traps. This study is expected to guide the future design of storage phosphor dosimeter readers, which could incorporate a thermal annealing pre‐read cycle. As a Result, a long wait time would be avoided.

SU‐E‐T‐513: Clinical Implementation of a GPU Accelerated Pencil Beam Dose Calculation Algorithm

PURPOSE This work reports a clinical implementation of a GPU accelerated pencil beam dose calculation algorithm (GPU-PB). METHODS Model parameters were determined using in-house scripts written in MATLAB. Dose distributions in a water phantom were calculated using a Pinnacle TPS for various open field sizes. Lateral profiles at 2-mm incremental depths were used to calculate PB kernel parameters. Weighted sum of squares were used for least-squares parameter fitting utilizing a Levenberg-marquardt algorithm. Weightings were adjusted based on goodness of fitting and in accordance with field size to suppress deviations due to horn effects. The scale factor was fitted iteratively. The calculated doses for two patient cases were analyzed with a 3D gamma method (3%/3mm). RESULTS Excellent agreement between Pinnacle calculation and GPU-PB calculation was achieved regarding PDD, profiles and output factor. For a head-neck 10-field step-and-shoot IMRT case, gamma passing rate was over 99% and maximum absolute dose value is 75.6 Gy vs. 73.9 Gy, differing by 2.3%. Similar results were obtained for a SBRT lung case. Gamma passing rate is a function of PB kernel cut-off distance and beamlet resolution. It appears that if the highest accuracy is desired, a resolution of 10 mm or better in the direction parallel to MLC travel and a cutoff distance of 10 cm or better should be used. The calculation time increases with both cut-off distance and beamlet resolution. For example, GPU calculation time is 1.36 seconds for 5 cm cut-off distance, and increases to 4.84 s for 15 cm cut-off distance. CONCLUSIONS A GPU accelerated PB dose calculation algorithm has been implemented using clinical measurement data. Excellent agreement with Pinnacle TPS has been achieved. Beamlet size and PB cut-off distance should be chosen according to desired dose calculation accuracy and speed.