bluuebunny/crossref_metadata_2025 huggingface dataset:
WE‐B‐116‐01: CT Tools and Protocols Available Through AAPM
Authors: Cody D
Published: July 2013

The AAPM CT Working Group on Nomenclature and Protocols (CT WGNP) was established in 2010; this working group is comprised of AAPM members with considerable CT experience, as well as representatives from each of the major CT manufacturers, ASRT, ACR, DICOM, FDA and Image Gently. The CT WGNP has two charges: 1. To develop consensus protocols for frequently performed CT examinations, summarizing the basic requirements of the exam and giving several model‐specific examples of scan and reconstruction parameters. 2. To develop by consensus a set of standardized terms for use on CT scanners, including all parameters that control the scan acquisition or reconstruction that are programmed by the user, are displayed on the final image or included in a DICOM‐specified tag, or describe a fundamental CT setting that is not always user selectable (such as a beam‐shaping filter). This working group has developed a set of tools for members and the general public that is available on the AAPM website. These tools include a Lexicon to allow users to translate important CT acquisition and reconstruction terms between different manufacturers' systems, thereby improving understanding of scan parameter meanings and decreasing the potential for parameter errors. A set of routine adult CT protocols have been provided that include Brain CT Perfusion, Routine Head, Routine Chest and Routine Abdomen/Pelvis exams. These provide a set of reasonable reference protocols for frequently performed CT examinations, summarizing the basic requirements of the exam and giving several model‐specific examples of scan and reconstruction parameters. These resources directly address the original charges of the working group and continue to be the main focus of effort. Pediatric versions of these protocols are now under development. In addition, the working group has been asked by MITA to develop additional resources specific to CT. The working group provides an excellent venue for discussion of issues that affect each of the primary stakeholders in the CT community.Recommendations were developed regarding default values for use in the CT Dose Check feature that has been incorporated into newer CT systems. Additionally, a set of educational slides specific to radiation dose issues in CT have been made created and made available. Versions of the slides that include manufacturer‐specific information, particularly with respect to radiation dose reduction features available on CT scanners, are under development. These resources will be reviewed during this lecture and directions regarding how to find and utilize this information will be given. Dr. Cody receives ‘in‐kind’ support of research from GE Healthcare.
WE‐C‐105‐01: Preparing for the ABR Diagnostic Exam
Authors: Becker S , Leng S , Supanich M
Published: July 2013

Preparing for all three parts of the Diagnostic ABR Physics boards is more than just studying as much material as possible. There will always be material that is missed and gaps in one's knowledge. Therefore it is crucial to understand how the materials relate to each other and to clinical experiences in order to fill those gaps. Each part of the board exam presents its own difficulties: Part I: Determining what material to study, most of which might have been learned a long time ago. Part II: Memorizing how all types of imaging devices work, how their techniques affect images, and the doses delivered by them. Part III: Gaining crucial clinical experiences and being able to explain orally how they are performed and what they mean. All three require different skill sets and preparation, from memorizing vast amounts of material, to rapidly recognizing and solving calculations, to being able to easily and confidently respond to oral questions about all aspects of working in the clinic and why those clinical methods and procedures are performed that way. This symposium is not a comprehensive review of all required study material. Instead it focuses on the previously mentioned problems and required skill sets that are needed for each part of the board exam. Expert Medical Physicists and Medical Faculty will share their experiences and methods to help students best prepare for the challenges of each individual exam along with helping students cope with the anxiety of the exam process.

Learning Objectives:

1. How to Prepare for Part 1 by determining study material

2. How to Prepare for Part 2 by learning to solve clinical calculations rapidly

3. How to Prepare for Part 3 by determining critical clinical experiences and learning how to answer questions orally
WE‐C‐137‐01: Professional Liability and Risk Management in Medical Physics Practice
Authors: Sherouse G , Masten J , Jordan D
Published: July 2013

Medical physicists can be exposed to significant liability risks when performing clinical duties. The nature and extent of such exposure is not always as clear to physicists as it is to physicians. The legal questions involved in resolving a case or claim involving patient harm are complex and are seldom governed according to common sense expectations. Whether a physicist is employed by a hospital or healthcare provider or functions independently as a contractor or consultant, the first line of defense against liability risk is contract negotiation. In this process, the physicist has the opportunity to establish a respectful, collaborative relationship with an employer or client while laying out clear rules and expectations for the handling of any incidents. Much legal, financial, and professional trouble can be avoided by handling this process carefully. This session explores methods and principles for establishing good relationships and contracts. Should a lawsuit occur, it is important to have a basic understanding of civil procedure and the “anatomy” of a professional liability case. This session will review common legal terminology and present examples from seminal cases that are used as the basis of many liability and malpractice cases today. In the event that a physicist is named as a defendant in a lawsuit, he or she should be well‐informed about the expected sequence of events and how they relate to the overall outcome of the case. Professional liability insurance is a useful tool to provide protection to an individual physicist's personal assets and income in the event of a lawsuit, and especially after an adverse legal decision or settlement. The choice of the appropriate amount and type of protection is an individual and complicated decision. Some employers of physicists also provide coverage to their employees, and the nature and extent of this coverage can be challenging to understand fully. This session discusses the common features and aspects of professional liability insurance products to enable the clinical medical physicist to evaluate their own situation and to choose an appropriate product for their needs.

Learning Objectives:

1. Describe a strategy for medical physicists to minimize liability risk by establishing and negotiating healthy contract relationships with employers or clients.

2. Recognize the common elements of civil procedure and explain how they are impacted by typical clinical medical physics activities, such as quality control and recordkeeping.

3. Evaluate commercial professional liability insurance offerings in the context of a physicist's personal asset and income protection needs and practice type and setting.
WE‐C‐108‐01: JUNIOR INVESTIGATOR WINNER — Towards Radiation Therapy with Very High‐Energy Electron Beams
Authors: Bazalova M , Hardemark B , Hynning E , Dunning M , McCormick D , Liu M , Tantawi S , Dolgashev A , Koong A , Maxim P , Loo B
Published: July 2013

Purpose: To experimentally validate Monte Carlo (MC) simulations for dose calculations with very high‐energy electron (VHEE) beams in the presence of heterogeneities and to use MC simulations for treatment planning optimization of ultra‐fast radiation therapy with VHEE beams. Methods: Polystyrene phantom with heterogeneities made of nine materials of various thicknesses was irradiated at a VHEE beam line at SLAC National Accelerator Laboratory. Films were used to measure the dose delivered to the phantom by 60–110MeV VHEE beams of 0.6–2.2mm in size. A MC model of the beam line was built and validated against the experimental data based on percentage depth‐dose (PDD) curves. The MC validated code was used for VHEE radiotherapy planning of three patients. More specifically, MC beamlets were exported into a modified version of RayStation for spot‐scanning optimization. The quality of VHEE dose distributions as a function of electron beam energy, beamlet size and spacing, source‐to‐isocenter distance, and number of beams was studied. Results: MC calculated PDD curves for heterogeneous phantoms were in agreement with the experimental PDD curves obtained at the VHEE beam line for all beam energies and beam sizes. VHEE treatment planning optimization study for a pediatric patient brain tumor revealed that significant normal tissue sparing was achieved in 60–100MeV VHEE plans compared to the state‐of‐the‐art 6MV VMAT plan. The mean dose to the cochleae was by 56–72% lower in the VHEE plans. Out of the studied parameters, electron beam energy had the largest effect on the quality of VHEE treatment plans. In order to generate a VHEE plan matching a VMAT plan for a large patient, VHEE beams of 100–150MeV are needed. Conclusion: We have validated VHEE beam MC dose calculations and we have developed a MC/RayStation treatment planning optimization tool for evaluation of the potential to rapidly treat cancer with VHEE beams. This work has been supported by the 2012 AAPM Research Seed Grant.
WE‐C‐108‐02: Current Methods for Calculating Air Density Can Overestimate Skin Dose
Authors: Best R , Libby B , Chen Q
Published: July 2013

Purpose: Following accepted procedures for configuring Image‐Value to Density Tables (IVDT) can lead to overestimation of skin dose due to exaggerated buildup and scatter in air. Treatment planning systems (TPS), such as Tomotherapy, which calculate dose to air before entering the body, can overestimate air density, leading to buildup and high skin dose. The purpose of this study is to evaluate the magnitude of errors in planned skin dose from incorrect air density assignment on various TPS. Methods: Air in a sample patient CT scan was contoured and manually assigned mass densities. Planning was performed using Pinnacle, Eclipse, and Tomotherapy TPS. Air densities were overridden to values between 0.0013 and 0.02 g/cc. A single anterior 6 MV breast tangent was applied to a sample patient to deliver 20 Gy, half the dose under RTOG protocol 1005. Results: Pinnacle and Eclipse assume negligible dose delivered along the beam path in air, so varying air density did not affect dose with these TPS. Using Tomotherapy, a test site 1 mm under the skin received 168% more dose with an air density of 0.02 g/cc than with normal air density of 0.0013 g/cc. The dose to contralateral breast was 1.40 Gy with air density of 0.0013 g/cc, but rose to 3.05 Gy with a slightly elevated density of 0.003 g/cc, greater than the limit of 2.4 Gy under RTOG 1005. Conclusion: Following procedure by adding a point at the average HU for air will assign half the air voxels excessive density. An IVDT that generated air density of 0.003 g/cc can increase the contralateral breast dose above the max dose limit of RTOG 1005. We recommend placing the IVDT point for air near the maximum image value for air to assure all air voxels are assigned accurate density.
WE‐C‐108‐04: A Monte Carlo Investigation of Low‐Z Targets in a TrueBeam Linear Accelerator Using Varian Virtualinac
Authors: Parsons D , Robar J , Sawkey D
Published: July 2013

Purpose: The focus of this work was to investigate the implementation of low‐Z targets in a TrueBeam linear accelerator (linac) using Monte Carlo simulations. Methods: Simulation of a TrueBeam linac was accomplished by using Varian Virtualinac. Virtualinac is a cloud based web application utilizing GEANT4 Monte Carlo code to model TrueBeam components. Phase spaces were recorded above the jaws and used as input to BEAMnrc simulations of the lower portion of TrueBeam. Dose distributions and spectral distributions were calculated using DOSXYZnrc and BEAMdp, respectively. For validation, 6 MV flattened and FFF photon beams were generated and compared to measurement. Two low‐Z targets were investigated: a carbon target operated at 2.35 MeV and the proposed 2.50 MeV commercial imaging target. The carbon target was also simulated in a 2100EX Clinac using BEAMnrc. Contrast simulations were made by scoring the dose in the phosphor layer of an IUD20 detector after propagating through 4 and 20 cm thick water and ICRP bone phantoms. Results: Measured and modeled depth dose curves for 6 MV flattened and FFF beams agree within 2% for 99.7% of points at depths greater than 0.85 cm. The two low‐Z target photon spectra produced in TrueBeam are harder than that from a Clinac. Percent dose at depth 10 cm is greater by 8.6% and 14.2%; the fraction of photons in the diagnostic energy range is lower by 7% and 25%; and contrasts are lower by factors of 1.2 and 1.5 (thin phantom) and 1.06 and 1.4 (thick), for the TrueBeam 2.35 MV/carbon and commercial imaging beams, respectively. Conclusion: Virtualinac is effective for Monte Carlo modeling of novel target designs. A significant spectral difference is observed between the low‐Z target beam on the Clinac platform, and the proposed imaging beam line on TrueBeam, with the former providing greater diagnostic energy content. This research was supported by Varian Medical Systems and was accomplished using Varian's Virtualinac Monte Carlo application.
WE‐C‐108‐06: Accelerating Collapsed Cone Convolution/Superposition Dose Calculation On GPU Using Spatial Decomposition
Authors: Xiao K , Zhou B , Hu XS , Chen DZ
Published: July 2013

Purpose: The performance of all known Collapsed Cone Convolution/Superposition (CCCS) dose calculation methods on Graphics Processing Units (GPUs) is bounded by memory throughput. One major cause of this bottleneck is the traditional uniform grid representation of the density volume for human body, which generally contains many homogeneous areas. The common ray traversal process of CCCS within a homogeneous area in a uniform grid repeatedly reads the same density information voxel by voxel, hence increasing the memory accessing workloads. We propose a new spatial decomposition based data structure, called Shell, to improve the CCCS performance on GPU. Our idea is to group voxels in homogeneous areas into rectangular regions such that the density of each such homogeneous region is acquired by one read. An efficient ray traversal method across different regions is developed to reduce the number of memory accesses. Methods: We use a spatial decomposition (e.g., octree) algorithm to adaptively partition a uniform grid density volume into a set of approximately homogeneous regions. Our Shell data structure consists of these regions, each containing its average density and pointers for its neighboring regions. The memory addresses of such pointers are associated with the voxels lying on the region boundaries, which allow any ray traversing in a region to efficiently find the next neighboring region to traverse using table lookup schemes. Results: We integrated our method with a CCCS program and tested using various clinical cases on NVIDIA GTX570 GPU. The program using Shell runs 1.32X to 1.44X faster than using a uniform grid, with less than 2% variance in the dose calculation results. Conclusion: The results show our proposed method effectively reduces the memory bottleneck and improves the CCCS performance on GPU. There is a trade off between the speedup and dose accuracy, which is determined by the spatial decomposition methods. The research of D.Z. Chen was supported in part by NSF under Grants CCF‐0916606 and CCF‐1217906.
WE‐C‐108‐08: Organ Doses in a Male Phantom Undergoing High‐Dose‐Rate Brachytherapy Applied to the Prostate
Authors: Candela‐Juan C , Granero D , Vijande J , Ballester F , Perez‐Calatayud J , Rivard M
Published: July 2013

Purpose: The aim of this study was to obtain equivalent doses to radiosensitive organs when applying high‐dose‐rate (HDR) brachytherapy to the prostate using60 Co or 192 Ir sources, and in comparison to external‐beam radiotherapy (EBRT) modalities. Methods: Monte Carlo simulations in Geant4 were performed using a voxelized adult reference man described in Publication 110 by the International Commission on Radiological Protection (ICRP). Point sources of 60Co or 192Ir with photon energy spectra corresponding to those exiting their capsules were placed in the center of the prostate. Equivalent doses per therapeutic absorbed dose to the prostate were obtained in several radiosensitive organs. Values were corrected to account for realistic source dwell times and positions throughout the prostate. This was repeated for a homogeneous water phantom to assess the adequacy of a homogeneous phantom for HDR brachytherapy dose calculations. Brachytherapy doses were compared to reported values from intensity‐modulated radiation therapy (IMRT) and proton therapy. Results: For the nearest organs considered (bladder, rectum, testes, small intestine, and colon), equivalent doses given by 60 Co source were smaller than from 192Ir. As the distance increased, 60Co delivered higher equivalent doses. Relative to 192Ir, the dose ratio increased up to a factor 5 for the brain. Differences between the heterogeneous and the water phantom became apparent at distances > 30 cm. For the closest organs, equivalent doses from the three EBRT modalities were within an order of magnitude to equivalent dosesdeposited by both brachytherapy sources. However, equivalent doses from brachytherapy to the farthest organs were up to two orders of magnitude smaller than EBRT doses. Conclusion: According to physical considerations, “ Co appears dosimetrically advantageous over 192Ir sources at close distances, but not for the farthest organs. Equivalent doses to distant healthy organs were lower for brachytherapy than the studied EBRT modalities.
WE‐C‐108‐09: Patient‐Specific Monte Carlo Dosimetry for I–125 Prostate Brachytherapy
Authors: Miksys N , Enger S , Xu C , Vigneault E , Beaulieu L , Thomson RM
Published: July 2013

Purpose: To investigate dose distributions in the prostate and surrounding tissues for I–125 brachytherapy using Monte Carlo (MC) calculations with patient‐specific phantoms derived from CT images. We explore the effectiveness of different techniques to mitigate streaking artifacts due to brachytherapy sources in post‐implant CT images. Methods: Nine patients (45–87 sources) are considered. Streaking artifacts in post‐implant CT images are mitigated using various metallic artifact reduction (MAR) techniques: raw sinogram, fan beam virtual sinogram and 3D median filter. Segmented structures (CTV, rectum and bladder) guide the assignment of tissues (air, muscle, prostate, calcification, average tissue and bone) to develop patient‐specific MC phantoms. The EGSnrc user‐code Brachydose and GEANT4 user‐code ALGEBRA are employed for MC dose calculations using patient‐specific phantoms derived from CT images. Dose distributions generated with patient models derived using different MAR techniques and different calculation methods (MC, TG‐43) are compared directly, and using recommended dose metrics. Results: Application of each MAR technique to patient CT data results in comparable mitigation of streaking artifacts within the treatment volume in CT images. MC calculations based on uncorrected CT data Result in high dose spikes (>200%) in the treatment volume and lower doses (40–70%) in surrounding tissues compared to MC calculations with corrected phantoms. Dose distributions within the prostate from MC simulations using phantoms generated with different MAR techniques are comparable to each other but differ from TG‐43 calculations with significant (10%+) local differences. Conclusion: Mitigation of streaking artifacts in CT images is necessary for patient‐specific MC dose calculations; however, dose distributions and clinical metrics in the target and OAR are insensitive to the particular MAR technique applied. Differences between MC and TG‐43 dose distributions within the target volume and surrounding organs for I‐125 prostate brachytherapy underline the importance of patient‐specific model‐based dose calculations for treatment planning and evaluation. NSERC, CCSRI, The Canada Research Chairs Program
WE‐C‐108‐11: Patient‐Specific Monte Carlo Simulation of Lung Brachytherapy: Metallic Artifact Reduction and Organ‐Constrained Tissue Assignment
Authors: Sutherland JGH , Miksys N , Furutani KM , Thomson RM
Published: July 2013

Purpose: To investigate techniques for developing accurate patient‐specific computational phantoms for Monte Carlo simulations of lung brachytherapy. Methods to mitigate streaking artifacts due to brachytherapy sources in CT images and organ‐constrained tissue assignment schemes are explored. Methods: Three different metallic artifact reduction (MAR) techniques are applied to lung brachytherapy patient CT data: thresholding replaces high CT values with estimated true values; fan‐beam virtual sinogram replaces artifact‐affected values in a virtual sinogram; and 3D median filter uses local voxel values to guide the replacement of outlying voxel values. Computational phantoms are generated from metallic artifact corrected and uncorrected images with voxel composition defined by CT number and density defined either by nominal tissue density or a CT number to density calibration. Multiple tissue assignment schemes are considered, including some with organ‐specific constraints. Dose distributions for I–125 and Cs‐131 seeds are calculated using the EGSnrc user‐code BrachyDose and are compared directly as well as through DVHs and dose metrics for target volumes surrounding surgical sutures. Results: The most effective MAR technique is the virtual sinogram method as it effectively mitigates artifacts while maintaining image integrity. Thresholding only removes artifacts in the vicinity of sources (leaving artifacts in other regions) and median filtering degrades tissue heterogeneities. Dose distributions for phantoms treated with various MAR techniques and tissue assignment schemes can vary significantly. Dose differences between MAR and uncorrected phantoms are reduced by constraining tissue assignments in lung contours; in one example, differences in D90 of 20% are reduced to 3%. Phantoms with nominal densities Result in higher doses than those with CT derived densities. Conclusion: Application of MAR techniques to CT data is necessary to develop realistic computational phantoms for Monte Carlo simulations; the virtual sinogram method is a promising image‐based technique. Organ‐constrained tissue assignment is necessary for accurate tissue assignment. This work was supported by the Natural Sciences and Engineering Research Council, the Canada Research Chairs program, and an Ontario Graduate Scholarship.
WE‐C‐108‐12: Accounting for Intrinsic Energy Dependence of TLDs When Establishing Dose Rate Constants for Brachytherapy Seeds
Authors: Rodriguez M , Rogers D
Published: July 2013

Purpose: To retrospectively account for the intrinsic energy dependence of TLDs when used to measure dose rate constants (DRC) for I–125 and Pd‐103 brachytherapy seeds, to establish new measured values for all seeds and compare the new values with Monte Carlo (MC) values and consensus values published by TG‐43U1. Methods: To calculate the relative absorbed‐dose energy dependence, f^rel, the ratio D(TLD)/D(w) at brachytherapy energies relative to Co‐60 is calculated using the EGSnrc BrachyDose code for TLD sized voxels at 1 cm from the seed centered in a water phantom. A similar geometry is used at high energy with the detector at 5 cm depth and DOSXYZnrc code. The original energy dependence applied to DRC measurements is replaced by the calculated f^rel values. By comparing the revised measured DRCs to the MC values, one can determine an appropriate relative intrinsic energy dependence, k^rel. The new relative absorbed‐dose sensitivity, S^rel, calculated as the product of f^rel and k^rel, is to give the measured DRC for comparison with Monte Carlo calculated values and TG‐43U1 consensus values. Results: The calculated f^rel values are 1.389 and 1.354 and the deduced k^rel values are 1.067 and 1.066 for I‐125 and Pd‐103 seed models, respectively. Using these new values with the literature DRC measurements, differences between calculated and revised measured DRCs are within 5% (typical measurement uncertainty) and the average discrepancy is 0.5%. The average of both DRC sets are on average 3.4% lower than the the consensus values reported in TG‐43U1. Conclusion: This work demonstrates that the energy response dependence, measured 25 years ago, applied to recent DRC measurements of I–125 and Pd‐103 brachytherapy seeds needs to be updated, thereby leading to a revision of the DRC consensus values reported by TG‐43U1.
WE‐C‐500‐01: Imaging Needs for Proton Therapy
Authors: Kruse J , Kooy H , Dong L
Published: July 2013

Proton therapy is a rapidly growing treatment modality. Superficially, proton therapy looks identical to x‐ray therapy ‐ the patient receives a treatment planning CT followed by daily image‐guided treatment sessions. However, the specifics of proton therapy impose unique demands on the imaging procedures at each phase of the treatment. In x‐ray therapy the Hounsfield Units (HU) of the CT are converted to relative electron density for megavoltage x‐ray dose calculations. In proton therapy the pertinent value to be determined from HU is proton stopping power for human tissue, usually different from stopping power in the phantom used for measurement. Accurate determination of stopping power from HU requires a unique calibration process and tight controls on CT imaging technique. A number of approaches have emerged in x‐ray therapy for daily localization of soft tissues — the most popular involving placement of fiducial markers or Cone Beam CT (CBCT). However, dose shadowing downstream of fiducials may disallow them from clinical application for proton therapy. CBCT is not yet used for proton therapy, but for some treatment sites daily localization of the tumor may not guarantee dosimetric coverage as variations in the relative positions of tumor and normal tissues in the beam path perturb the proton dose distribution. Direct proton beam imaging — radiography and tomography — are in development. Nuclear activation of tissues by the proton beam presents a unique quality assurance measure in which imaging of the activation could verify the range of the protons in the patient.

Learning Objectives:

1. Present the unique demands of the treatment simulation process for proton therapy, including HU to stopping power calibration, technique standardization, and artifact reduction.

2. Discuss the proton specific challenges in daily target localization including dose shadowing from fiducials, dose perturbation by anatomical changes, and the need for respiratory correlated IGRT.

3. Introduce the possibility of proton beam range verification by direct proton beam imaging and post‐treatment activation imaging.

Research grant from Varian Medical Systems
WE‐C‐WAB‐01: Intratumor Correlations of FDG, FLT, and Cu‐ATSM PET in Canine Tumors: Implications for Dose Painting
Authors: Bradshaw T , Bowen S , Jallow N , Forrest L , Jeraj R
Published: July 2013

Purpose: In dose painting, it is uncertain which of a tumor's biological properties should be targeted, and if plans for different tumor histologies are equally sensitive to the choice of biological target. This study characterizes the relationships between three potential biological targets ‐ glucose metabolism, proliferation, and hypoxia — in two different tumor histologies using PET/CT imaging. Methods: Twenty canine patients with sinonasal tumors (7 sarcomas and 13 carcinomas) were imaged using FDG, FLT, and Cu‐ATSM PET/CT on three consecutive days. Patients were immobilized and precisely positioned, and resulting images were rigidly registered. Within each tumor volume, voxel SUV distributions from different tracers were compared and inter‐tracer correlations were evaluated using voxel‐based Spearman correlation coefficients. Correlation coefficients were then Fisher‐transformed, and a two‐sided t‐test was applied to determine if sarcoma and carcinoma populations differed significantly in inter‐tracer correlations. SUV measures such as SUVmax, SUVpeak, and SUVmean were also compared between sarcomas and carcinomas using Mann‐Whitney U‐tests. Results: Significant differences in inter‐tracer correlations were observed between sarcoma and carcinoma tumors. Population‐averaged Spearman correlation coefficients were significantly higher for carcinoma tumors than sarcoma tumors in comparisons of FLT:Cu‐ATSM (0.83 vs. 0.38; p<0.0001), FDG:FLT (0.80 vs. 0.61; p=0.02), and FDG:Cu‐ATSM (0.82 vs. 0.69; p=0.04). Tracer distributions generally overlapped in carcinomas; in sarcomas, however, different tracers clustered in different tumor regions. Carcinomas also had significantly higher average FDG SUVmax (11.1 vs. 5.0; p=0.01) and higher Cu‐ATSM SUVmean (2.6 vs. 1.2; p=0.02) than sarcoma tumors. Conclusion: Carcinoma tumors, with high spatial correlations between tumor metabolism, proliferation, and hypoxia, are robust targets for therapies that target a single biological property. Sarcomas may not be well‐suited for such therapies. Histology‐specific robustness in biological target definition has large implications for dose painting strategies, as well as for other biologically targeted therapies.
WE‐E‐108‐03: Relative Biological Effectiveness (RBE) of Protons in Pristine Bragg Peaks
Authors: Streitmatter S , Stewart R , Sandison G
Published: July 2013

Purpose: Quantify spatial variations in proton RBE in the plateau region and near the distal edge of pristine Bragg peaks. Methods: The linear energy transfer (LET) and dosimetric characteristics of pristine Bragg peaks were simulated for 30 and 200 MeV proton beams using MCNPX Version 2.7.0. To assess proton RBE, the Monte Carlo Damage Simulation (MCDS) was used to determine the double strand break (DSB) yield as a function of proton energy. To model reproductive cell death, mechanistic formulas derived from the Repair‐Misrepair‐Fixation (RMF) model were used to explicitly link DSB induction to estimates of linear‐quadratic (LQ) model parameters α and β. These formulas were input into MCNPX (DE/DF tally modifier) and used to compute dose‐averaged estimates of α and β at selected locations in pristine Bragg Peaks. The modified MCNPX dose tallies provide all of the information needed to compute the RBE‐weighted dose (RWD) for the endpoints of reproductive cell death and DSB induction. Results: The RBE for reproductive cell death is greater than or equal to the RBE for DSB induction, regardless of location within the Bragg Peak. RBE and RWD are predicted to decrease as α/β increases. The ratio of the maximum RWD (distal edge of peak) to minimum RWD (plateau region) is 12.1 for a 30 MeV beam and 3.7 for a 200 MeV beam (α/β = 1.5 Gy). The lateral spread in RWD is much more significant for the degraded clinical beam than for the 30 MeV beam. Conclusion: A low‐energy proton beam has attractive physical and biological characteristics for the precise delivery of a wide range of RWD in vitro and in vivo. However, models are needed to translate the results of preclinical studies with low‐energy protons to the more clinically relevant proton beam energies used on human patients.
WE‐E‐108‐10: Validating a 192Ir‐Based Small Animal Irradiation Apparatus Using a 3D‐Printed Applicator: Comparison Between TG‐43, Monte Carlo and Films Dosimetry
Authors: Nie K , Fekete C Collins , Pinnaduwage D , Cunha JAM , Mellis K , Descovich M , Beaulieu L , Pouliot J
Published: July 2013

Purpose: Accurate assessment of dose delivered is key to early prediction of radiation‐induced anatomic and physiologic changes vital in providing the most accurate patient‐specific treatments. We are conducting a translational small‐animal study using functional Hyperpolarized‐13C‐Urea with DCE‐MRI to evaluate tumor perfusion changes following local targeted Ir‐192 irradiation using the Leipzig applicator. The purpose of this study is to present and evaluate a novel Monte Carlo (MC) tool, which provides heterogeneous dose predictions for this specific application, and to compare the results against the TG‐43 dose calculation. Methods: CT scans were obtained with a Leipzig applicator mold (fabricated using a 3D‐printer to avoid metal artifact) centrally placed on top of the CyberKnife Ball Cube QA phantom . The CT density of the mold was overridden to be that of the applicator material, Tungsten. A Monte Carlo platform, ALGEBRA (ALgorithm for heterogeneous dosimetry based on GEANT4 for BRAchytherapy) was used for simulation. Dose measurements were done using Gafchromic EBT2 film placed orthogonally inside the Ball Cube exposed to Ir‐192 with the Leipzig applicator in place. Simple TG‐43 calculations for a free source in water was done using the Oncentra treatment planning system. Results: The two‐dimensional planar dose distributions obtained from MC simulation showed strong agreement (within 4–5%) with dose measurements while TG‐43 calculations showed differences up to 20%. Compared to TG‐43, the MC Result was attenuated less at the surface because of the Leipzig air cavity, but penetrated less due to the collimation effect. Conclusion: We have validated a new MC simulation tool using a Leipzig applicator for small animal irradiation. This tool will provide the basis for studies associating tumor response with the actual dose delivered. The tool can be further used to construct dosimetry information for clinical treatments using the Leipzig applicator as an alternative to superficial/orthovoltage radiation treatment.
WE‐F‐105‐01: On the Importance of Nuclear Models On the Accuracy of Fast Monte Carlo Methods for Proton‐Therapy
Authors: Souris K , Lee J , Sterpin E
Published: July 2013

Purpose: Parallel architectures like graphical processor units (GPU) with high computational power allow dose distributions for proton beams to be calculated with Monte Carlo (MC) simulations within the time constraints of clinical practice. However, physical models often need to be simplified to exploit the full potential of GPUs. This study aims at evaluating comprehensively the impact on computation accuracy of the simplified model for nuclear inelastic collisions described by Fippel et al (Med Phys 2004) and used by Jia et al (PMB 2012). Methods: Three MC codes were used: 1) an in‐house version of Fippel's algorithm, which uses an empirical method to reproduce approximately ICRU 63 nuclear inelastic cross‐sections for oxygen; 2) an extension of PENELOPE to protons (PENH) that samples directly ICRU 63 cross sections (but neglects neutrons); 3) GEANT4 with Binary‐Cascade.The integral depth‐dose deposited by a 200 MeV proton beam into water was then calculated using those MC codes. The simulations were performed in water phantoms of dimensions 4.1×4.1×40, 8×8×40 and 60×60×40 cm3 with a scoring resolution of 1 mm in depth. Results: In the 4.1×4.1×40 cm3 phantom, deviations between Fippel's algorithm and GEANT4 were within 2%, whereas this bound grew up to 3% between PENH and GEANT4. However, when the phantom dimensions are increased, the deviations between PENH and GEANT4 remained within 3%, while they reached 5% for the 60×60×40 cm3 phantom using Fippel's method. Conclusion: Our in‐house implementation of Fippel's algorithm did not reproduce consistently GEANT4 results for varying phantom sizes. This was attributed to the over‐simplified sampling of the angular distributions of secondary protons resulting from nuclear inelastic reactions. However, this was not observed for PENH that implements an exact sampling of ICRU 63 cross‐sections. Therefore, the details of nuclear models may have a significant impact on accuracy. Kevin Souris is financed by the Walloon Region under the project name InVivoIGT, convention number 1017266. Kevin Souris is sponsored by a public‐private partnership IBA ‐ Walloon Region
WE‐G‐WAB‐05: Upgrading the MRI‐Linac Prototype to a Gantry‐Based System: Impact On Magnetic Field Homogeneity
Authors: Crijns S , Raaymakers B , Lagendijk J
Published: July 2013

Purpose: In cooperation with Elekta AB (Stockholm, Sweden) and Philips (Best, The Netherlands) the UMC Utrecht has constructed a prototype MRI‐linear accelerator. This prototype was recently upgraded to a continuously rotating gantry‐based system with a 160‐leaf MLC. Ferro‐magnetic parts on the gantry around the MR scanner may induce gantry position dependent magnetic field inhomogeneities. Here the aim is to investigate the influence of the field inhomogeneity variations on geometrical accuracy of images as well as the feasibility of possible solutions. Methods: Magnetic field maps were acquired with and without additional shimming at a gantry position resolution of 5° for static gantry positions using a dual‐echo GRE‐based sequence. From these maps the minimum read‐out gradient strength GR,min to achieve an accuracy δ at position (x,y) is Gread,min(x,y)=ΔB(x,y)/δ. Moreover, the measured maps were used as input for a for simulation of a dynamic shimming solution, that updates the scanner shims in real‐time while the gantry is rotating. Results: With shimming, GR,min was below 10mTm−1 for δ=1.0mm for all measurements. Shim settings for the x and y directions displayed a sinusoidal pattern with varying gantry position and are highly reproducible. The z‐shim remained gantry position independent. The simulations showed a great reduction of image artifacts due to dynamic shimming in case of gantry rotation during image acquisition. Conclusion: Gantry position dependent field inhomogeneity in our upgraded MRI‐linac prototype can be mitigated by shimming in combination with a clinically feasible minimum read‐out gradient strength. The shim settings are reproducible, such that look‐up‐table‐based shimming is feasible for static gantry positions, reducing preparation time needed before imaging after changing the gantry angle. Simulations show that for simultaneous gantry rotation and MR imaging, a look‐up‐table‐based dynamic shimming solution could be used to preserve image quality.
WE‐G‐WAB‐07: Commissioning of a Novel Integrated Platform for Online MRIgRT
Authors: Tadic T , Jaffray D , Winter J , Petropoulos L , Marle J , Sweitzer M , Stanescu T
Published: July 2013

Purpose: An integrated system for online MRIgRT comprising an MR scanner‐on‐rails proximal to a Varian TrueBeam linac is currently under installation at our institution. This study presents the development of a procedure to commission this platform along with the synthesis, testing, and validation of several unique tools/methods central to this challenging frontier. Methods: The commissioning procedure is organized into three phases: 1) the linac and a modified patient couch are commissioned in the absence of magnetic fields; 2) the MR magnet is energized, followed by experimental field mapping and verification of linac/couch performance proximal to the magnet; 3) MR imaging is evaluated and integrated guidance functionalities are tested. Novel methods/technologies embedded in these steps include: magnetic field finite element simulations to confirm magnetic decoupling of the MR scanner, linac, and couch; development of a novel harmonic distortion analysis/shim evaluation technique and associated phantom design incorporating comprehensive imaging QA; simulation of patient‐induced susceptibility distortions and pulse sequence optimization; development of a phantom/method to evaluate the accuracy/stability of MR scanner/couch travel. Results: The finite element simulations of the MRIgRT platform indicated acceptable magnetic forces on critical couch/linac components, undisturbed x‐ray beam production, and acceptable field uniformity within the MRI active volume (via active shimming). Validation of the harmonic distortion analysis demonstrated that complete volumetric mapping of the geometric distortion could be performed with an error bound by the imaging voxel size. Susceptibility‐induced distortions were simulated for various anatomical sites and can be reduced to within 1–2 mm by increasing the frequency encoding gradient strength with acceptable SNR losses. Lastly, the accuracy of translation and rotation measurements was established for evaluating MR scanner/couch travel. Conclusion: A comprehensive procedure has been established for the commissioning of an integrated MRIgRT platform, and various key tools/methods for assessing system performance have been developed and tested. J. Winter and L. Petropoulos are employees of IMRIS. J. Marle and M. Sweitzer are employees of Varian Medical Systems. IMRIS and Varian are companies co‐developing the MRgRT system. Funding for the MRgRT project at Princess Margaret Hospital was provided by the Canadian Foundation for Innovation.
TH‐A‐103‐08: System Optimization of Spectral Breast CT Based On An Energy‐Resolved Photon‐Counting Si Strip Detector: A Simulation Study
Authors: Cho H , Ding H , Masaki F , Barber W , Iwanczyk J , Molloi S
Published: July 2013

Purpose: To determine the optimal design parameters for a high resolution spectral breast CT system based on an energy‐resolved photon counting Si strip detector for breast imaging. Methods: A simulation package was developed for a spectral breast CT system. The focal spot size (100 to 300 μm), magnification (1.1 to 1.4), motion blurring and detector pixel size (intrinsic pixel pitch and 4×4 pixel binning) were considered to optimize the system design. Simulations were conducted using single slice fan beam geometry at 60 kVp with 2 mm Al filter. The system modulation transfer functions (MTFs) were calculated from reconstructed image of a 10 μm diameter tungsten wire. The performance of the system with optimal design parameters was evaluated with a simulated breast phantom. A 14 cm diameter cylindrical phantom made of breast tissue with 20% glandularity was used to simulate an average‐sized breast. The soft tissue lesions and microcalcifications were simulated with spheres of glandular tissues (1,2, and 4 mm in diameter) and hydroxyapatite (100 to 250 μm in diameter), respectively. Results: The spatial frequencies at 10 % of the maximum MTF was calculated to be 1.6 lp/mm and 6.4 lp/mm with and without a 4 x 4 pixel binning, respectively. The optimal system magnification was estimated to be 1.2. There was minimal improvement in system MTF when the x‐ray tube focal spot size was changed from 300 to 100 μm. In the simulated breast phantom image, 100 μm microcalcifications can be clearly identified and the soft tissue contrast‐to‐noise ratio was improved with separate image processing, where 4×4 pixel binning was used at a dose level of 6 mGy. Conclusion: The simulation results show that the proposed system can potentially detect micro‐calcifications and soft tissue lesions with a dose level equivalent to that of a standard two‐view mammography.
TH‐A‐103‐10: Improved Segmentation of Low‐Contrast Fibroglandular Structures in High‐Noise Breast CT Volumes for XCAT Modeling
Authors: Wells J , Segars P , Dobbins J
Published: July 2013

Purpose: This work improves the accuracy and realism of automated breast computed tomography (bCT) tissue segmentation by refining the detection of low‐contrast fibroglandular structures to produce high‐resolution realistic computer‐generated (XCAT) breast phantoms from empirical human subject data. Methods: Previous work by Hsu et al. [Med. Phys. 38, 5756‐5770 (2011)] produced high‐resolution realistic computer‐generated breast phantoms from empirical human subject data but challenges were encountered with the accurate segmentation of fine, low‐contrast glandular structures. The current work addresses those challenges. A 3‐D anisotropic diffusion algorithm was used to denoise fourteen bCT datasets. After breast masking, two adipose non‐uniformity correction techniques were applied. The first has been described by Altunbas, et al. [Med. Phys. 34, 3109‐3118 (2007)]. The second approach employed an original technique using higher‐order polynomials to correct for residual adipose non‐uniformity. Histogram thresholding then produced initial gland and skin segmentations. This was followed by a novel glandular linking and extension protocol based on skeletonization of the skin and glandular segmentations and a pixel gray‐level‐weighted distance transform. Skin mask definition and glandular density differentiation completed the segmentation. Results: Volumetric denoising reduced the standard deviation of the adipose background by an average of 60.4%. The Altunbas method corrected for radially symmetric, quadratic non‐uniformities in breasts with circular coronal cross sections, but performed poorly on high‐density breasts and breasts with asymmetric adipose non‐uniformity. Follow‐up correction using the novel method improved adipose uniformity by an average of 24.6%. The new fibroglandular linking and extension protocol improved the detection of low‐contrast fibroglandular structures, including Cooper's ligaments. The total number of fibroglandular tissue islands was also reduced. Conclusion: The semi‐automated bCT segmentation protocol improved low‐contrast glandular fiber detection in high‐noise reconstructions. Linking of disparate fibroglandular tissue islands and capture of Cooper's ligaments will contribute to the overall accuracy and realism of empirically‐derived XCAT breast phantoms. This work was supported by NIH Grant 5R01‐CA‐134658.
TH‐A‐116‐02: Radiobiological Implications of Various Target and Beam Geometry Utilization in Treatment Planning for Intensity Modulated Proton Therapy
Authors: Giantsoudi D , Unkelbach J , Craft D , Zeng C , Trofimov A , Paganetti H
Published: July 2013

Purpose: To investigate how different target definition and beam utilization techniques in intensity modulated proton therapy (IMPT) treatment planning (TP) can influence the radiobiological effectiveness of a deliverable patient plan. Methods: Our multi‐criteria optimization (MCO) IMPT planning system (ASTROID) allows assigning separate target volume for each beam, such as in split target plans. For pediatric chordoma cases, a variety of target sub‐volumes (TSV) were generated by 4mm and 8mm uniform constriction of the original clinical target volume (CTV), and their subtraction from CTV and each other, forming a co‐centric shell structure arrangement. In addition to the original plan, utilizing three beams all assigned to the full CTV, five plans were optimized for the same beam angles but different assigned TSV combinations. To increase the degrees of freedom the original beams were duplicated to allow for TSVs to be assigned to multiple beam‐angles, having a total of up to 9 beams. All plans utilized all three beam angles and single dose prescription to CTV. Dose and linear energy transfer (LET) distributions were calculated by TOPAS (TOol for PArticle Simulation) Monte Carlo system and evaluated via dose‐and LET‐volume histograms and a relative biological effectiveness (RBE) model. Results: While dose variations for the CTV were negligible, mean LET value deviations of more than 30% were noticed among the different plans. For the organs at risk (OARs), variations in dosimetric, LET and RBE values were more prominent. The amount of variation among the plans seemed to be dependent on their volume and location relative to the target. Conclusion: We show that the definition of sub‐targets at the treatment plan optimization level may impact the biological effectiveness of proton treatments.