bluuebunny/crossref_metadata_2025_split huggingface dataset:
SU‐D‐BRA‐02: Penumbra Measurement for a Flattening‐Filter‐Free Artiste Linear Accelerator
Authors: Gayou O , Johnson M 
Published: November 2011

Purpose: The primary benefit of removing the flattening filter of a linear accelerator is to increase the dose rate by a factor of 3 to 6. Additionally, change in scatter conditions in the head of the accelerator may also decrease the penumbra, which could lead to better target conformality. The purpose of this work is to evaluate this effect on a Siemens Artiste linear accelerator. Methods: Profiles of 10 ×10 cm2 flat and flattening‐filter‐free (FFF) fields were measured in water using a scanning diode with 0.3 mm steps. The penumbra created by both the 160‐leaf multileaf collimator (MLC) and the backup jaw (Y‐jaw) was measured. The profile of the FFF beam was renormalized so that for both flat and FFF beams, the penumbra was defined as the distance between the points with 20% and 80% of the central flat beam dose. The penumbra was measured every 5 cm along the major axes and around the central axis at four depths from 1.5 to 20 cm. Results: The average penumbra defined by the MLC and Y‐jaw was 4.9 mm and 5.9 mm, respectively for the flat beam, and 4.9 mm and 5.2 mm for the FFF beam. The flat beam penumbra increased with depth from 4.6 to 9.2 mm and 5.5 to 10.1 mm for the MLC and Y‐jaw respectively. The corresponding increase for the FFF beam penumbra was 4.6 to 9.2 mm and 4.6 to 8.8 mm. The average paired difference was <0.1 mm for the MLC and 0.8 mm for the Y‐jaw. Conclusion: The MLC penumbra was not affected by removing the flattening filter, while the decreased scattering in the FFF beam reduced the Y‐jaw penumbra by 0.8 mm. Removing the flattening filter does not clinically significantly improve the way the MLC can conform to the target.This research was partially supported by Siemens Medical Solutions.
SU‐D‐BRA‐03: Gantry Angle Sensitivity for IMRT Plan Delivery
Authors: Atwal P , Cao F , Ansbacher W , Kohli K 
Published: November 2011

Purpose: Intensity Modulated Radiation Therapy (IMRT) is currently the modality of choice for optimum dose conformability for head and neck (H&N) patients. We have quantified the sensitivity of H&N treatment delivery to errors in the gantry angle (GA) using EPIDose[1], an IMRT QA tool used in our centre. Methods: EPIDose was modified to read in the planned GA for each field, add an ‘error’ term, and continue with analysis. Systematic errors (shifting all fieldsˈ GA together by same value) and a subset of random errors (using different values for each field) up to 5 degrees were studied. Analysis consisted of recreating the dose based on the captured images (including the modified GAs), and creating Chi distributions (using 3% dose‐difference, 3mm distance‐to‐agreement parameters) between planned and reconstructed doses for two dose ranges: dose above 80% of prescription dose, for primary tumour volume, and the region with 40% to 80% of prescription dose, focusing on organ‐at‐risk. For the planned and reconstructed dose to be considered clinically equivalent at least 90% of the data‐points being compared in either region needed to have a Chi value between −1 and +1. Results: Based on 20 plans, the average systematic error that can be introduced into H&N IMRT plans without having the planned and reconstructed dose considered clinically different, via the metric discussed above, is 2.4±0.5 degrees. Random error affected 13 of these plans, with the average error being 4.4±0.8 degrees. Conclusion: During delivery of H&N treatments, discrepancies in the GA less than approximately 1 degree (3 standard deviations from average systematic error) are not flagged as clinically relevant by our QA procedure, suggesting no accuracy impact to IMRT plan delivery. Fortunately, this is greater than 0.5 degrees standard tolerance set by CAPCA (Canadian Association of Provincial Cancer Agencies).Medical Physics 33 (2006) 3369‐3382 W.Ansbacher
SU‐D‐BRA‐05: Evaluation of the Accuracy of 3DVH+Mapcheck2 Predictions of Dose to Virtual Ion Chamber and Film in Composite IMRT QA
Authors: Olch A 
Published: November 2011

Purpose: Two common methods of performing patient‐specific IMRT QA are the composite dose (ion chamber and film) and beam‐by‐beam dose measurement (diode array). Neither method by itself gives a complete enough measure of the dose distribution. DVH+Mapcheck2 was evaluated for its ability to provide both types of doses with just the array measurement. Methods: The dosimetric results from 15 patient‐specific IMRT QA tests performed by composite irradiation of ion chamber and EDR film (in coronal plane) in phantom were compared to the predicted doses for the same locations based on Mapcheck 2 measured doses of each beam within each plan and interpretation by 3DVH software. The ratio of the measured to predicted ion chamber doses was calculated and the Gamma metric was used to compare the relationship between the film doses to the Eclipse or 3DVH predicted planar doses. Also, the 3D gamma value was calculated in the 3DVH software which compares the Eclipse to 3DVH predicted dose distributions. For the 2D and 3D Gamma metrics, 2% dose and 2mm DTA were used. In addition, a simple dose difference was performed using either a 2% or 3% dose difference tolerance. Results: The mean ratio(SD) of the chamber to 3DVH predicted dose was 1.013(.015) while the ratio of the measured to Eclipse calculated dose to the chamber was 1.003(.012). The mean percentage of pixels failing the 3D gamma=1 metric was 1.2%(1.4%) while the failure rate for the 2D Gamma metric (3DVH to Eclipse) was 1.1%(1%). The Gamma failure rate for the film vs. Eclipse was 1.6%(2%). Mean dose differences, highly sensitive measures of dosimetric difference when not paired with DTA, were not significantly different between 3DVH+Mapcheck2 or film vs. Eclipse. Conclusions: 3DVH+Mapcheck2 accurately predicts the doses one would have measured had one instead used an ion chamber and coronal film in plastic phantom.Sun Nuclear has provided support for this project
SU‐D‐BRB‐01: On the Dependence of Scanned Proton Beam Dose Output Factors on the Field Size and the Accuracy of the Lateral Dose Profiles
Authors: Titt U , Mirkovic D , Anand A , Perles L , Sawakuchi G , Mohan R 
Published: November 2011

Purpose: To evaluate the dependence of central axis doses and output factors of square fields formed by scanning proton beamlets on the lateral profiles of the beamlets. Method and Materials:Square field dose distributions in a water phantom, delivered by proton beamlets used for scanned beam proton therapy, were simulated with MCNPX. Normalized central axis doses as a function of field size and depths in the phantom, the output factors, were analyzed and compared to measured values. Lateral profiles of the simulated beamlets were adjusted to closer fit characteristics of measured beamlet profiles, and to improve the agreement of output factors for measured and simulated doses and output factors. Results: Our results revealed that small changes in the low dose parts of lateral profiles far from the central axis (i.e., the “halo”) of proton beamlets produced significant changes in the central axis doses of square fields. This indicates the importance of low‐dose halo around individual beamlets in the context of modeling beams for dose prediction purposes.Conclusion: Our findings suggest that the accuracy of dose prediction algorithms for scanned beam proton therapy is highly dependent on correct modeling of the halo doses far from the beam axis. Because contemporary dose prediction algorithms do not calculate doses with a large enough lateral extent for reasons of maintaining acceptable calculation times, results of this research may help develop novel and more accurate algorithms for dose computations for scanned charged particle beams.This project was supported in part by P01CA021239 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.
SU‐D‐110‐01: A Method for 2D‐To‐4D Sorting of Dynamic MRI to Derive an Anatomic Representation of the Average Breathing Cycle
Authors: Tryggestad E , Flammang A , Roland T , Shea S , Wong J 
Published: November 2011

Purpose: Beneficial “4D” treatment planning and delivery methods, most of which are under development, rely on robust characterizations of motion. Implementation of these 4D methods may be hindered by deficiencies in current pre‐treatment imaging. 4D‐CT is the current standard for deriving patient‐specific margins. However, 4D‐CT is sub‐optimal because it represents a single‐cycle “snapshot” motion per slice location which results in slice‐to‐slice inconsistencies for variable breathing. Moreover, 4D‐CT involves ionizing radiation. MRI is not thusly encumbered. We have developed a 2D‐to‐4D sorting technique for dynamic MRI to derive an average‐breathing‐cycle anatomic representation for radiotherapy planning. Methods: Healthy lung and abdominal volunteers were imaged continuously for 10–30 minutes with balanced steady‐state free precession (bSSFP) and/or half‐Fourier single‐shot turbo spin echo (HASTE) on a 1.5T MRI system (MAGNETOM Espree, Siemens). 3D volumes were scanned in sagittal or coronal planes by changing the slice location per frame. Voxels were 2×2×5 mm3 and sequence parameters were adjusted to achieve repetition times of 200–300 ms. Synchronous with image acquisition, an external respiratory trace was collected at 50 Hz. Results: A two‐step sorting technique based on the respiratory trace was developed. In the first step, all frames per slice location and phase bin were averaged, producing an “average 4D‐MRI.” From this we derived corresponding “standard deviation 4D‐MRI.” Frame averaging resulted in blurring. In the second step, raw frames were compared to the set of images per slice location from the average 4D‐MRI; a normalized cross‐correlation scoring criterion over a user‐selected rectangular ROI indicated the best‐matching respiratory phase anatomically. The subset of best‐matching raw frames per slice location and phase were averaged to produce a final representation with improved contrast. Conclusions: Dynamic MRI shows great promise as a potential standard for motion management. Future efforts will involve modifications to the bSSFP sequence for significant improvements in speed.This work has received research personnel support from Siemens
SU‐D‐110‐05: MR‐Based Dose Calculation for Prostate Radiotherapy Using Atlas‐ Based Auto‐Segmentation
Authors: Lambert J , Dowling J , Menk F , Parker J , Capp A , Denham J , Salvado O , Greer PB 
Published: November 2011

Purpose: A method that could enable dose calculations to be performed using magnetic resonance (MR) images for conventional treatment planning and adaptive planning using MR‐accelerator systems would be to apply bulk electron densities to the MR images. However currently bone must be manually segmented making this impractical. This work develops and tests an atlas‐based method to automatically segment bone on pelvic MR images for dose calculations. Methods: An MR whole‐pelvic atlas was created using manually delineated scans from 39 patients. Atlas‐based pelvic‐bone auto‐segmentations were then created for 25 patient scans using deformable image registration of the atlas to each patients scan with a leave‐one‐out atlas approach. These and corresponding expert manual segmentations were compared using the Dice similarity coefficient. Bone was assigned a density of 1.19 g/cm3 and all other tissues a water equivalent density. Treatment plans were generated on the whole‐pelvis MR images and doses compared for the manual and auto‐segmented bone plans. Results: The average Dice coefficient was 0.83 (standard deviation = 0.05). The average manual bone volume was 834.6 cm3 compared to the atlas based average volume of 840.0 cm3, with a mean difference of 5.4 cm3 (0.64%). The average ICRU point dose calculated on the MR images using the atlas‐based bone segmentations was 0.2% lower (standard deviation 0.3%) than the dose calculated using the manual bone segmentations. Conclusions: The atlas based method for auto‐segmentation of pelvic bone enables MR‐based prostate radiotherapy dose calculations for treatment and adaptive planning.This work was partially funded by a Cancer Council New South Wales Grant RG07‐06
SU‐D‐301‐01: Performance Limits of Large Area Poly‐Si X‐Ray Imagers Employing In‐Pixel Amplification
Authors: El‐Mohri Y , Liang A , Koniczek M , Antonuk L , Zhao Q , Jiang H 
Published: November 2011

Purpose: Active matrix flat‐panel imagers (AMFPIs) — based on amorphous silicon thin‐film transistors (TFTs) — have been developed for a wide variety of x‐ray imaging applications at diagnostic energies. However, under conditions of low exposure or small pixel sizes, the additive electronic noise associated with readout of the pixel signal of these imagers leads to relatively low levels of signal‐to‐noise ratio — resulting in significant loss of detective quantum efficiency (DQE) and image quality. An increasingly promising approach for overcoming such limitations involves the incorporation of in‐pixel amplification circuits, referred to as active pixel (AP) architectures, based on low‐temperature polycrystalline silicon (poly‐ Si) TFTs. Methods: Theoretical performance limits for additive noise and DQE for large area imagers employing various AP designs have been explored. The methodology involves SPICE simulations of pixel circuits in an array environment employing realistic inputs for signal, as well as noise derived from individual poly‐Si TFT test devices. Pixel circuit noise performance derived from these simulations, along with other imaging parameters, are input to cascaded systems models for calculations of the corresponding DQE performance. Results: For large area imagers employing architectures comprising one‐ and two‐stage in‐pixel amplification, the calculations suggest that the effect of additive noise can be significantly diminished, leading to substantial improvement in DQE compared to that of conventional AMFPIs, especially at the low exposures associated with fluoroscopy. Such improved performance is largely maintained even for more sophisticated pixel designs incorporating additional features such as higher frame rate and selectable amplification gain. Conclusions: Active pixel imagers based on poly‐Si TFTs offer a potential alternative to conventional AMFPIs for overcoming the decline of DQE at low exposures in fluoroscopy. The combination of detailed circuit simulations and cascaded systems modeling provides a powerful tool for guiding the development of future generations of ever more sophisticated imagers.Work Supported by NIH grant R01‐EB000558
SU‐D‐301‐05: Angular Dependency of the Slice Sensitivity Profile in Tomographic Breast Imaging
Authors: Nosratieh A , Yang K , Boone J 
Published: November 2011

Purpose: To study the effect of angular acquisition on the slice sensitivity profile (SSP) in tomographic breast imaging, for both breast CT and tomosynthesis. Methods: Brass disks (0.025 mm thick) ranging in diameter from 2.5–20 mm were placed within mammography phantoms of varying thicknesses (3–6 cm). The disks were imaged using a prototype digital tomosynthesis system (Selenia Dimensions, Hologic Corporation, Bedford, Ma). The tomosynthesis system acquires images over an angular acquisition range of 15‐degrees. The same brass disks were used to measure the SSP in breast CT, using a 14cm‐diameter cylindrical polyethylene phantom. The bCT angular range spanned from 15 to 360‐degrees. A subset of the projection images were reconstructed with a filtered back projection algorithm to simulate limited‐angle acquisition. The SSP was determined by measuring the mean gray‐scale (GS) in the tomographic images along the z axis of the scan corresponding to the disk position. Normalized profiles of the background‐corrected mean GS values were generated as a function of the distance away from the detector. The full‐width at half maximum (FWHM) GS intensity was determined. Results: As the angular acquisition range increases, the SSP narrows and results in a smaller FWHM value. The FWHM values of the SSP for the smallest disk at (2.5 mm‐diameter) were 8.6 mm, 4.2 mm, and 0.8 mm for 15, 40, and 180 degrees, respectively. Conclusions: Limited angle tomography results in a broadening of the SSP, inversely proportional to the angular range of acquisition. The SSP approaches a minimum as the angular coverage exceeds 180 degrees.One author receives funding from Hologic (Bedford, Mass).
SU‐E‐E‐05: The Compute Cloud, a Massive Computing Resource for Patient‐Independent Monte Carlo Dose Calculations and Other Medical Physics Applications
Authors: Constantin M , Sawkey D , Mansfield S , Svatos M 
Published: November 2011

Purpose: Massive computing resources are needed to generate an extensive linac phase‐space (PHSP) database for different radiotherapy beams that could serve as a starting point for several user‐applications, such as patient‐dependent dose calculations or Monte‐Carlo‐based quality assurance procedures. The goal of this project is to disseminate our knowledge about performing patient‐independent Monte‐Carlo dose calculations on the Amazon Elastic Compute Cloud (EC2) and educate medical physicists on technical aspects such as simulation instance management and cluster coordination while ensuring data security for a low CPU cost. Methods: Amazon‐Web‐Services, along with Google, Microsoft, IBM, etc, offers resizable compute capacity in the cloud changing the economics of computing by eliminating the server fixed expenses. A cost‐efficient elastic cluster is available for a wide users community who can perform Monte Carlo simulations or other numerical applications using Amazon Machine Images. These virtual machines can be configured to work in a particular scientific environment, i.e. GEANT4 or PENELOPE. GEANT4 patient‐independent dose simulations were launched in parallel on EC2 using high CPU extra‐large spot instances (7 GB of memory, 8 virtual cores at 2.13 to 2.44GHz). Data storage was performed using the Simple Storage Service (S3). The S3 Organizer was used to upload the simulation or download multiple files. The 4‐layer EC2 security system, composed of the host/guest operating systems, Firewall, and signed secret access keys, ensures data privacy. Results: A local script was set‐up to launch multiple dose calculation instances on EC2 and automatically save the results on S3. Geant4‐application development, testing, and debugging were performed on a local workstation, along with data analysis and comparison with experiment. IAEA‐compliant patient‐independent PHSP files were generated and dose calculations were successfully validated against experiment. Conclusions: Cloud computing offers a low‐cost alternative for improved radiotherapy planning and optimization algorithms by enabling beam transport and Monte Carlo‐based dose calculations.Work supported by Varian Medical Systems
SU‐E‐I‐16: CBCT with Flexible Imaging Trajectories to Improve Image Quality and Longitudinal Coverage — Concept, Simulation and Digital Phantom Study
Authors: Yang D , Goddu S , Li H , Klein E , Thorstad W , Parikh P 
Published: November 2011

Purpose: Cone beam CT (CBCT) image quality and volumetric coverage are very important for patient treatments. LINAC ‐mounted CBCT (Varian Trilogy) has limited 17 cm longitudinal coverage, not enough for many clinical sites including head‐neck. CBCT also suffers from poor image quality especially in the most superior and inferior slices. These problems hold back the usage of CBCT in many quantitative studies, including tumor response evaluation and daily dose computation. In this study, we evaluated a new idea of flexible X‐ray source trajectory CBCT to overcome these limitations. Methods: The core concept is to combine gantry rotation with simultaneous couch motion during CBCT acquisition. Longitudinal coverage can therefore be extended without interruption. Image quality can be enhanced by improving the image reconstruction algorithms which supports the flexible imaging‐trajectory utilizing all the projection data. To test these new ideas, we implemented CBCT simulation software tools to compute the projection images of digital phantoms according to any flexible source trajectories. We improved the FDK algorithm to reconstruct images from the simulation projection data. Results: We studied a few different source trajectory models including double circle and helical. The initial results were encouraging. The longitudinal coverage was extended. Image quality has been improved even with simple FDK reconstruction algorithm, comparing to multiple individual scans with a couch shift in‐between. Conclusions: We are still at the beginning of the project. It is quite clear that the new CBCT method has great potential. We are developing better reconstruction algorithms to fully take advantages of the new methods for image quality improvement. We are optimizing the source trajectory designs, mathematically and with simulation, to improve image quality, dose exposure and scan time. Anthropomorphic phantom studies with Varian TrueBeam, which could support this proposed CBCT method, are also planned.This projection is partially supported by a Varian MRA grant with Washington University in Saint Louis.
SU‐E‐I‐33: Low‐Dose CBCT by Iterative Image Reconstruction Using Non‐Local Edge‐Preserving Prior
Authors: Wang J , Ouyang L , Lu W , Solberg T 
Published: November 2011

Purpose: Statistical iterative image reconstruction (SIR) algorithms are effective for reconstructing high‐quality low‐dose cone beam computed tomography (CBCT). The performance of SIR strongly depends on the prior/penalty term. In this work, we develop and evaluate an edge‐preserving penalty term in SIR that incorporates high‐order neighbors. Methods: CBCT projection data of a CatPhan 600 phantom was acquired on a Varian Acuity simulator (Varian Medical Systems, Palo Alto, CA) using two protocols: 80mA/12ms/projeciton for high dose and 10mA/10ms/projection for low dose. Low‐dose CBCT images were reconstructed by minimizing an objective function based on the penalized weighted least‐squares (PWLS) criterion. Conventionally, the penalty term is used to enforce the smoothness constraint within a local neighborhood (e.g. up to third‐order neighbors). In this work, we propose a non‐local anisotropic quadratic penalty term that incorporates higher‐order neighbors. The relative contribution from different neighbors is controlled by the absolute difference between the selected neighbors. For neighbors associated with larger differences, equivalence between them will be discouraged. Contrast‐to‐noise ratio (CNR) at different regions of interest (ROIs) is calculated to quantitatively evaluate the proposed high‐order neighborhood penalty. Results: Noise is substantially reduced in low‐dose CBCT reconstructed using the iterative PWLS algorithm, while the edges are well‐preserved. The CNRs in selected ROIs in PWLS‐reconstructed images are higher than that of the corresponding FDK reconstructed high‐dose images. Using the higher‐order neighbor penalty, the average CNR of the selected ROIs is 24% higher than the image reconstructed by PWLS using the penalty with the conventional local neighbor. Conclusions: A higher‐order neighborhood methodology is proposed for the penalty term in PWLS iterative image reconstruction for enhancement of low‐dose CBCT image quality. PWLS coupled with higher‐order neighbor penalty outperforms the PWLS with conventional local neighbor penalty.This work was partially supported by American Cancer Society Institutional Research Grant (ACS‐IRG‐02‐196).