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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

A real-time in vivo dosimetric verification method for high-dose rate intracavitary brachytherapy of nasopharyngeal carcinoma

Purpose: A real-time in vivo dosimetric verification method using metal-oxide-semiconductor field effect transistor (MOSFET) dosimeters has been developed for patient dosimetry in high-dose rate (HDR) intracavitary brachytherapy of nasopharyngeal carcinoma (NPC). Methods: The necessary calibration and correction factors for MOSFET measurements in {sup 192}Iridium source were determined in a water phantom. With the detector placed inside a custom-made nasopharyngeal applicator, the actual dose delivered to the tumor was measured in vivo and compared to the calculated values using a commercial brachytherapy planning system. Results: Five MOSFETs were independently calibrated with the HDR source, yielding calibration factors of 0.48 {+-} 0.007 cGy/mV. The maximum sensitivity variation was no more than 7% in the clinically relevant distance range of 1-5 cm from the source. A total of 70 in vivo measurements in 11 NPC patients demonstrated good agreement with the treatment planning. The mean differences between the planned and the actually delivered dose within a single treatment fraction were -0.1%{+-} 3.8% and -0.1%{+-} 3.7%, respectively, for right and left side assessments. The maximum dose deviation was less than 8.5%. Conclusions: In vivo measurement using the real-time MOSFET dosimetry system is possible to evaluate the actual dose to the tumor received by the patient during a treatment fraction and thus can offer another line of security to detect and prevent large errors.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Dosimetric properties of an amorphous silicon EPID for verification of modulated electron radiotherapy

Purpose: To investigate the dosimetric properties of an electronic portal imaging device (EPID) for electron beam detection and to evaluate its potential for quality assurance (QA) of modulated electron radiotherapy (MERT). Methods: A commercially available EPID was used to detect electron beams shaped by a photon multileaf collimator (MLC) at a source-surface distance of 70 cm. The fundamental dosimetric properties such as reproducibility, dose linearity, field size response, energy response, and saturation were investigated for electron beams. A new method to acquire the flood-field for the EPID calibration was tested. For validation purpose, profiles of open fields and various MLC fields (square and irregular) were measured with a diode in water and compared to the EPID measurements. Finally, in order to use the EPID for QA of MERT delivery, a method was developed to reconstruct EPID two-dimensional (2D) dose distributions in a water-equivalent depth of 1.5 cm. Comparisons were performed with film measurement for static and dynamic monoenergy fields as well as for multienergy fields composed by several segments of different electron energies. Results: The advantageous EPID dosimetric properties already known for photons as reproducibility, linearity with dose, and dose rate were found to be identical for electron detection. The flood-field calibration method was proven to be effective and the EPID was capable to accurately reproduce the dose measured in water at 1.0 cm depth for 6 MeV, 1.3 cm for 9 MeV, and 1.5 cm for 12, 15, and 18 MeV. The deviations between the output factors measured with EPID and in water at these depths were within {+-}1.2% for all the energies with a mean deviation of 0.1%. The average gamma pass rate (criteria: 1.5%, 1.5 mm) for profile comparison between EPID and measurements in water was better than 99% for all the energies considered in this study. When comparing the reconstructed EPID 2D dose distributions at 1.5 cm depth to film measurements, the gamma pass rate (criteria: 2%, 2 mm) was better than 97% for all the tested cases. Conclusions: This study demonstrates the high potential of the EPID for electron dosimetry, and in particular, confirms the possibility to use it as an efficient verification tool for MERT delivery.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2013

Characterization of a novel EPID designed for simultaneous imaging and dose verification in radiotherapy

Purpose: Standard amorphous silicon electronic portal imaging devices (a-Si EPIDs) are x-ray imagers used frequently in radiotherapy that indirectly detect incident x-rays using a metal plate and phosphor screen. These detectors may also be used as two-dimensional dosimeters; however, they have a well-characterized nonwater-equivalent dosimetric response. Plastic scintillating (PS) fibers, on the other hand, have been shown to respond in a water-equivalent manner to x-rays in the energy range typically encountered during radiotherapy. In this study, the authors report on the first experimental measurements taken with a novel prototype PS a-Si EPID developed for the purpose of performing simultaneous imaging and dosimetry in radiotherapy. This prototype employs an array of PS fibers in place of the standard metal plate and phosphor screen. The imaging performance and dosimetric response of the prototype EPID were evaluated experimentally and compared to that of the standard EPID.Methods: Clinical 6 MV photon beams were used to first measure the detector sensitivity, linearity of dose response, and pixel noise characteristics of the prototype and standard EPIDs. Second, the dosimetric response of each EPID was evaluated relative to a reference water-equivalent dosimeter by measuring the off-axis and field size response in a nontransit configuration, along with the off-axis, field size, and transmission response in a transit configuration using solid water blocks. Finally, the imaging performance of the prototype and standard EPIDs was evaluated quantitatively by using an image quality phantom to measure the contrast to noise ratio (CNR) and spatial resolution of images acquired with each detector, and qualitatively by using an anthropomorphic phantom to acquire images representative of human anatomy.Results: The prototype EPID's sensitivity was 0.37 times that of the standard EPID. Both EPIDs exhibited responses that were linear with delivered dose over a range of 1–100 monitor units. Over this range, the prototype and standard EPID central axis responses agreed to within 1.6%. Images taken with the prototype EPID were noisier than those taken with the standard EPID, with fractional uncertainties of 0.2% and 0.05% within the central 1 cm{sup 2}, respectively. For all dosimetry measurements, the prototype EPID exhibited a near water-equivalent response whereas the standard EPID did not. The CNR and spatial resolution of images taken with the standard EPID were greater than those taken with the prototype EPID.Conclusions: A prototype EPID employing an array of PS fibers has been developed and the first experimental measurements are reported. The prototype EPID demonstrated a much morewater-equivalent dose response than the standard EPID. While the imaging performance of the standard EPID was superior to that of the prototype, the prototype EPID has many design characteristics that may be optimized to improve imaging performance. This investigation demonstrates the feasibility of a new detector design for simultaneous imaging and dosimetry treatment verification in radiotherapy.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Characterization of a two-dimensional liquid-filled ion chamber detector array used for verification of the treatments in radiotherapy

Purpose: The purpose of the study is to investigate the characteristics of a two-dimensional (2D) liquid-filled ion chamber detector array, which is used for the verification of radiotherapy treatment plans that use small field sizes of up to 10 × 10 cm. Methods: The device used in this study was Octavius 1000 SRS model (PTW, Freiburg, Germany). Its 2D array of detectors consists of 977 liquid-filled ion chambers arranged over an area of 11 × 11 cm. The size of the detectors is 2.3 × 2.3 × 0.5 mm (volume of 0.003 cm{sup 3}) and their spacing in the inner area of 5.5 × 5.5 cm is 2.5 mm center-to-center, whereas in the outer area it is 5 mm center-to-center. The detector reproducibility, dose linearity, and sensitivity to positional changes of the collimator were tested. Also, the output factors of field sizes ranging from 0.5 × 0.5 to 10 × 10 cm{sup 2} both for open and wedged fields have been measured and compared against those measured by a pin-point ionization chamber, liquid filled microchamber, SRS diode, and EDR2 film. Results: Its short-term reproducibility was within 0.2% and its medium and long-term reproducibility was within 0.5% (verified with air ionization chamber absolute dose measurements), which is an excellent result taking into account the daily fluctuation of the linear accelerator and the errors in the device setup reproducibility. The dose linearity and dose rate dependence were measured in the range of 0.5–85 Gy and 0.5–10 Gy min{sup −1}, respectively, and were verified with air ionization chamber absolute dose measurements was within 3%. The measurements of the sensitivity showed that the 2D Array could detect millimetric collimator positional changes. The measured output factors showed an agreement of better than 0.3% with the pinpoint chamber and microliquid filled chamber for the field sizes between 3 × 3 and 10 × 10 cm{sup 2}. For field sizes down to 1 × 1 cm{sup 2}, the agreement with SRS diode and microliquid filled chamber is better than 2%. The measurements of open and wedge-modulated field profiles were compared to the film and ionization chamber in water measurements. Conclusions: The Octavius Detector 1000 SRS is an accurate, precise, and reliable detector, very useful for the daily performance of the patient specific quality assurance of radiotherapy treatment plans.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2014

Absolute calibration of the Gamma Knife{sup ®} Perfexion™ and delivered dose verification using EPR/alanine dosimetry

Purpose: Elekta Leksell Gamma Knife{sup ®} (LGK) is a radiotherapy beam machine whose features are not compliant with the international calibration protocols for radiotherapy. In this scope, the Laboratoire National Henri Becquerel and the Pitié-Salpêtrière Hospital decided to conceive a new LKG dose calibration method and to compare it with the currently used one. Furthermore, the accuracy of the dose delivered by the LGK machine was checked using an “end-to-end” test. This study also aims to compare doses delivered by the two latest software versions of the Gammaplan treatment planning system (TPS). Methods: The dosimetric method chosen is the electron paramagnetic resonance (EPR) of alanine. Dose rate (calibration) verification was done without TPS using a spherical phantom. Absolute calibration was done with factors calculated by Monte Carlo simulation (MCNP-X). For “end-to-end” test, irradiations in an anthropomorphic head phantom, close to real treatment conditions, are done using the TPS in order to verify the delivered dose. Results: The comparison of the currently used calibration method with the new one revealed a deviation of +0.8% between the dose rates measured by ion chamber and EPR/alanine. For simple fields configuration (less than 16 mm diameter), the “end-to-end” tests showed out average deviations of −1.7% and −0.9% between the measured dose and the calculated dose by Gammaplan v9 and v10, respectively. Conclusions: This paper shows there is a good agreement between the new calibration method and the currently used one. There is also a good agreement between the calculated and delivered doses especially for Gammaplan v10.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2014

SU-E-J-58: Dosimetric Verification of Metal Artifact Effects: Comparison of Dose Distributions Affected by Patient Teeth and Implants

Purpose: Implant-supported dentures seem particularly appropriate for the predicament of becoming edentulous and cancer patients are no exceptions. As the number of people having dental implants increased in different ages, critical dosimetric verification of metal artifact effects are required for the more accurate head and neck radiation therapy. The purpose of this study is to verify the theoretical analysis of the metal(streak and dark) artifact, and to evaluate dosimetric effect which cause by dental implants in CT images of patients with the patient teeth and implants inserted humanoid phantom. Methods: The phantom comprises cylinder which is shaped to simulate the anatomical structures of a human head and neck. Through applying various clinical cases, made phantom which is closely allied to human. Developed phantom can verify two classes: (i)closed mouth (ii)opened mouth. RapidArc plans of 4 cases were created in the Eclipse planning system. Total dose of 2000 cGy in 10 fractions is prescribed to the whole planning target volume (PTV) using 6MV photon beams. Acuros XB (AXB) advanced dose calculation algorithm, Analytical Anisotropic Algorithm (AAA) and progressive resolution optimizer were used in dose optimization and calculation. Results: In closed and opened mouth phantom, because dark artifacts formed extensively around the metal implants, dose variation was relatively higher than that of streak artifacts. As the PTV was delineated on the dark regions or large streak artifact regions, maximum 7.8% dose error and average 3.2% difference was observed. The averaged minimum dose to the PTV predicted by AAA was about 5.6% higher and OARs doses are also 5.2% higher compared to AXB. Conclusion: The results of this study showed that AXB dose calculation involving high-density materials is more accurate than AAA calculation, and AXB was superior to AAA in dose predictions beyond dark artifact/air cavity portion when compared against the measurements.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-J-119: What Effect Have the Volume Defined in the Alignment Clipbox for Cervical Cancer Using Automatic Registration Methods for Cone- Beam CT Verification?

Purpose: To investigate the impact of different clipbox volumes with automated registration techniques using commercially available software with on board volumetric imaging(OBI) for treatment verification in cervical cancer patients. Methods: Fifty cervical cancer patients received daily CBCT scans(on-board imaging v1.5 system, Varian Medical Systems) during the first treatment week and weekly thereafter were included this analysis. A total of 450 CBCT scans were registered to the planning CTscan using pelvic clipbox(clipbox-Pelvic) and around PTV clip box(clipbox- PTV). The translations(anterior-posterior, left-right, superior-inferior) and the rotations(yaw, pitch and roll) errors for each matches were recorded. The setup errors and the systematic and random errors for both of the clip-boxes were calculated. Paired Samples t test was used to analysis the differences between clipbox-Pelvic and clipbox-PTV. Results: . The SD of systematic error(σ) was 1.0mm, 2.0mm,3.2mm and 1.9mm,2.3mm, 3.0mm in the AP, LR and SI directions for clipbox-Pelvic and clipbox-PTV, respectively. The average random error(Σ)was 1.7mm, 2.0mm,4.2mm and 1.7mm,3.4mm, 4.4mm in the AP, LR and SI directions for clipbox-Pelvic and clipbox-PTV, respectively. But, only the SI direction was acquired significantly differences between two image registration volumes(p=0.002,p=0.01 for mean and SD). For rotations, the yaw mean/SD and the pitch SD were acquired significantly differences between clipbox-Pelvic and clipbox-PTV. Conclusion: The defined volume for Image registration is important for cervical cancer when 3D/3D match was used. The alignment clipbox can effect the setup errors obtained. Further analysis is need to determine the optimal defined volume to use the image registration in cervical cancer. Conflict of interest: none.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-24: A Simple Correction-Based Method for Independent Monitor Unit (MU) Verification in Monte Carlo (MC) Lung SBRT Plans

Purpose: Lung-SBRT uses hypo-fractionated dose in small non-IMRT fields with tissue-heterogeneity corrected plans. An independent MU verification is mandatory for safe and effective delivery of the treatment plan. This report compares planned MU obtained from iPlan-XVM-Calgorithm against spreadsheet-based hand-calculation using most commonly used simple TMR-based method. Methods: Treatment plans of 15 patients who underwent for MC-based lung-SBRT to 50Gy in 5 fractions for PTV V100%=95% were studied. ITV was delineated on MIP images based on 4D-CT scans. PTVs(ITV+5mm margins) ranged from 10.1- 106.5cc(average=48.6cc). MC-SBRT plans were generated using a combination of non-coplanar conformal arcs/beams using iPlan XVM-Calgorithm (BrainLAB iPlan ver.4.1.2) for Novalis-TX consisting of micro-MLCs and 6MV-SRS (1000MU/min) beam. These plans were re-computed using heterogeneity-corrected Pencil-Beam (PB-hete) algorithm without changing any beam parameters, such as MLCs/MUs. Dose-ratio: PB-hete/MC gave beam-by-beam inhomogeneity-correction-factors (ICFs):Individual Correction. For independent-2nd-check, MC-MUs were verified using TMR-based hand-calculation and obtained an average ICF:Average Correction, whereas TMR-based hand-calculation systematically underestimated MC-MUs by ∼5%. Also, first 10 MC-plans were verified with an ion-chamber measurement using homogenous phantom. Results: For both beams/arcs, mean PB-hete dose was systematically overestimated by 5.5±2.6% and mean hand-calculated MU systematic underestimated by 5.5±2.5% compared to XVMC. With individual correction, mean hand-calculated MUs matched with XVMC by - 0.3±1.4%/0.4±1.4 for beams/arcs, respectively. After average 5% correction, hand-calculated MUs matched with XVMC by 0.5±2.5%/0.6±2.0% for beams/arcs, respectively. Smaller dependence on tumor volume(TV)/field size(FS) was also observed. Ion-chamber measurement was within ±3.0%. Conclusion: PB-hete overestimates dose to lung tumor relative to XVMC. XVMC-algorithm is much more-complex and accurate with tissues-heterogeneities. Measurement at machine is time consuming and need extra resources; also direct measurement of dose for heterogeneous treatment plans is not clinically practiced, yet. This simple correction-based method was very helpful for independent-2nd-check of MC-lung-SBRT plans and routinely used in our clinic. A look-up table can be generated to include TV/FS dependence in ICFs.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-138: Dosimetric Verification For Volumetric Modulated Arc Therapy Cranio-Spinal Irradiation Technique

Purpose: Dosimetric feasibility of cranio-spinal irradiation with volumetric modulated arc therapy (VMAT-CSI) technique in terms of dose distribution accuracy was investigated using a humanlike phantom. Methods: The OARs and PTV volumes for the Rando phantom were generated on supine CT images. Eclipse (version 8.6) TPS with AAA algorithm was used to create the treatment plan with VMAT-CSI technique. RapidArc plan consisted of cranial, upper spinal (US) and lower spinal (LS) regions that were optimized in the same plan. US field was overlapped by 3cm with cranial and LS fields. Three partial arcs for cranium and 1 full arc for each US and LS region were used. The VMAT-CSI dose distribution inside the Rando phantom was measured with thermoluminescent detectors (TLD) and film dosimetry, and was compared to the calculated doses of field junctions, target and OARs. TLDs were placed at 24 positions throughout the phantom. The measured TLD doses were compared to the calculated point doses. Planar doses for field junctions were verified with Gafchromic films. Films were analyzed in PTW Verisoft application software using gamma analysis method with the 4 mm distance to agreement (DTA) and 4% dose agreement criteria. Results: TLD readings demonstrated accurate dose delivery, with a median dose difference of -0.3% (range: -8% and 12%) when compared with calculated doses for the areas inside the treatment portal. The maximum dose difference was 12% higher in testicals that are outside the treatment region and 8% lower in lungs where the heterogeinity was higher. All planar dose verifications for field junctions passed the gamma analysis and measured planar dose distributions demonstrated average 97% agreement with calculated doses. Conclusion: The dosimetric data verified with TLD and film dosimetry shows that VMAT-CSI technique provides accurate dose distribution and can be delivered safely.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-140: Dynamic Wave Arc Trajectory Verification Using KV X-Ray Fluoroscopy

Purpose: Purpose: This study investigates the geometric accuracy of simultaneous Gantry/Ring rotation during Dynamic Wave Arc (DWA) delivery. Methods: The Vero SBRT system consists of a 6MV LINAC mounted on an O-ring gantry that can rotate around the vertical axis (±60°), similar to couch rotation on C-arm gantries. To provide CBCT and fluoroscopy imaging functionalities, two orthogonal kV imaging units are attached to the O-ring at −45°/+45° from the beam axis.Dynamic Wave Arc maximizes Vero's motion capabilities by employing synchronized gantry and ring motion on a complex non-coplanar trajectory in combination with aperture based optimized MLC segments.Four wave arc trajectories (T1-4) were delivered using a cubic phantom with a configuration of five lead beads. O-ring gantry position information was retrieved through continuous dual-source kV X-ray image acquisition during DWA. An in-house algorithm read in the image set, extracted the projected marker positions and determined the angulation through reconstruction of the beam source position. The geometric error was quantified as the distance between the independently detected positions from kV-images and reference trajectory derived from the treatment plan in the Ring-Gantry space. Results: The average displacement between the 3D gantry/ring positions reconstructed from the fluoroscopy images and the reference trajectory was 0.346 mm (SD 0,171) for T1. A mean offset of 0.348 mm (SD 0,182) and 0.357 mm (SD 0.194) was observed for trajectory T2(2segmens) and T3(4segments), respectively. The saw shape T4 presented a mean geometric error of 0.363 (SD 0.156). The overall systematic error of 0.350 was caused by the difference between planned reference trajectory created by linear interpolation between CP, and the machine delivery following a spline curve. Conclusion: An independent geometric QA approach has been developed for DWA delivery verification, successfully applied on diverse trajectories and disclosed that the Vero system is capable of accurately follow complex trajectories.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

SU-E-T-189: First Experimental Verification of the Accuracy of Absolute Dose Reconstruction From PET-CT Imaging of Yttrium 90 Microspheres

Purpose: In the past few years there have been numerous proposals for 3D dose reconstruction from the PET-CT imaging of patients undergoing radioembolization treatment of the liver with yttrium-90 microspheres. One of the most promising techniques uses convolution of the measured PET activity distribution with a pre-calculated Monte Carlo dose deposition kernel. The goal of the present study is to experimentally verify the accuracy of this method and to analyze the significance of various error sources. Methods: Optically stimulated luminescence detectors (OSLD) were used (NanoDot, Landauer) in this experiment. Two detectors were mounted on the central axis of a cylinder filled with water solution of yttrium-90 chloride. The total initial activity was 90mCi. The cylinder was inserted in a larger water phantom and scanned on a Siemens Biograph 16 Truepoint PET-CT scanner. Scans were performed daily over a period of 20 days to build a calibration curve for the measured absolute activity spanning 7 yttrium-90 half-lives. The OSLDs were mounted in the phantom for a predetermined period of time in order to record 2Gy dose. The measured dose was then compared to the dose reconstructed from the activity density at the location of each dosimeter. Results: Thorough error analysis of the dose reconstruction algorithm takes into account the uncertainties in the absolute PET activity, branching ratios, and nonlinearity of the calibration curve. The measured dose for 105-minute exposure on day 10 of the experiment was 219(11)cGy, while the reconstructed dose at the location of the detector was 215(47)cGy. Conclusion: We present the first experimental verification of the accuracy of the convolution algorithm for absolute dose reconstruction of yttrium-90 microspheres. The excellent agreement between the measured and calculated point doses will encourage the broad clinical adoption of the convolution-based dose reconstruction algorithm, making future quantitative dose/outcome studies possible.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-474: IMRT Verification Using the On-Board EPID

Purpose: To verify IMRT and Rapid Arc (RA) plan's agreement with treatment delivery for radiation therapy using an on-board EPID compared to MapCHECK2 and ArcCHECK. Methods: On-board EPID (Exact arm) is commissioned using manufacturer's Preconfigured Data Input Package (PDIP). Portal dose calculation engine for comparison with measurement was commissioned with PDIP. The absolute dosimetric calibration of the EPID is performed using 10×10cm field size at 105cm source to image detector distance with 90.7 CU in the integrated mode. The dark and flood field of EPID were also calibrated in the integrated mode for all energies and dose rates. The output dose-response data from the manufacturer was verified. The improved gamma test method was used for EPID data analysis. Gamma test were used for MapCHECK2 (or ArcCHECK). The sensitivity of the method was tested using collimator rotation at 3 degree and 5 degree. Student T-test was used to distinguish the difference among the groups. Results: IMRT: The average of Gamma passing rate using EPID and MapCHECK2 is 98.9±1.6% and 98.7±2.0% (p = 0.342). There is no statistical difference between two groups.RA: The average of Gamma passing rate using EPID and ArcCHECK is 97.2±2.3% and 93.2±4.1%. (p = 0.002). EPID significantly improved the RapidARC plan passing rate compared to the ArcCHECK. 5% dose change typically causes between 3 – 55% change in Gamma. Conclusion: Using the EPID, we were able to perform QA verification on IMRT plans comparable to MapCHECK2 (p=0.342) and superior to that of ArcCHECK (p=0.002) using Gamma 3%, 3mm criteria. The sensitivity of EPID was found to be comparable to that of the MapCHECK2 and ArcCHECK. EPID dosimetry is faster (4.3 mins vs. 10 mins) due to easier setup and integrated data processing.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

SU-E-T-506: Dosimetric Verification of Photon MLC Delivered Electron Fields for Implementing MERT On An Artiste Linac

Purpose: To verify the dose accuracy of photon MLC delivered electron fields for implementing energy-intensity modulated electron radiotherapy (MERT) on an Artiste linac. Methods: It was proposed to deliver MERT on an Artiste linac at a short SSD (60 cm) to reduce beam penumbra caused by electron scatters. An in-house developed Monte Carlo (MC)-based dose calculation/optimization planning code was used for treatment planning. Our previous study showed that the measured dose distribution of a breast plan showed good agreement with the calculations in low-medium dose regions while the differences in high dose regions were outstanding. A continuous work found that the discrepancy is mainly caused by improper modeling in MC for the single focused MLC in the Artiste which was simplified as double focused in the previous MC simulations. With this remodeled MLC in the calculations, an energy-intensity modulated electron plan using 6, 9, 12 and 15 MeV was generated for a breast treatment on a breast phantom at a 60 cm SSD and recalculated regarding a solid water phantom. For a test study, four of MLC segments (each with a different energy) generated in the plan were delivered to the phantom and a film measurement was performed at the depth of 2 cm. The measured 2D dose distribution was then compared with calculations. Results: For composite doses of the four segments, measured 2D dose distributions overall agree well with the calculations (3mm/3%) in most area. The separate measurement for a single MLC segment for each of energies also showed the consistence with the calculations. Conclusion: After remodeling MLC in the MC calculations, the measured dose distribution for a subset of MLC segments from a MERT plan showed good agreement. Further detailed verification for the full plan will be the work in the next step.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

WE-E-18A-08: Towards a Next-Generation Electronic Portal Device for Simultaneous Imaging and Dose Verification in Radiotherapy

Purpose: This work forms part of an ongoing study to develop a next-generation electronic portal imaging device (EPID) for simultaneous imaging and dose verification in radiotherapy. Monte Carlo (MC) simulations were used to characterize the imaging performance of a novel EPID that has previously been demonstrated to exhibit a water-equivalent response. The EPID ' s response was quantified in several configurations and model parameters were empirically validated against experimental measurements. Methods: A MC model of a novel a-Si EPID incorporating an array of plastic scintillating fibers was developed. Square BCF-99-06A scintillator fibers with PMMA cladding (Saint-Gobain Crystals) were modelled in a matrix with total area measuring 150×150 mm{sup 2}. The standard electromagnetic and optical physics Geant4 classes were used to simulate radiation transport from an angled slit source (6 MV energy spectrum) through the EPID and optical photons reaching the photodiodes were scored. The prototype's modulation transfer function (MTF) was simulated and validated against experimental measurements. Several optical transport parameters, fiber lengths and thicknesses of an air gap between the scintillator and photodiodes were investigated to quantify their effects on the prototype's detection efficiency, sensitivity and MTF. Results: Simulated EPID response was more sensitive to variations in geometry than in the optical parameters studied. The MTF was particularly sensitive to the introduction of a 0.5–1.0 mm air gap between the scintillator and photodiodes, which lowered the MTF relative to that simulated without the gap. As expected, increasing the fiber length increased the detector efficiency and sensitivity while decreasing the MTF. Conclusion: A model of a novel water-equivalent EPID has been developed and benchmarked against measurements using a physical prototype. We have demonstrated the feasibility of this new device and are continuing to optimize the design to achieve an imaging response that warrants the development of a next-generation prototype.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Poster — Thur Eve — 55: An automated XML technique for isocentre verification on the Varian TrueBeam

Isocentre verification tests, such as the Winston-Lutz (WL) test, have gained popularity in the recent years as techniques such as stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments are more commonly performed on radiotherapy linacs. These highly conformal treatments require frequent monitoring of the geometrical accuracy of the isocentre to ensure proper radiation delivery. At our clinic, the WL test is performed by acquiring with the EPID a collection of 8 images of a WL phantom fixed on the couch for various couch/gantry angles. This set of images is later analyzed to determine the isocentre size. The current work addresses the acquisition process. A manual WL test acquisition performed by and experienced physicist takes in average 25 minutes and is prone to user manipulation errors. We have automated this acquisition on a Varian TrueBeam STx linac (Varian, Palo Alto, USA). The Varian developer mode allows the execution of custom-made XML script files to control all aspects of the linac operation. We have created an XML-WL script that cycles through each couch/gantry combinations taking an EPID image at each position. This automated acquisition is done in less than 4 minutes. The reproducibility of the method was verified by repeating the execution of the XML file 5 times. The analysis of the images showed variation of the isocenter size less than 0.1 mm along the X, Y and Z axes and compares favorably to a manual acquisition for which we typically observe variations up to 0.5 mm.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Sci—Thur AM: YIS - 06: An EPID-based 3D patient dose verification method for SBRT-VMAT delivery

Purpose: Stereotactic body radiation therapy (SBRT) delivered via volumetric modulated arc therapy (VMAT) can strongly benefit from an in vivo patient dose verification due to the large doses per fraction. Electronic portal imaging devices (EPIDs) can be utilized as a patient dose dosimeter. In this work we present a physics-based model which utilizes on-treatment EPID images to reconstruct the dose delivered to an anthropomorphic phantom during SBRT-VMAT delivery. Methods: An SBRT linac beam was modeled using Monte Carlo methods and verified with measured data. Our dose reconstruction model back-projects EPID measured focal fluence upstream of the patient and adds a predicted extra-focal fluence component. This fluence is forward projected onto the patient's density matrix and convolved with dose kernels to calculate dose. The model was validated for two prostate, three lung, and two spine SBRT-VMAT treatments. Results were compared to the treatment planning system's calculation. Results: 2%/2 mm chi comparison calculations gave pass rates for the whole volume, infield, and high dose region respectively, and no lower than: 98%, 95%, 99% for the prostate plans, 99%, 92%, 85% for the lung plans, and 91%, 85%, 81% for the spine plans. A 3%/3mm calculation gave pass rates no lower than 99%, 94%, and 90% for all dose regions for the prostate, lung, and spine respectively. Conclusions: We have developed a physics-based model which calculates delivered dose to phantom (or patient) for SBRT-VMAT delivery using on treatment EPID images. The accuracy of the results has allowed us to test this model clinically.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-D-BRD-07: Evaluation of the Effectiveness of Statistical Process Control Methods to Detect Systematic Errors For Routine Electron Energy Verification

Purpose: To evaluate the ability of statistical process control methods to detect systematic errors when using a two dimensional (2D) detector array for routine electron beam energy verification. Methods: Electron beam energy constancy was measured using an aluminum wedge and a 2D diode array on four linear accelerators. Process control limits were established. Measurements were recorded in control charts and compared with both calculated process control limits and TG-142 recommended specification limits. The data was tested for normality, process capability and process acceptability. Additional measurements were recorded while systematic errors were intentionally introduced. Systematic errors included shifts in the alignment of the wedge, incorrect orientation of the wedge, and incorrect array calibration. Results: Control limits calculated for each beam were smaller than the recommended specification limits. Process capability and process acceptability ratios were greater than one in all cases. All data was normally distributed. Shifts in the alignment of the wedge were most apparent for low energies. The smallest shift (0.5 mm) was detectable using process control limits in some cases, while the largest shift (2 mm) was detectable using specification limits in only one case. The wedge orientation tested did not affect the measurements as this did not affect the thickness of aluminum over the detectors of interest. Array calibration dependence varied with energy and selected array calibration. 6 MeV was the least sensitive to array calibration selection while 16 MeV was the most sensitive. Conclusion: Statistical process control methods demonstrated that the data distribution was normally distributed, the process was capable of meeting specifications, and that the process was centered within the specification limits. Though not all systematic errors were distinguishable from random errors, process control limits increased the ability to detect systematic errors using routine measurement of electron beam energy constancy.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2015

SU-E-J-16: Automatic Image Contrast Enhancement Based On Automatic Parameter Optimization for Radiation Therapy Setup Verification

Purpose: In RT patient setup 2D images, tissues often cannot be seen well due to the lack of image contrast. Contrast enhancement features provided by image reviewing software, e.g. Mosaiq and ARIA, require manual selection of the image processing filters and parameters thus inefficient and cannot be automated. In this work, we developed a novel method to automatically enhance the 2D RT image contrast to allow automatic verification of patient daily setups as a prerequisite step of automatic patient safety assurance. Methods: The new method is based on contrast limited adaptive histogram equalization (CLAHE) and high-pass filtering algorithms. The most important innovation is to automatically select the optimal parameters by optimizing the image contrast. The image processing procedure includes the following steps: 1) background and noise removal, 2) hi-pass filtering by subtracting the Gaussian smoothed Result, and 3) histogram equalization using CLAHE algorithm. Three parameters were determined through an iterative optimization which was based on the interior-point constrained optimization algorithm: the Gaussian smoothing weighting factor, the CLAHE algorithm block size and clip limiting parameters. The goal of the optimization is to maximize the entropy of the processed Result. Results: A total 42 RT images were processed. The results were visually evaluated by RT physicians and physicists. About 48% of the images processed by the new method were ranked as excellent. In comparison, only 29% and 18% of the images processed by the basic CLAHE algorithm and by the basic window level adjustment process, were ranked as excellent. Conclusion: This new image contrast enhancement method is robust and automatic, and is able to significantly outperform the basic CLAHE algorithm and the manual window-level adjustment process that are currently used in clinical 2D image review software tools.

60 APPLIED LIFE SCIENCES↗