Zhaohui Han

Quantitative megavoltage radiation therapy dosimetry using the storage phosphor KCl: Eu2+.

This work, for the first time, reports the use of europium doped potassium chloride (KCl:Eu2+) storage phosphor for quantitative megavoltage radiation therapy dosimetry. In principle, KCl:Eu2+ functions using the same photostimulatated luminescence (PSL) mechanism as commercially available BaFBr0.85I0.15:Eu2+ material that is used for computed radiography (CR) but features a significantly smaller effective atomic number-18 versus 49-making it a potentially useful material for nearly tissue-equivalent radiation dosimetry. Cylindrical KCl:Eu2+ dosimeters, 7mm in diameter and 1mm thick, were fabricated in-house. Dosimetric properties, including radiation hardness, response linearity, signal fading, dose rate sensitivity, and energy dependence, were studied with a laboratory optical reader after irradiation by a linear accelerator. The overall experimental uncertainty was estimated to be within ±2.5%. The findings were (1) KCl:Eu2+ showed satisfactory radiation hardness. There was no significant change in the stimulation spectra after irradiation up to 200Gy when compared to a fresh dosimeter, indicating that this material could be reused at least 100 times if 2Gy per use was assumed, e.g., for patient-specific IMRT QA. (2) KCl:Eu2+ exhibited supralinear response to dose after irradiation from 0to800cGy. (3) After x ray irradiation, the PSL signal faded with time and eventually reached a fading rate of about 0.1%∕h after 12h. (4) The sensitivity of the dosimeter was independent of the dose rate ranging from 15to1000cGy∕min. (5) The sensitivity showed no beam energy dependence for either open x ray or megavoltage electron fields. (6) Over-response to low-energy scattered photons was comparable to radiographic film, e.g., Kodak EDR2 film. By sandwiching dosimeters between low-energy photon filters (0.3mm thick lead foils) during irradiation, the over-response was reduced. The authors have demonstrated that KCl:Eu2+ dosimeters have many desirable dosimetric characteristics that make the material conducive to radiation therapy dosimetry. In the future, a large-area KCl:Eu2+-based CR plate with a thickness of the order of a few microns, created using modern thin film techniques, could provide a reusable, quantitative, high-resolution two-dimensional dosimeter with minimal energy dependence.

Theoretical and empirical investigations of KCl:Eu2+ for nearly water-equivalent radiotherapy dosimetry

PURPOSE The low effective atomic number, reusability, and other computed radiography-related advantages make europium doped potassium chloride (KCl : Eu2+) a promising dosimetry material. The purpose of this study is to model KCl : Eu2+ point dosimeters with a Monte Carlo (MC) method and, using this model, to investigate the dose responses of two-dimensional (2D) KCl : Eu2+ storage phosphor films (SPFs). METHODS KCl : Eu2+ point dosimeters were irradiated using a 6 MV beam at four depths (5-20 cm) for each of five square field sizes (5 x 5-25 x 25 cm2). The dose measured by KCl : Eu2+ was compared to that measured by an ionization chamber to obtain the magnitude of energy dependent dose measurement artifact. The measurements were simulated using DOSXYZnrc with phase space files generated by BEAMnrcMP. Simulations were also performed for KCl : Eu2+ films with thicknesses ranging from 1 microm to 1 mm. The work function of the prototype KCl : Eu2+ material was determined by comparing the sensitivity of a 150 microm thick KCl : Eu2+ film to a commercial BaFBr0.85 I0.15 : Eu(2+)-based SPF with a known work function. The work function was then used to estimate the sensitivity of a 1 microm thick KCl : Eu2+ film. RESULTS The simulated dose responses of prototype KCl : Eu2+ point dosimeters agree well with measurement data acquired by irradiating the dosimeters in the 6 MV beam with varying field size and depth. Furthermore, simulations with films demonstrate that an ultrathin KCl : Eu2+ film with thickness of the order of 1 microm would have nearly water-equivalent dose response. The simulation results can be understood using classic cavity theories. Finally, preliminary experiments and theoretical calculations show that ultrathin KCl : Eu2+ film could provide excellent signal in a 1 cGy dose-to-water irradiation. CONCLUSIONS In conclusion, the authors demonstrate that KCl : Eu(2+)-based dosimeters can be accurately modeled by a MC method and that 2D KCl : Eu2+ films of the order of 1 microm thick would have minimal energy dependence. The data support the future research and development of a KCl : Eu2+ storage phosphor-based system for quantitative, high-resolution multidimensional radiation therapy dosimetry.

Low-cost flexible thin-film detector for medical dosimetry applications.

The purpose of this study is to characterize dosimetric properties of thin film photovoltaic sensors as a platform for development of prototype dose verification equipment in radiotherapy. Towards this goal, flexible thin‐film sensors of dose with embedded data acquisition electronics and wireless data transmission are prototyped and tested in kV and MV photon beams. Fundamental dosimetric properties are determined in view of a specific application to dose verification in multiple planes or curved surfaces inside a phantom. Uniqueness of the new thin‐film sensors consists in their mechanical properties, low‐power operation, and low‐cost. They are thinner and more flexible than dosimetric films. In principle, each thin‐film sensor can be fabricated in any size (mm2 – cm2 areas) and shape. Individual sensors can be put together in an array of sensors spreading over large areas and yet being light. Photovoltaic mode of charge collection (of electrons and holes) does not require external electric field applied to the sensor, and this implies simplicity of data acquisition electronics and low power operation. The prototype device use for testing consists of several thin film dose sensors, each of about 1.5 cm×5 cm area, connected to simple readout electronics. Sensitivity of the sensors is determined per unit area and compared to EPID sensitivity, as well as other standard photodiodes. Each sensor independently measures dose and is based on commercially available flexible thin‐film aSi photodiodes. Readout electronics consists of an ultra low‐power microcontroller, radio frequency transmitter, and a low‐noise amplification circuit implemented on a flexible printed circuit board. Detector output is digitized and transmitted wirelessly to an external host computer where it is integrated and processed. A megavoltage medical linear accelerator (Varian Tx) equipped with kilovoltage online imaging system and a Cobalt source are use to irradiate different thin‐film detector sensors in a Solid Water phantom under various irradiation conditions. Different factors are considered in characterization of the device attributes: energies (80 kVp, 130 kVp, 6 MV, 15 MV), dose rates (different ms × mA, 100–600 MU/min), total doses (0.1 cGy‐500 cGy), depths (0.5 cm–20 cm), irradiation angles with respect to the detector surface (0°‐180°), and IMRT tests (closed MLC, sweeping gap). The detector response to MV radiation is both linear with total dose (~1‐400 cGy) and independent of dose rate (100‐600 Mu/min). The sensitivity per unit area of thin‐film sensors is lower than for aSi flat‐panel detectors, but sufficient to acquire stable and accurate signals during irradiations. The proposed thin‐film photodiode system has properties which make it promising for clinical dosimetry. Due to the mechanical flexibility of each sensor and readout electronics, low‐cost, and wireless data acquisition, it could be considered for quality assurance (e.g., IMRT, mechanical linac QA), as well as real‐time dose monitoring in challenging setup configurations, including large area and 3D detection (multiple planes or curved surfaces). PACS number: 87.56.Fc

Nuclear magnetic resonance study of the Ru∕Mn valence states and magnetic interactions in SrRu0.9Mn0.1O3

The substitution of Mn for Ru in SrRuO3 suppresses the ferromagnetic (FM) order monotonically at low concentration ( 0.39). In an attempt to understand how the Mn substitution initially modifies the magnetic interactions in SrRu1−xMnxO3, a polycrystalline SrRu0.9Mn0.1O3 sample was prepared and studied by Mn55 and Ru99,101 spin-echo nuclear magnetic resonance (NMR). Mn55 NMR reveals that the Mn dopants are in an intermediate Mn3+∕4+ valence state due to fast electron hopping. The magnetic interaction between the Mn ions is FM, mediated by the double exchange mechanism. The Ru4+ hyperfine field is reduced due to an AFM coupling with the Mn moments, which shifts the Ru NMR peaks to lower frequencies. The suppression of the FM ordering temperature in SrRu1−xMnxO3 is attributed to the interruption of the itinerancy of the Ru 4d electrons by Mn substitution.

High-resolution transmission electron microscopy studies of planar defects in the magnetic superconductor RuSr2EuCu2O8

The defect structure in powders of the 1212-type phase RuSr2EuCu2O8 was investigated using high-resolution transmission electron microscopy. Both 90° domain boundaries and planar “faults” on {100} were observed. By comparing images of the faults with simulations for various plausible atomic models, it was shown that they are crystallographic shear planes with a displacement vector of R=(1∕6)⟨332⟩. Such defects correspond to a local deficiency in Ru, Cu, and O, and give a local discontinuity in the Ru-O charge reservoir planes. As such, these defects may be responsible for the sensitivity of the superconducting and magnetic properties to processing conditions.

Evaluation of initial setup accuracy and intrafraction motion for spine stereotactic body radiation therapy using stereotactic body frames

PURPOSE The purposes of this study were (1) to evaluate the initial setup accuracy and intrafraction motion for spine stereotactic body radiation therapy (SBRT) using stereotactic body frames (SBFs) and (2) to validate an in-house-developed SBF using a commercial SBF as a benchmark. METHODS AND MATERIALS Thirty-two spine SBRT patients (34 sites, 118 fractions) were immobilized with the Elekta and in-house (BHS) SBFs. All patients were set up with the Brainlab ExacTrac system, which includes infrared and stereoscopic kilovoltage x-ray-based positioning. Patients were initially positioned in the frame with the use of skin tattoos and then shifted to the treatment isocenter based on infrared markers affixed to the frame with known geometry relative to the isocenter. ExacTrac kV imaging was acquired, and automatic 6D (6 degrees of freedom) bony fusion was performed. The resulting translations and rotations gave the initial setup accuracy. These translations and rotations were corrected for by use of a robotic couch, and verification imaging was acquired that yielded residual setup error. The imaging/fusion process was repeated multiple times during treatment to provide intrafraction motion data. RESULTS The BHS SBF had greater initial setup errors (mean±SD): -3.9±5.5mm (0.2±0.9°), -1.6±6.0mm (0.5±1.4°), and 0.0±5.3mm (0.8±1.0°), respectively, in the vertical (VRT), longitudinal (LNG), and lateral (LAT) directions. The corresponding values were 0.6±2.7mm (0.2±0.6°), 0.9±5.3mm (-0.2±0.9°), and -0.9±3.0mm (0.3±0.9°) for the Elekta SBF. The residual setup errors were essentially the same for both frames and were -0.1±0.4mm (0.1±0.5°), -0.2±0.4mm (0.0±0.4°), and 0.0±0.4mm (0.0±0.4°), respectively, in VRT, LNG, and LAT. The intrafraction shifts in VRT, LNG, and LAT were 0.0±0.4mm (0.0±0.3°), 0.0±0.5mm (0.0±0.4°), and 0.0±0.4mm (0.0±0.3°), with no significant difference observed between the 2 frames. CONCLUSIONS These results showed that the combination of the ExacTrac system with either SBF was highly effective in achieving both setup accuracy and intrafraction stability, which were on par with that of mask-based cranial radiosurgery.

SU‐GG‐T‐242: The Use of a Commercial Multi‐Detector Device for Daily QA of Linear Accelerators

Purpose: With the increasing number of complex treatments there is a pressing need for QA tools that can help improve QA efficiency and comprehensiveness. The clinical use of a commercial multi‐detector device for daily QA of linear accelerators is presented. Method and Materials: The DailyQA 3™ (Sun Nuclear, Melbourne, FL) device consists of 25 detectors: 5 ionization chambers for flatness, symmetry, and output check; 4 curved ionization chambers for photon energy check by studying beam flatness; 4 circular chambers with inherent attenuators for electron energy verification; 4 sets of three diodes with 5 mm spacing for light‐radiation coincidence check. During morning output check, the open 20×20 cm2 field is formed by MLC in order to check the SMLC positioning. For DMLC delivery check, a slit with 2 cm gap is programmed to move across the detector array.Results: With one exposure for each modality, we can verify that the output is within 3%, the symmetry (both axial ant transverse) and flatness are within 3%, light‐radiation coincidence is within 3 mm, and electron energy is within 3%. The flatness is an extremely sensitive indicator to the change of X‐ray energy. Our data showed that a 10% tolerance for X‐ray energy was clinical acceptable, which translates to 1MV for 6MV and 2 MV for 18 MV. This meets 2% tolerance in PDD as recommended by AAPM protocol. The QA device is able to detect 1 mm SMLC error on a daily basis. The DMLC output typically is within 3%, which approximately translates to 0.6 mm leaf positioning/motion consistency. Combined with monthly picket‐fence QA, it provides confidence in MLC positioning. Conclusion: The clinical use of this multi‐detector device has improved the efficiency and thoroughness of linear accelerator daily QA. Reasonable action level has been established by considering dosimetric relevance and clinic flow.

Nuclear magnetic resonance and magnetization studies of the ferromagnetic ordering temperature suppression in Ru deficient SrRuO3

Abstract The synthesis of SrRuO 3 under high-pressure oxygen produces a nonstoichiometric form with randomly distributed vacancies on the Ru sites, along with a significantly reduced ferromagnetic ordering temperature. In order to gain additional insight into the suppression of the ferromagnetism, local studies utilizing 99,101 Ru zero-field spin-echo NMR, Ru K-edge XAFS and XANES, along with complimentary magnetization and X-ray diffraction measurements, have been carried out on samples of SrRuO 3 annealed at both (ambient) atmospheric pressure and ‘high-pressure’ oxygen (600 atm). Consistent with previous work, the NMR spectrum for ambient SrRuO 3 consists of two well-defined peaks at 64.4 and 72.2 MHz corresponding to the 99 Ru and 101 Ru isotopes, respectively, and a hyperfine field of 329 kG. Although the magnetization measurements show a lower ferromagnetic ordering temperature for the high-pressure oxygen sample (90 K compared to 160 K for the ambient sample), the NMR spectrum shows no significant shift in the two peak frequencies. However, the two peaks exhibit considerable broadening, along with structure on both the low and high frequency sides which is believed to be quadrupolar in origin. Analysis of the Ru K-edge XAFS reveals more disorder in the Ru–O bond for the high-pressure oxygen sample compared to the ambient sample. Furthermore, XANES of Ru K-edge analysis indicates no difference in the valence of Ru between the two samples. The magnetic behavior indicates the existence of some vacancies on the Ru sites for the high-pressure oxygen sample.

Topological detector: measuring continuous dosimetric quantities with few-element detector array.

A prototype topological detector was fabricated and investigated for quality assurance of radiation producing medical devices. Unlike a typical array or flat panel detector, a topological detector, while capable of achieving a very high spatial resolution, consists of only a few elements and therefore is much simpler in construction and more cost effective. The key feature allowing this advancement is a geometry-driven design that is customized for a specific dosimetric application. In the current work, a topological detector of two elements was examined for the positioning verification of the radiation collimating devices (jaws, MLCs, and blades etc). The detector was diagonally segmented from a rectangular thin film strip (2.5 cm  ×  15 cm), giving two contiguous but independent detector elements. The segmented area was the central portion of the strip measuring 5 cm in length. Under irradiation, signals from each detector element were separately digitized using a commercial multichannel data acquisition system. The center and size of an x-ray field, which were uniquely determined by the collimator positions, were shown mathematically to relate to the difference and sum of the two signals. As a proof of concept, experiments were carried out using slit x-ray fields ranging from 2 mm to 20 mm in size. It was demonstrated that, the collimator positions can be accurately measured with sub-millimeter precisions.

SU-E-T-440: Assessing the Accuracy of Treatment Planning System Calculated Out-of-Target Doses in IMRT and VMAT Plans through 2D Measurements

Purpose: To quantify the dose uncertainties in the out‐of‐target regions for both VMAT and IMRT. Methods: 12 clinical IMRT (6) and VMAT (6) plans for six patients were created in Varian Eclipse 8.6 and delivered to solid water phantoms containing a 2D ion chamber array — MatriXX. The measured doses in the out‐of‐target regions were compared with TPS calculations. Two types of out‐of‐target regions were considered in this study: type I, with direct irradiations and type II, without direct irradiations. Results: The TPS was found to generally underestimate the out‐of‐target doses for both IMRT and VMAT plans. The mean underestimate was 48.0% and 35.5% for IMRT and VMAT, respectively, in regions without direct irradiations (type II), where the dominant dose contributions are scatter. In regions where direct irradiations are present (type I), the underestimate was significantly smaller, which was 6.5% and 4.0% for IMRT and VMAT, respectively. The percentage dose discrepancy increases as the distance to the field edge increases for both IMRT and VMAT. The dose discrepancies between TPS and measurements also demonstrate a clear dependence on the equivalent square jaw size for both IMRT and VMAT plans. As the jaw size increases, the observed discrepancy increases. This phenomenon is attributed to both open beam and dynamic MLC modeling in the TPS. Conclusions: The accuracy of the out‐of‐target dose by Varian Eclipse TPS for IMRT and VMAT was investigated through 2D phantom measurements. Eclipse was found to underestimate the doses in those regions. The underestimate shows dependence on the types of plans, equivalent field size, and the distance to the field edge. Such an underestimate can have impacts on the evaluation of the OAR and normal tissue sparing.