bluuebunny/crossref_metadata_2025 huggingface dataset:
WE‐G‐BRCD‐02: Speed Up Standard Optimization of IMPT Treatment Planning Through the Application of GPUs
Authors: Ge S , Liu W , Quan E , Mohan R
Published: June 2012

Purpose: While intensity‐modulated proton therapy (IMPT) has great potential to deliver highly conformal tumoricidal dose to targets whilst minimizing dose to nearby organs‐at‐risk, IMPT optimization is very time consuming and memory extensive due to finer dose grids and a large number of energy layers used compared to intensity‐modulated radiation therapy (IMRT). In this presentation, for the first time, a new approach is introduced to speed up the IMPT treatment planning through application of parallel computing with Graphic Processor Units (GPUs). Methods: Parallel computation with GPUs, which are affordable and can be plugged in a workstation easily, is potentially a good way to improve the computation efficiency. In our approach, we used the standard quadratic objective function to optimize the intensity map of beamlets. The objective function and gradient equations, which are the most time consuming parts of the optimization, were calculated with GPUs. We compared the computation time of optimization done by an Intel ® Core™ i7 CPU and that by the same CPU accelerated by GPUs (TESLA C1060). The influence matrix was pre‐ calculated before optimization with an in‐house proton pencil beam dose calculation engine. Two clinical cases were studied: one base‐of‐skull (BOS) case (clivus chordoma) and one prostate case (adenocarcinoma). The dose volume histogram (DVH) data for the tumor and critical organs were derived for comparison of optimization results generated by CPU and GPUs. Results: For the BOS case, application of GPUs for the optimization and overall gained 54 and 36.5 times speedup. For the prostate case, application of GPUs for the optimization and overall gained 69 and 28.5 times speedup. Conclusions: The application of GPUs for the parallel computing of IMPT treatment plan optimization can dramatically improve the computation efficiency. The optimization time can be reduced from typically half to one hour to only several minutes.
WE‐G‐BRCD‐04: Development of Automated Determination Method of Robust Beam Directions against Patient Setup Errors Based on Spatial Distribution of Electron Density in Particle Therapy
Authors: Kakiuchi G , Arimura H , Shioyama Y , Minohara S , Mizoguchi A , Nakamura K , Honda H , Toyofuku F , Ohki M , Hirata H
Published: June 2012

Purpose: The three‐dimensional (3D) dose distribution covering a tumor region tends to be more breakable if the beam's eye view (BEV) of the 3D electron density (ED) map in a beam direction changes more abruptly with large fluctuations. Our aim of this study was to develop an automated determination method of robust beam directions against the patient setup error based on the ED‐based BEV in the beam direction in the particle therapy. Methods: The basic idea of our proposed method was to find the robust beam directions, whose the ED‐based BEV has the spatial fluctuations with low special frequency and small amplitude. For evaluation of the spatial fluctuation in the ED‐based BEV in a beam direction, we obtained power spectra of the ED‐based BEVs in all directions, i.e., 0 to 355 degree, with an interval of 5 degree. It was assumed that as the average spatial frequency and amplitude of the fluctuation in the ED‐based BEV in a beam direction is lower and smaller, respectively, the absolute value of a gradient of the power spectrum becomes larger. Therefore the gradient of the power spectrum was calculated for determination of the robust beam direction. The ED‐based BEV was produced by projecting a 3D electron density map derived from the computed tomography (CT) image from a beam source to the distal end of a planning target volume (PTV). Four cases of head and neck cancer patients were selected for evaluation of the proposed method. Results: As a preliminary result, radiation oncologists agreed with most beam directions, which seem to be robust against patient setup errors, suggested by the proposed method. Conclusions: Our proposed method could be feasible to suggest the robust beam directions against patient setup errors in hadron particle therapy.
WE‐G‐BRCD‐07: IMRT Re‐Planning by Adjusting Voxel‐Based Weighting Factors for Adaptive Radiotherapy
Authors: Li N , Zarepisheh M , Tian Z , Uribe‐Sanchez A , Zhen X , Graves Y , Gautier Q , Zhou Linghong , Jia X , Jiang S
Published: June 2012

Purpose: Adaptive radiation therapy (ART) can reduce normal tissue toxicity and/or improve tumor control through treatment adaptations based on the current patient anatomy. Developing an efficient re‐planning algorithm is an important step for ART. For the re‐planning process, manual trial‐and‐error approach to fine‐tune planning parameters is a time‐ consuming and resource‐requiring task. However, prior information in the original plan, such as dose distribution or dose‐volume histogram (DVH) can be employed to facilitate the re‐planning. The goal of this work is to develop a re‐planning algorithm that automates voxel weighting factor adjustments to generate a plan with close, or possibly better, DVH curves compared with original plan. Methods: Our algorithm iterates the following two loops. The inner loop is the traditional fluence map optimization, in which we optimize a quadratic objective function penalizing the deviation of the dose received by each voxel from its prescribed or threshold dose with a set of fixed weighting factors. In outer loop, the weighting factors in the objective function for each voxel are heuristically adjusted according to the deviation of the DVH curves in the calculated plan from those in the original plan. The process is repeated until the result converges, or the maximum iteration step is reached. The whole algorithm is implemented on GPU for high efficiency. Results: We have tested our algorithm on three 8‐field head‐and‐neck cases. Compared with the DVH curves in original plan, the DVH curves in the resulting plan using our algorithm with 30 iterations are almost better for every structure. We can finish the re‐optimization process around 30 seconds. Conclusions: Adjusting voxel‐based weighting factors automatically, by comparing the DVH curves, seems to be a promising approach to avoid the tedious trial‐and‐error scheme for ART re‐planning. This work is supported by Varian Medical Systems through a Master Research Agreement.
WE‐G‐BRB‐04: BEST IN PHYSICS (THERAPY) ‐ A Novel Multi‐Point Plastic Scintillation Detector for in Vivo Dosimetry and Quality Assurance in Radiation Therapy
Authors: Therriault‐Proulx F , Beaulieu L , Archambault L , Beddar S
Published: June 2012

Purpose: To develop a novel multi‐point plastic scintillation detector (mPSD) capable of accurately measuring dose at multiple positions simultaneously with the use of a single optical guide. Methods: We built a new generation of plastic scintillation detectors composed of multiple scintillating elements along a same optical transmission line. Three different scintillating fibers were optically coupled to a single collecting optical fiber. A primary challenge for this new type of detector is that the output signal is a superposition of multiple scintillation spectra and contaminating elements. Acquisition with a spectrometry setup allows for the implementation of a new hyperspectral approach that accounts for each light‐emitting component separately, and allows spectral unmixing. The mPSD and an ion chamber were irradiated in a water phantom with a 6 MV photon beam. Profiles and depth‐dose curves were measured and compared between detectors. This detector and the corresponding calibration approach were also applied to Ir‐ 192 HDR brachytherapy. Results: Doses measured with the mPSD were in good agreement with the ion chamber measurements for external beam irradiations. Average relative differences of (2.3±1.1)%, (1.6±0.4)% and (0.32±0.19)% were observed for each scintillating element. The mPSD measurements tended to be at least as accurate as published measurements from single‐point PSDs. For the Ir‐192 HDR brachytherapy application, the average difference between the treatment planning system and the measurements were (4.6±1.0)% per dwell‐position and (2.1±1.0)% per catheter. The accuracy of each scintillating element was shown to depend on light attenuation and on the similarity of its scintillation spectrum in comparison to the other light emitters. Conclusions: The feasibility and accuracy of mPSDs using a single transmission line was demonstrated. In addition to well‐documented advantages of single‐point PSDs, the multi‐point capability of this single‐fiber detector makes mPSDs a very promising new technique for quality assurance and on‐line in vivo dosimetry.
WE‐G‐BRB‐06: Real‐Time Radiation Field Tracking Using Long Scintillating Fibers
Authors: Goulet M , Gingras L , Depauw N , Archambault L , Seco J , Beaulieu L
Published: June 2012

Purpose: To ensure the quality assurance of small field, dynamic radiotherapy, we present and validate a radiation tracking system based on long scintillating fibers that allows for the real‐time measurement of the position and energetic fluence of a small incident radiation field. Method: We aligned 60 parallel scintillating fibers on a thin grooved acrylic slab with a 100‐cm source‐to‐fibers distance. Both ends of each scintillating fiber were coupled to clear optical fibers to enable light collection by a single CCD camera using an f/0.95, 50 mm focal length lens. Using a small, static photon radiation field of 2×2 cm2 of a Varian Clinac iX, we changed the interaction position on the prototype using the linac treatment couch. The interaction position parallel and perpendicular to the scintillating fiber array were deduced using the optical attenuation of the scintillating fibers. The energetic fluence of the incident field was calculated from the fibers light fluxes, corrected for the position dependent optical attenuation and scintillation efficiency. Results: Considering a treatment couch positioning error of ±0.5 mm, the system was able to measure the field position with a mean error of 0.1 mm perpendicular and 0.8 mm parallel to the scintillating fiber array. The maximum error measured using this setup was of 0.13 mm perpendicular and 3.2 mm parallel to the scintillating fiber array. The energetic fluence was determined with a mean error of 0.5% and a maximum error of 2.2%. Conclusions: This work demonstrates the capacity of a long scintillating fibers array to detect in real‐time both the position and the energetic fluence of an incident small radiation field. Such methodology would allow for the real‐time tracking of small field in both photon and particle radiation therapy.
WE‐G‐BRB‐07: Application of RADPOS in Vivo Dosimetry for QA of High Dose Rate Brachytherapy
Authors: Cherpak A , Kertzscher G , Cygler J
Published: June 2012

Purpose: The RADPOS in vivo dosimetry system combines an electromagnetic positioning sensor with MOSFET dosimetry, allowing for simultaneous online measurements of dose and spatial position. In this work, we assess the potential use of RADPOS for measurements of motion and dose during prostate HDR brachytherapy. Methods: The RADPOS was positioned in the centre of a thin plastic tube supported by two parallel acrylic plates submerged in water. The detector was calibrated by sending an Ir‐192 source into an adjacent tube, to positions ranging from 8.0 to −8.0 cm along the vertical axis in increments of 0.5 cm. The dwell time at each position was 20 s and the change in threshold voltage of the RADPOS dosimeter was recorded for each position. The expected dose for each source position was calculated and compared to RADPOS measurements to obtain a detector calibration coefficient (cGy/mV). The procedure was then repeated sending the Ir‐192 source into 5 other tubes, located 1 to 10 cm away from the RADPOS. Source positions, dwell times, and position of the RADPOS detector were the same as for the calibration. The dose for each source position was determined by RADPOS measurements and calibration coefficient and compared to the expected dose. Results: An average calibration coefficient of 0.74±0.11 cGy/mV was calculated for RADPOS measurements of Ir‐192 irradiations. The absolute difference between RADPOS values and expected dose for subsequent measurements averaged over all source positions was 0.7±5.4 cGy. Conclusions: In vivo dosimetry can potentially signal errors in catheter placement or numbering before entire dose is delivered. The demonstrated accuracy of RADPOS dose measurements and its ability to simultaneously measure displacement makes it a powerful tool for HDR brachytherapy treatments for prostate cancer, where high dose gradients and movement of the prostate gland can present unique in vivo dosimetry challenges. Financial and technical support has been received from Best Medical Canada and Ascension Technology Corporation.
WE‐G‐BRB‐08: TG‐51 Calibration of First Commercial MRI‐Guided IMRT System in the Presence of 0.35 Tesla Magnetic Field
Authors: Goddu S , Green O Pechenaya , Mutic S
Published: June 2012

Purpose: The first real‐time‐MRI‐guided radiotherapy system has been installed in a clinic and it is being evaluated. Presence of magnetic field (MF) during radiation output calibration may have implications on ionization measurements and there is a possibility that standard calibration protocols may not be suitable for dose measurements for such devices. In this study, we evaluated whether a standard calibration protocol (AAPM‐ TG‐51) is appropriate for absolute dose measurement in presence of MF. Methods: Treatment delivery of the ViewRay (VR) system is via three 15,000Ci Cobalt‐60 heads positioned 120‐degrees apart and all calibration measurements were done in the presence of 0.35T MF. Two ADCL‐ calibrated ionization‐chambers (Exradin A12, A16) were used for TG‐51 calibration. Chambers were positioned at 5‐cm depth, (SSD=105cm: VR's isocenter), and the MLC leaves were shaped to a 10.5cm × 10.5 cm field size. Percent‐depth‐dose (PDD) measurements were performed for 5 and 10 cm depths. Individual output of each head was measured using the AAPM‐ TG51 protocol. Calibration accuracy for each head was subsequently verified by Radiological Physics Center (RPC) TLD measurements. Results: Measured ion‐recombination (Pion) and polarity (Ppol) correction factors were less‐than 1.002 and 1.006, respectively. Measured PDDs agreed with BJR‐25 within ±0.2%. Maximum dose rates for the reference field size at VR's isocenter for heads 1, 2 and 3 were 1.445±0.005, 1.446±0.107, 1.431±0.006 Gy/minute, respectively. Our calibrations agreed with RPC‐ TLD measurements within ±1.3%, ±2.6% and ±2.0% for treatment‐heads 1, 2 and 3, respectively. At the time of calibration, mean activity of the Co‐60 sources was 10,800Ci±0.1%. Conclusions: This study shows that the TG‐ 51 calibration is feasible in the presence of 0.35T MF and the measurement agreement is within the range of results obtainable for conventional treatment machines. Drs. Green, Goddu, and Mutic served as scientific consultants for ViewRay, Inc. Dr. Mutic is on the clinical focus group for ViewRay, Inc., and his spouse holds shares in ViewRay, Inc.
WE‐G‐BRA‐02: Visual Demonstrations of Medical Physics Concepts of Transmission Imaging for Resident Education
Authors: Sechopoulos I
Published: June 2012

Purpose: To improve the radiology residents' understanding of medical physics concepts through visualization of physical phenomena. Methods: Several medical physics concepts in x‐ray transmission imaging are relevant to many radiographic modalities, not only to planar radiography. Therefore, it is important that the diagnostic radiology residents obtain a good understanding of these concepts. However, standard PowerPoint slides or blackboard‐based graphical representations are not always effective ways to communicate these novel concepts to the residents. To improve upon the understanding of these concepts, the computer, projector and screen in the lecture room are used as surrogates of an x‐ray imaging system. The projector is the source of light (x‐rays) with PowerPoint slides defining the pattern emitted (x‐ray field) on to the projector screen (detector/monitor). Several different transparencies and acrylic objects are used to demonstrate varied medical physics phenomena relevant to transmission imaging, such as: straight‐line travel of electromagnetic radiation; tissue superimposition; object, subject, image and display contrast; linear systems; point spread functions; frequency domain; contrast and modulation transfer functions; quantum and image noise; noise frequency and noise power spectrum; anatomical noise; magnification and geometric unsharpness; inverse square distance relationship; sampling and aliasing; and x‐ray scatter. Results: The residents' comprehension and ability to explain these concepts has substantially improved, in addition to their interest in these topics. This was reflected on improved test scores and on anonymous feedback surveys post‐ lectures. Conclusions: The use of demonstrations that mimic the conditions and physical phenomena found in transmission imaging by taking advantage of the projector and screen together with transparencies and other objects improves the residents' grasp of basic radiographic concepts and promotes live interactions between the residents and the medical physicist. Additional concepts that can be demonstrated in this manner are being sought.
WE‐G‐BRA‐03: Enhancing Medical Imaging Physics Education on a Global Basis with Shared Resources
Authors: Sprawls P
Published: June 2012

Purpose: To enhance and enrich medical imaging physics education by first translating personal professional experience into highly‐effective educational resources and then sharing with both learners and medical physics educators on a global basis as an open and free resource. Methods: Effective medical imaging physics education that contributes to high‐quality and safe clinical procedures requires medical physicists and students, anywhere in the world, having access to the science of contemporary imaging methods and procedures in a form that enriches both the teaching and learning activities. This is being achieved in two phases. The first (and continuing) phase is the translation of personal experience as a practicing clinical medical physicist in to a synergistic collection of educational resources including textbooks, online modules, and especially highly‐ effective visuals that can be used in classroom presentations and small group discussions. It is the availability of these visuals that enhances the performance of medical physicists as teachers who can devote their efforts to guiding the class learning activities and sharing their personal experiences and not be burdened with the production of extensive teaching materials. The second (and overlapping) phase is the development of a model and method for sharing as an open and free resource. This is achieved through a web site developed for that purpose, www.(blinded for review).org. Results: These resources are used to support medical physics educators, programs, and individual learners both in the US and in over 100 different countries with an average of over 1000 users each day. Conclusions: The production and sharing of educational resources by an individual medical physicist complements and extends beyond the programs and activities of institutions and organizations. It is of significant value to learners anywhere in the world because the availability is not limited by costly course enrollment, membership, meeting registration, or geographic restrictions.
WE‐G‐BRA‐05: Simulated Troubleshooting Session for Medical Physics Resident Rotations
Authors: Tang X , Wang S , Chang S , Burkhardt K , Schreiber E , Lawrence M
Published: June 2012

Purpose: Troubleshooting is a difficult to train, but critical skill for a clinical medical physicist. It is unlikely that residents will have opportunities to troubleshoot many real clinical scenarios during a typical 3 month rotation. The ability to analyze and correct abnormal clinical situations is imperative to providing safe radiation therapy care. We have developed a novel strategy to train medical physics residents to gain such important clinical physics skill. Methods: Simulated troubleshooting sessions were designed for each rotation, featuring problematic clinical scenarios. The resident was required to troubleshoot each scenario in a given time frame. Using a brachytherapy rotation as an example, the transfer tubes connecting the afterloader to the Tandem and Ovoids (T&O) applicators were purposely inserted incorrectly. The resident was asked to deliver a dummy T&O plan in a reasonable time frame including troubleshooting all problems that may occur. Many simulated troubleshooting situations were presented to the resident. Two residents went through a 3 month brachytherapy rotation, one with and the other without the simulated troubleshooting session. All other rotation activities were the same. At the end of the rotation, an oral exam was given to test how well the residents understood the brachytherapy clinical practice. Results: Both residents were able to pass the exam. However, the resident that received the troubleshooting session performed better'‴a superior understanding of the reasons behind the clinical practice and better troubleshooting skills were demonstrated. Conclusions: To train residents to better handle the clinical situations, it is important to include a simulated troubleshooting session into clinical rotations. Simulated troubleshooting sessions are excellent and necessary supplements to the clinical rotations for the residents to gain practical clinical skills.
WE‐G‐213CD‐03: A Dual Complementary Verification Method for Dynamic Tumor Tracking on Vero SBRT
Authors: Poels K , Depuydt T , Verellen D , De Ridder M
Published: June 2012

Purpose: to use complementary cine EPID and gimbals log file analysis for in‐vivo tracking accuracy monitoring. Methods: A clinical prototype of dynamic tracking (DT) was installed on the Vero SBRT system. This prototype version allowed tumor tracking by gimballed linac rotations using an internal‐external correspondence model. The DT prototype software allowed the detailed logging of all applied gimbals rotations during tracking. The integration of an EPID on the vero system allowed the acquisition of cine EPID images during DT. We quantified the tracking error on cine EPID (E‐EPID) by subtracting the target center (fiducial marker detection) and the field centroid. Dynamic gimbals log file information was combined with orthogonal x‐ray verification images to calculate the in‐vivo tracking error (E‐kVLog). The correlation between E‐kVLog and E‐EPID was calculated for validation of the gimbals log file. Further, we investigated the sensitivity of the log file tracking error by introducing predefined systematic tracking errors. As an application we calculate gimbals log file tracking error for dynamic hidden target tests to investigate gravity effects and decoupled gimbals rotation from gantry rotation. Finally, calculating complementary cine EPID and log file tracking errors evaluated the clinical accuracy of dynamic tracking. Results: A strong correlation was found between log file and cine EPID tracking error distribution during concurrent measurements (R=0.98). We found sensitivity in the gimbals log files to detect a systematic tracking error up to 0.5 mm. Dynamic hidden target tests showed no gravity influence on tracking performance and high degree of decoupled gimbals and gantry rotation during dynamic arc dynamic tracking. A submillimetric agreement between clinical complementary tracking error measurements was found. Conclusions: Redundancy of the internal gimbals log file with x‐ray verification images with complementary independent cine EPID images was implemented to monitor the accuracy of gimballed tumor tracking on Vero SBRT. Research was financially supported by the Flemish government (FWO), Hercules Foundation and BrainLAB AG.
WE‐G‐213CD‐07: Enhancing Respiratory Motion Prediction Accuracy Using Audiovisual (AV) Biofeedback
Authors: Pollock S , Lee D , Keall P , Kim T
Published: June 2012

Purpose: Prediction of respiratory‐related tumor motion is hampered by irregularities present in the patient breathing patterns. Audiovisual (AV) biofeedback reduces irregularities, thereby producing a less complex breathing pattern. The aim of this project is to improve respiratory motion prediction accuracy using an AV biofeedback system. Methods: An AV biofeedback system combined with real‐time MRI was implemented in this project (4 human subjects across 5 studies (one subject had both an initial and follow‐up study)). The AV biofeedback system consists of external marker positioned on the abdomen of human subjects, being tracked using an RPM system (Real‐time Position Management, Varian) to guide the subject's breathing. Acquired respiratory data has been used as input for motion prediction through a dynamic multi‐leaf collimator (DMLC) simulator developed by Prof. Keall. The prediction algorithm utilized was a kernel density estimation‐based real‐time prediction algorithm. A variety of prediction parameters were tested to determine optimum prediction performance. Prediction parameters adjusted were the delay time (DT) and training examples (TE); the parameters tested here were: DT/TE = 2500/1500, 2500/100, 1000/250, 500/250; Given that the data sampling rate was kept at 30 Hz, the resultant prediction training window lengths were 49.5, 8.25, 3.3 and 3.3seconds respectively. Results: The mean difference between measured and predicted data for free breathing was 1.98±2.32mm; and 0.65±0.65mm for when AV biofeedback was implemented (reduction of error of 67%). The most accurate prediction results were attained using the parameters: DT/TE = 500 ms/250. Conclusions: This study demonstrates the improvement of respiratory motion prediction accuracy when AV biofeedback is implemented to produce a more regular breathing pattern.
WE‐G‐217BCD‐01: BEST IN PHYSICS (IMAGING) ‐ High‐Quality CT Imaging in the Presence of Surgical Instrumentation Using Spectral System Models and Knowledge of Implanted Devices
Authors: Zbijewski W , Stayman J , Otake Y , Carrino J , Khanna A , Siewerdsen J
Published: June 2012

Purpose: Imaging in the presence of implants (instrumentation and prostheses) presents a notoriously difficult challenge to CT because of photon starvation and beam hardening. To alleviate these limitations, a statistical reconstruction approach that includes knowledge of implant shape and composition was previously reported. This work extends the approach to modeling of photon transport, including polychromatic x‐ray beams and scatter, and evaluates the method in simulated and real data. Methods: Previous work on Known‐Component Reconstruction (KCR) is first extended to include a polyenergetic beam (KCR‐POLY). The method simultaneously estimates the unknown background volume and the position of implants with known attenuation and shape. Simulations included an anthropomorphic knee with a Co‐Cr‐Mo implant and system model for an extremities CT system (110 kVp+0.2 mm Cu). Experimental validation was performed on an imaging bench in which a Titanium spine fixation rod (65 mm long, 5.5 mm diameter) was imaged within a 20.5 cm diameter water cylinder (120 kVp+0.2 mm Cu) in geometry simulating an interventional C‐ arm. Results: The polyenergetic system model was essential to high image quality in KCR reconstructions of large, highly attenuating implants such as knee prostheses and spine instrumentation, where standard penalized‐ likelihood and monoenergetic variants of KCR fail. The first application of KCR‐POLY in real data demonstrates the potential of the algorithm in practice, reducing or eliminating artifacts and restoring image uniformity. Conclusions: The KCR‐POLY algorithm yielded major reduction in metal artifacts, owing both to a priori component knowledge (the implant) and account of the polyenergetic beam, object attenuation, and x‐ray scatter. Ongoing research focuses on improvements to the registration algorithm, scatter, and experimental studies with complex, deformable implants. The work supports application of CT to a range of applications conventionally prohibited by metal implants ‐ e.g. surgical guidance or diagnostic imaging of joints with prostheses. This work was supported in part by NIH 2R01‐CA‐112163
WE‐G‐217BCD‐05: Fiducial Marker‐Based Motion Compensation for the Acquisition of 3D Knee Geometry Under Weight‐Bearing Conditions Using a C‐Arm CT Scanner
Authors: Choi J‐H , Keil A , Maier A , Pal S , McWalter EJ , Fahrig R
Published: June 2012

Purpose: Imaging the knee under realistic load‐bearing conditions can be carried out in a horizontal plane using a C‐arm CT scanner. Human subjects can be scanned in a standing position and acquired data successfully reconstructed. However, reconstructing this data is a challenge due to significant artifacts that are induced due to involuntary motion. Here, we propose motion correction methods in 2D and 3D. Methods: Four volunteers were scanned for 8 seconds while squatting with ∼30 degree flexion. Eight tantalum fiducial markers suitably attached around the knee were used to track motion. The marker position in each projection was semi‐ automatically detected. Each marker's static 3D position, which served as a reference to correct temporal motion, was estimated by triangulating each marker's 2D position from 248 projections using known projection matrices. Motion was corrected in 3 ways: 1) 2D projection shifting based on the mean position of markers, 2) 2D projection warping using approximate thin‐ plate splines, 3) 3D rigid body warping. Results: The original reconstruction was severely motion‐corrupted which made it impossible to distinguish the boundaries of bones. Reconstruction with projection shifting and warping in 2D improved visualization of edges of soft tissue as well as bone. A simple numerical metric of residual bead deviation from static position was reduced from 3.2mm to 0.4mm. The 2D‐based methods are inherently limited in that they cannot fully accommodate different 3D movements at different depths from the X‐ray source. Reconstruction with 3D warping shows clearer edges and less streak artifact than the 2D methods. Conclusions: The proposed three motion correction methods effectively reduced motion‐induced artifacts in the reconstruction and are therefore suitable for weight‐bearing scanning. Future work includes scanning patients in standing position after contrast injection for evaluating the soft tissue structure and constructing 3D finite element models for the estimation of joint cartilage stress. This study was supported by Center for Biomedical Imaging at Stanford, by Siemens AG, Healthcare Sector, and by the Lucas Foundation at Stanford. The concepts and contents proposed here are based on research and are not commercially available.
WE‐G‐217BCD‐09: Calibration of a DRR Algorithm
Authors: Staub D , Sampson A , Williamson J , Murphy M
Published: June 2012

Purpose: To calibrate an algorithm for digitally reconstructed radiographs (DRRs) such that synthetic projections cast through an object in a fan‐beam CT (FBCT) optimally match cone‐beam CT projections of the same object. Methods: Our DRR algorithm models the transmission of primary photons through an object with a ray‐tracing algorithm that samples the CT at small steps along each source‐detector pixel path. Sampled CT values are converted to linear attenuation coefficients (LACs), scaled by the step‐size, and added to a running sum for each detector pixel. We made attenuation measurements of materials of known CT number to calibrate a CT to LAC conversion function for our FBCT and CBCT systems. DRRs are post‐ processed to reproduce the tube output, source fluence distribution, and detector response of the CBCT system. In addition, they are modified to account for scatter, beam hardening, and detector veiling glare. Finally, we determined CBCT geometry parameters using a specially designed phantom. Results: We analyzed the quality of our DRR algorithm by directly comparing synthetic DRRs cast through the FBCT of a Catphan® phantom with actual CBCT projections of the phantom. Line plots comparing the intensity profiles of the DRRs and CBCT projections show that uncorrected DRRs do not align perfectly with the projections. However, when scatter, beam hardening, and veiling glare are modeled there is much closer agreement. Simulations with different geometry phantom shapes and sizes recommend a cylindrical phantom 150 mm in diameter. Computational times under one second per DRR are achieved using a GPU for a simulated 1024×768 pixel detector with a 512×512×281 mm CT volume and 0.5 mm ray step‐size. Conclusions: Our DRR algorithm is able to closely reproduce actual CBCT projections taken through a test object. It is optimized specifically for the FBCT and CBCT systems in our department. Funded in part by NCI grants R01CA123299 and P01CA116602. The authors report no conflicts of interest.
WE‐G‐217A‐06: Spatial Accuracy Quantification of an MR System
Authors: Hwang K , McKinnon G , Lorbiecki J , Maier J
Published: June 2012

Purpose: To develop a phantom and measurement protocol for quantifying spatial accuracy of an MR imaging system over its entire imaging volume. Methods: The measurement protocol is comprised of a phantom, a set of MR sequence parameters for imaging the phantom, and analysis software for calculating spatial errors in the acquired phantom images. The phantom covers the entire imaging volume of the scanner above the patient table. It consists of layers of tooling foam which does not produce any detectable signal on conventional MR images, embedded with a matrix of oil capsules to serve as markers. To account for possible spatial errors in the construction of the phantom, the phantom was imaged with CT to create a gold standard data set. On MR scanners, the phantom is acquired with a 3D FGRE sequence that covers an extended FOV of 61.44 mm and with bandwidth = ±62.5 kHz. Error measurements are performed by detecting markers in the image sets and identifying them based on their known locations on the phantom. The spatial error of a marker is defined as the difference between its locations on the MR and CT image sets. Results: The phantom was constructed and the measurement protocol was executed on two different MR scanners. Some markers were located in areas of severe field inhomogeneity or gradient nonlinearity, and could not be adequately detected for analysis. Maximum errors over concentric spherical regions were observed by plotting the error of each marker as a function of their distance from isocenter. Conclusion: The proposed phantom and protocol can be an effective tool for verifying the spatial accuracy of an MR system, which in turn can improve the accuracy and confidence of MR guided therapies. Data from this protocol may also be used in the development of advanced distortion correction algorithms. Employed by General Electric Healthcare
WE‐G‐217A‐07: Increased MR Spatial Accuracy with Improved Gradient Nonlinearity and Magnet Inhomogeneity Correction
Authors: Hwang K , Maier J , Slavens Z , McKinnon G
Published: June 2012

Purpose: To develop improved distortion correction of MR images based on higher degree spherical harmonic models of the gradient system and the main magnetic field. Methods: The induced field gradient along all three axes can be modeled by first order spherical harmonics. These models provide a true encoding of the physical location of a spin to the frequency at which it is detected. Currently on many commercial systems, only the lower 5 degrees of the model are used for gradient nonlinearity correction. While this provides sufficient accuracy for diagnostic imaging, the gradient nonlinearity correction was extended to include all first order harmonics up to the 9th degree to improve the spatial accuracy of the images. Using zeroth degree spherical harmonics up to the 20th order, a model of the main magnetic field was also incorporated into the correction algorithm. Shifts caused by field inhomogeneity were calculated using knowledge of the receiver bandwidth, frequency encode direction, and the magnetic field at any given point. These corrections were applied to images of a 50 cm diameter phantom, acquired with an extended FOV 3D FGRE sequence. Any improvements in spatial accuracy were measured in the resulting images. Results: Visual improvements in spatial accuracy were observed with both corrections. With standard gradient nonlinearity correction, edges of the phantom were distorted in a wave‐like fashion. With accurate models, almost all of the errors at the edges of the phantom were corrected when both gradient and field homogeneity corrections were applied. Conclusion: With accurate models of the gradient and magnetic field, the two greatest system‐induced spatial errors can be corrected. This improved spatial accuracy enables the use of widebore MR scanners for therapy planning on large FOV images and guidance of percutaneous devices. Further applications include extended FOV imaging for combined PET‐MR systems. All authors are employed by General Electric Healthcare.
WE‐G‐218‐01: Management of RT Patients with Implanted Cardiac Devices: Updated Recommendations?
Authors: Mihailidis D
Published: June 2012

It has been long time since AAPM published TG‐34 on cardiac pacemakers of older technology, which has been the standard document for clinical use, even today, for pacemakers (ICPs) and for defibrillators (ICDs), alike. Management of RT patients with recent technology cardiac devices has been widely published in literature without the provision of a new comprehensive and concise set of recommendations. The various effects of interaction of non‐ionizing and ionizing radiations with those devices are crucial to be studied and accounted for during RT treatment deliveries. Thus, AAPM has formed the new TG‐203 to work on this issue and provide recommendations to the clinical user for management of patients with cardiac devices when receiving RT. It has been numerous postings that we see in medical physics list server groups inquiring advices on dealing with these devices during patient imaging and radiation treatments. As treatment delivery technologies (IMRT, SBRT, dose escalations, proton beams, etc) and ICP/ICD technology advance, the need to address the management of patients with such devices receiving radiation treatment becomes increasingly important. ICDs offer the same functionality as ICPs, but they are also able to deliver a high‐voltage shock to the heart, if needed. Finally, major discrepancies exist among manufacturer recommendations and wide variations exist among radiation therapy facilities regarding patient management precautions.

Learning Objectives:

1. Provide a review on sources of potential malfunctions of modern ICPs and ICDs, including malfunction mechanisms from high‐LET radiations and transient effects attributed to medical imaging procedures for radiotherapy.

2. Provide a review on management of radiotherapy patients with cardiac implanted devices.

3. Utilize recently available data and computation methods of out‐of‐field/peripheral dose by scattered photons and secondary neutrons in order to assess cumulative doses on the ICPs and ICDs, during current treatment deliveries (IMRT, SBRT, proton beam therapy, etc). Risk of failure associated with these doses will be discussed.

4. Provide recommendations for management of radiotherapy patients with implanted cardiac devices including the initial patient evaluation stage, dosimetric evaluation to the ICP/ICD during treatment simulation, treatment planning and treatment delivery. Recommendations for the final evaluation of the integrity and functionality of the device after treatment completion will be assessed.
WE‐G‐218‐03: Management of RT Patients with Implanted Cardiac Devices: From Recommendation to Practical Implementation
Authors: Hurkmans CW
Published: June 2012

Guidelines for the management of patients with an internal cardiac pacemaker (ICP) or internal cardiac defibrillator (ICD) referred for radiotherapy exist but are outdated, as both ICP and ICD technology and radiation therapy techniques have evolved. (ICPs and ICDs are both CIEDs). Furthermore, the clinical practice of patient management varies widely and the number of patient with a ICP or ICD referred for radiation therapy is increasing. There is thus a clear need for updated guidelines, which are currently developed by the new AAPM TG 203.An overview of the literature, existing recommendations and dose measurement methods has been given the previous speakers of this session. In this presentation, the focus will be more on the management aspects. Which questions need to be answered to decide if a patient can receive radiation therapy? How can this be weighted against e.g. CIED relocation or replacement? It is clear that good communication between the patient, the treating cardiologist and radiation therapy department is of importance to come to the best multidisciplinary treatment approach. When radiotherapy is given, the practical measures that need to be taken should preferably be categorised based on the chance on CIED failure combined with the consequence for the patient in case of such failure. Thus, an important distinction may be made between pacing dependent and pacing independent patients. Examples of such risk classifications with corresponding practical implications will be presented, including the new Dutch national guideline recently authorised by the national cardiology, pacemaker technologists, medical physics and radiation oncology societies.

Learning Objectives:

1. Understand and list the questions that need to be answered to tailor the treatment to the individual patient and whom should be involved in this process.

2. Appreciate the reasons why only a crude categorisation of risks can be given.

3. Know how to interpret new CIED guidelines, their strengths and weaknesses and have sufficient knowledge to translate and incorporate these guidelines into your own institution.
TH‐A‐213AB‐03: Inverse Planning for 3D Intensity‐Modulated Grid‐Therapy
Authors: Chen J , Rashid D , Yu E , Fakir H , Karnas S , Jordan K
Published: June 2012

Purpose: To develop an inverse optimization method for 3D intensity‐modulated grid‐therapy to improve dose distribution of grid therapy of advanced stage tumors. Methods: The following process was used to generate a 3D intensity‐modulated grid‐therapy plan. First, based on the geometry of the target volume and organs at risk (OAR), three to six radiation fields were selected to minimize the overlap between the target volume and OARs. Typically, three orthogonal fields were used to minimize the overlap between the fields to maintain grid‐like dose distribution for each field. Second, a step‐and‐shoot IMRT plan was generated with selected fields using Pinnacle treatment planning system (Philips Medical Systems). Third, each MLC segment was converted to a grid field defined by additional MLC segments using an in‐house developed program. Finally, the plan was further optimized using segment weight optimization in Pinnacle treatment planning system, maintaining the grid shape for each IMRT field with optimized intensities for the opening grids. The method was tested for a few clinical cases with bulky tumors. Dose distributions and dose volume histograms were compared between conventional single‐field grid‐therapy plan and 3D intensity‐modulated grid‐therapy plan. Results: Compared to conventional single‐field grid‐therapy plan, 3D intensity‐modulated grid‐therapy plan gives higher minimum dose to the target volume for potential improved tumor control probability and lower space‐fractionated doses to OARs for potential reduction of normal tissue complication probability. The drawback of the method is longer radiation treatment time. Conclusions: A method was developed to generate 3D intensity‐modulated grid‐therapy plan. Comparing to single‐field grid‐therapy plan, it gives higher dose to the target volume and lower space‐fractionated dose to OARs for a potential therapeutic advantage.
TH‐A‐213AB‐05: Robust Plan Optimization for Calypso Gated Hypo‐Fractionated Prostate Treatment Using Volumetric Modulated Arc Therapy
Authors: Zhang P , Hunt M , Happersett L , Cox B , Mageras G
Published: June 2012

Purpose: To develop a robust optimization algorithm which incorporates a Calypso‐guided gating strategy into the optimization process to maximize clinical target volume (CTV) coverage and organs at risk (OAR) sparing in the presence of intra‐fractional motion. Method: In a computer simulation, intra‐fractional motion traces from prior treatments with Calypso were converted to a probability distribution function (PDF), truncated using a patient specific gating window, and renormalized to yield a new PDF with Calypso‐gated interventions. In lieu of a conventional PTV, multiple instances of CTV and OARs were replicated and displaced to extreme positions inside the gating window representing possible delivery scenarios. When optimizing the volumetric modulated arc therapy plan, doses to the CTV and OARs were calculated as a sum of doses to the replicas weighted by the PDF to account for motion. A robust plan meeting the clinical constraints was produced and compared to the counterpart conventional margin (PTV) plan. Calypso traces from a separate testing database served to simulate motion during gated delivery. Dosimetric end points extracted from 4D dose accumulations for each trace were utilized to evaluate potential clinical benefit. Results: Five prostate cases from a hypofractionated protocol (42.5 Gy in 5 fractions) were retrospectively investigated. The patient specific gating window resulted in tight anterior and inferior action levels (∼1mm) to protect rectal wall and bladder wall, and resulted in an average of four beam interruptions per fraction in the simulation. The robust‐optimized plans achieved the same average CTV D95 coverage of 40.5Gy as the PTV‐optimized plans, but with reduced patient‐averaged rectum wall D1cc by 2.2 Gy and bladder wall mean dose by 2.9Gy. Conclusion: Integration of an intra‐fractional motion management strategy into the robust optimization process is feasible and may yield improved OAR sparing compared to the standard margin approach.