bluuebunny/crossref_metadata_2025 huggingface dataset:
WE‐G‐BRCD‐06: Knowledge‐Based Intensity Modulated Radiotherapy (IMRT) Treatment Planning for Prostate Cancer
Authors: Dick D , Das S , Lo J
Published: June 2012

Purpose: To verify that a knowledge‐based approach to intensity modulated radiotherapy (IMRT) treatment planning can create clinically acceptable plans of higher or comparable dosimetric quality than prior clinically approved plans. Methods: Each case in a database of 140 IMRT prostate plans treated to 54 – 74Gy (28–41 fractions) is used as a query case and is compared to the other 139 cases in the database (match cases) using a case‐ similarity algorithm. 2D beam's eye view (BEV) projections of the query and match cases anatomies (planning target volume (PTV), rectum, bladder, right and left femoral heads) at treatment gantry angles are captured. To quantify similarity between the query and match cases, mutual information (MI) values of the query vs. match BEVs are averaged over all treatment gantry angles. The BEV PTV projections of the best match case, identified by the highest average MI value, are deformed to the query's PTV BEVs. The deformation maps are applied to deform the match case's fluences to suit the query case and further tweaked by running 50–100 iterations (without manual intervention) of the Eclipse optimization engine with constraints directly imported from the match case. Results: Approximately 84.3% of the bladder and rectum dose constraints were achieved for the original plans whereas 86% were achieved for the post‐optimized plans. On average, the rectum and bladder volume receiving 65Gy was smaller for the original plans than the post‐optimized plans by (4.52±30.84) % and (3.44±33.55) % respectively. However, the rectum and bladder volume receiving 40Gy was smaller for the post optimized plans than the original plans by (1.53±24.66) % and (3.65±24.44) % respectively. Conclusions: The knowledge‐based approach produces treatment plans of greater or equivalent dosimetric quality to prior clinically approved plans. This work has the potential to semi‐automatically provide high quality plans while dramatically reducing treatment planning time.
WE‐G‐BRCD‐08: Virtual Couch Shift (VCS) by Online Plan Re‐Optimization for the MRI Linear Accelerator
Authors: Bol G , Hissoiny S , Lagendijk J , Raaymakers B
Published: June 2012

Purpose: With the MRI accelerator it will be possible to get continuous patient anatomy updates, ranging from organ deformation to patient translation. To compensate for translations, one can re‐optimize the treatment plan based on the online MRI images. Consequently, the IMRT optimization system should be fast and robust enough to generate daily a clinically acceptable plan to perform this ‘virtual couch shift (VCS)’. Methods: The system uses a GPU based Monte‐Carlo dose engine (GPUMCD) for online beamlet generation in a 1.5 T magnetic field and a fast inverse dose optimization algorithm (FIDO). For four phantom and two clinical cases (cervix and kidney), we generated clinically acceptable plans. The given plans are regenerated after a series of x, y, and z translations (up to 34 mm) of the patient anatomy, without adapting the optimization constraints as used during the initial optimization. The differences between the original plan and the regenerated plans are evaluated by using the gamma criterion and the relative D99 target coverage. Results: The system accurately reproduced the initial dose distribution after translating the phantom and patient anatomies. The gamma criterion of 2%/2 mm is satisfied for 99.2% all target voxels and for 97.2% for all OAR voxels. The relative D99 differences are almost 0.0 with a small standard deviation. With current hardware, a 7 beam cervix beamlet generation and IMRT optimization takes 141 seconds, the kidney case takes only 14 seconds. Conclusions: We developed a system which is fast and accurate enough to perform a VCS by online re‐optimization for the MRI accelerator. Currently we are adding sequencing to the system. We expect that this method can also be used for compensating patient rotations and tissue deformation and with this go towards realtime adaptive treatment planning and delivery.
WE‐G‐BRCD‐09: Assessment of Quantitative Differences Between Different Deformable Image Registration Algorithms for Image Guided Radiotherapy of Head and Neck Cancer Patients
Authors: Fatyga M , Dogan N , Sleeman W , Williamson J , Schutzer M , Song S
Published: June 2012

Purpose: DIR algorithms differ by physical principles employed in their design which in turn determines complexity of the transformation that the algorithm allows. Indiscriminate application of different algorithms without regard for their limitations may lead to significant discrepancies in research results and in clinical procedures. We compare two different algorithms in Head and Neck (H&N) patients to assess what aspects of DIR use are most sensitive to differences between algorithms. Methods: H&N patients are enrolled in a protocol which acquires weekly FBCT and daily double CBCT. Each FBCT study is contoured by the same physician. Two DIR algorithms are compared: Small Deformation Inverse‐Consistent Linear Elastic (SICLE), and the ITK Diffeomorphic Demons (ITKDD) as implemented in the ITK package. Both algorithms employ significantly different physical principles in their design and consequently impose different restrictions on the complexity of transformation they allow. We compare Jacobian Volume Histograms (JVH), Spatial Discrepancy Volume Histograms (SDVH), BED and physical dose accumulation results and resulting plan evaluation indices. Results: Analysis of mean Jacobian shows that both algorithms are able to detect changes in structure volumes, though they differ quantitatively from one another and from the ground truth as established by the analysis of changes in contours. Width of Jacobian distributions is very different indicating that Jacobian should not be used as a measure of volume change at a voxel level without independent validation. Analysis of SDVHs shows that dose lookup points implied by both algorithms are separated by 5mm – 10mm over approximately 30% of most volumes. These differences translate into clearly visible though not very significant differences in BED and dose accumulation. We further observe that physical dose accumulation in external beam H&N patients is a good proxy for direct BED accumulation. Conclusions: Different DIR algorithms may have to be applied selectively in different areas of treatment planning. Acknowledgments: Supported by NIH Grant P01 CA11602
WE‐G‐BRB‐01: Per‐Frame Analysis of MatriXX Data
Authors: Veltchev I , Price R , Ma C
Published: June 2012

Purpose: One of the most widely used IMRT QA devices (MatriXX) is an array of ionization chambers which are periodically read during plan delivery. Although the ionization chambers are not expected to exhibit strong angular dependence, the measured dose distribution is often found to significantly differ from the planned dose distribution. We identify the origin of all factors that affect the measurement accuracy of the MatriXX and develop a per‐frame post‐processing strategy that reduces their impact on the passing rate of IMRT and VMAT plans. Methods: We developed software that reads the dose frame sequence recorded by the MatriXX and applies a number of correction factors to each frame. Angular correction factors are computed as ratios of measured dose at the isocenter of the phantom and planned dose at the same location for all clinically used photon energies. For every clinical case, the recorded movie file is read and the dose for every frame is corrected according to the angle of the beam. In addition, the background evolution is tracked in the ‘beam‐off’ frames which are subsequently subtracted from the ‘beam‐on’ frames according to a predictive model. Machine output correction is also implemented, which significantly improves the absolute dose measurements. The IMRT effective plane of measurement of the MatriXX was identified and found not to coincide with the isocentric plane. Results: The clinical passing rates are significantly improved when the per‐frame analysis software was introduced in our IMRT QA procedure. For a group of 800 patients with no corrections the average passing rate was 93.6%, while for the first 300 cases with per frame corrections the average passing rate was 97.3%. Conclusions: We identify all factors that impact the measurement accuracy of the MatriXX (angular effects, background evolution, machine output, plane of measurement) and propose a strategy for their elimination.
WE‐G‐BRB‐02: MU‐EPID an EPID Based Tool for IMRT Quality Assurance
Authors: Quino L Vazquez , Stathakis S , Gutierrez A , Esquivel C , Alkhatib H , Papanikolaou N
Published: June 2012

Purpose: A software program (MU‐EPID) has been developed to perform patient specific IMRT pre‐treatment QA verification using an electronic portal imaging device. Methods: The software converts measured images of intensity modulated beams delivered to an EPID, into fluence maps that can be imported in the treatment planning system. The dose can then be calculated in the patient anatomy and compared against the patient's treatment plan for QA purposes. We first benchmarked the software using as a patient a cylindrical phantom. An aSi‐1000 EPID mounted on a Varian Novalis linear accelerator was used for the image acquisition. Finally, IMRT plans from different treatment sites were used to further validate this in‐ house software. QA analysis was performed by evaluation of isodose distributions, DVH comparison and 2D gamma analysis. Results: The validation study with the cylindrical phantom showed that the dose to the ion chamber measurement point was in good agreement with both the original treatment plan and the MU‐EPID reconstructed dose. Similar results were found for the clinical cases that we studied. A gamma analysis of the dose to the isocenter plane was performed for each plan. Using 3% and 3 mm as the evaluation criteria, resulted in an average of 97.44% of pixels passing the analysis (gamma<1). Good agreement was also observed for the DVH, isodose and profile comparisons between the clinically delivered IMRT plan and the MU‐EPID derived dose calculation. Conclusions: The results of the present investigation suggest that MU‐EPID can be used in a clinical environment and can be used for patient specific QA for IMRT plans. This work has been supported by the SCOA
WE‐G‐BRB‐03: On the Detectability of Jaw Decalibrations with the DAVID System
Authors: Poppe B , Looe H , Harder D
Published: June 2012

Purpose: To evaluate if the recently described problems with the calibration of jaws at Varian linear accelerators can be detected with the DAVID system. Methods: Recently Varian Clinac field size issues resulted in a widespread medical device correction notice, stating that incorrect jaw positioning due to excessive connector resistances may result in clinically relevant deviations from the intended field size. Since field size checks via the light field, separately for each administered field segment, are not practicable, it might be helpful to know that an independent, permanently active field size surveillance system is routinely used in our department since four years. The DAVID system (PTW‐Freiburg, Germany) is a translucent multiwire transmission ionisation chamber, placed in the linac accessory holder. Each detection wire of the chamber is positioned in theprojection of the midline of the associated MLC leaf pair, resulting in a signal proportional to the leaf pair aperture. High lateral resolution has been achieved by a deconvolution software (Looe et al, PMB 55, 2010, 3981– 3992). MLC decalibrations are detected by comparison between measured and reference signal profiles. Results: The side wings of the DAVID system signal profiles are sensibly influenced by the position of the backup jaw blocks limiting the field size in the direction at right angles to the MLC travel direction. Systematic variation of the backup jaw positions for several field sizes and IMRT plans has revealed high sensitivity of the signal profile side wings with respect to jaw decalibrations. Conclusions: The DAVID system is able to detect, without any delay, suddenly appearing jaw decalibrations down to positioning errors less than 1mm. Use of the DAVID system provides the independent and permanent surveillance of correct field sizes including the position of the backup jaws, making checks of light‐field congruence for each field segment unnecessary.
WE‐G‐BRB‐09: CyberKnife Patient Specific QA Using a 4D Cylindrical Diode Array System
Authors: Koren S , Veltchev I , Fan J , Lin M , Tafo A Guemnie , Ma C
Published: June 2012

Purpose: To examine and facilitate the feasibility of the ArcCheck cylindrical diode array system as a patient specific QA device for CyberKnife radiosurgery delivery. Methods: There is an obvious necessity for CyberKnife robotic radiosurgery patient QA procedures for hypofractionated treatment of larger planned treatment volumes (PTV), e.g. prostate. This need will increase when the future CyberKnife MLC is introduced. The small unflattened CyberKnife fields, along with the variation of beam‐to‐detector spatial angles, pose a significant detection challenge for dosimetric systems. The feasibility of the ArcCheck (Sun Nuclear Inc.) cylindrical diode array system for patient‐specific QA on the CyberKnife is demonstrated using a beam‐to‐diode specific angular correction that was developed and has been applied. For localization and tracking, four gold seed fiducial markers were embedded in the system's central plug. We used a Monte Carlo 1% uncertainty for the dose calculation. Results: By disabling the Linac based corrections and applying the custom CyberKnife correction that we developed, the passing rate increased from 39.6% to 99.8% using a 3%3mm gamma criteria for a given lung case. An additional lung case passed 98.5%. In both cases, a 10% dose threshold was used. In addition, brain, trigeminal nerve and lung cases with synchrony tracking are being investigated. Conclusions: We demonstrated the ArcCheck feasibility for CyberKnife patient specific QA performance. The custom CK angular correction that we developed and applied showed a high passing rate for the lung cases. A verification of the polar angle response should be conducted, in addition to the azimuthal angle that was verified for Linacs. Any data that is being retrieved is additional data to the current chamber point measurement procedures.
WE‐G‐BRA‐04: The Development of a Virtual Reality Dosimetry Training Platform for Physics Training
Authors: Beavis A , Ward J
Published: June 2012

Purpose: Recently there has been a great deal of interest in the application of Simulation methodologies for training. We have previously developed a Virtual Environment for Radiotherapy Training, VERT, which simulates a fully interactive and functional Linac. Patient and plan data can be accessed across a DICOM interface, allowing the treatment process to be simulated. Here we present a newly developed range of Physics equipment, which allows the user to undertake realistic QC processes. Five devices are available: 1) scanning water phantom, 2) ‘solid water’ QC block/ion chamber, 3) light/ radiation field coincidence phantom, 4) laser alignment phantom and 5) water based calibration phantom with reference class and ‘departmental’ ion chamber. Methods: The devices were created to operate realistically and function as expected, each has an associated control screen which provides control and feedback information. The dosimetric devices respond appropriately to the beam qualities available on the Linac. Geometrical characteristics of the Linac, e.g. isocentre integrity, laser calibration and jaw calibrations can have random errors introduced in order to enable the user learn and observe fault conditions. In the calibration module appropriate factors for temperature and pressure must be set to correct for ambient, simulated, room conditions. Results: The dosimetric devices can be used to characterise the Linac beams. Depth doses with Dmax of 15mm/29mm and d10 of 67%/77% respectively for 10cm square 6/15MV beams were measured. The Quality Indices (TPR20/10 ratios) can be measured as 0.668 and 0.761 respectively. Conclusions: At a simple level the tools can be used to demonstrate beam divergence or the effect of the inverse square law; They are also designed to be used to simulate the calibration of a new ion chamber. We have developed a novel set of tools that allow education of Physics processes via simulation training in our virtual environment. Both Authors are Founders and Directors of Vertual Ltd, a spin‐out company that exists to commericalise the results of the research work presented in this abstract.
WE‐G‐BRA‐06: Calibrating an Ionisation Chamber: Gaining Experience Using a Dosimetry ‘flight Simulator’
Authors: Beavis A , Saunderson J , Ward J
Published: June 2012

Purpose: Recently there has been great interest in the use of simulation training, with the view to enhance safety within radiotherapy practice. We have developed a Virtual Environment for Radiotherapy Training (VERT) which facilitates this, including the simulation of a number of ‘Physics practices’. One such process is the calibration of an ionisation chamber for use in Linac photon beams. Methods: The VERT system was used to provide a life sized 3D virtual environment within which we were able to simulate the calibration of a departmental chamber for 6MV and 15 MV beams following the UK 1990 Code of Practice. The characteristics of the beams are fixed parameters in the simulation, whereas default (Absorbed dose to water) correction factors of the chambers are configurable thereby dictating their response in the virtual x‐ray beam. When the simulation is started, a random, realistic temperature and pressure is assigned to the bunker. Measurement and chamber positional errors are assigned to the chambers. A virtual water phantom was placed on the Linac couch and irradiated through the side using a 10 × 10 field. With a chamber at the appropriate depths and irradiated iso‐centrically, the Quality Indices (QI) of the beams were obtained. The two chambers were ‘inter‐compared’, allowing the departmental chamber calibration factor to be calculated from that of the reference chamber. Results: For the virtual 6/15 MV beams, the QI were found to be 0.668/ 0.761 and the inter‐comparison ratios 0.4408/ 0.4402 respectively. The departmental chamber calibration factors were calculated; applying these and appropriate environmental corrections allowed the output of the Linac to be confirmed. Conclusions: We have shown how a virtual training environment can be used to demonstrate practical processes and reinforce learning. The UK CoP was used here, however any relevant protocol could be demonstrated. Two of the authors (Beavis and Ward) are Founders of Vertual Ltd, a spin‐out company created to commercialise the research presented in this abstract.
WE‐G‐213CD‐02: 4D‐PET Maximum Intensity Projections Improve Accuracy of Mobile Tumor Volume Delineation
Authors: Jani S , Lamb J , Dahlbom M , Robinson C , White B , Low D
Published: June 2012

Purpose: To examine the efficacy and accuracy of respiratory‐gated positron emission tomography (4D‐PET) maximum intensity projections (MIPs) in segmenting mobile tumor internal target volumes (ITVs) through a phantom study. Methods: An acrylic phantom was used for PET list‐mode acquisition, consisting of 11C‐filled spheres affixed inside a cylinder containing an 18‐fluorodeoxyglucose solution. The phantom was attached to a robotic arm that underwent 1D motion based on clinically‐derived patient breathing trajectories. An amplitude‐based gating was performed on sequential list‐mode sub‐files of varying signal‐to‐background ratios, and PET‐MIPs were generated from the gated images. ITVs were segmented by first denoising and deblurring images using a custom post‐processing module and then applying an absolute SUV threshold. ITV accuracy was assessed using the volume recovery fraction (VRF) ‐ the ratio of measured ITV to true volume occupied by the moving phantom spheres ‐ which was used to compare PET‐MIP, ungated PET, and static sphere images. In addition, the effects of tumor trajectories, number of gating windows, and margin additions were investigated. Results: VRFs of ITVs generated from PET‐MIPs were consistently higher than those from ungated PET. They also demonstrated a closer agreement to VRFs of static spheres (up to 99% similarity vs. 72% for ungated PET), suggesting that tumor motion had very little effect on the accuracy of PET‐MIP measurements. Trajectories with higher amplitude and baseline drift decreased VRF by up to 14% for both PET‐MIP and ungated PET. Increasing the number of gating windows (up to eight windows) resulted in higher VRFs for PET‐MIPs without producing excessive image noise. PET‐MIPs required a smaller margin addition than ungated PET to achieve a better overlap with the ground truth ITV. Conclusions: Compared to ungated PET, PET‐MIPs are not significantly affected by tumor motion. 4D‐PET imaging is a promising and viable methodology to better delineate mobile tumor volumes.
WE‐G‐213CD‐04: A Triggering System to Guide 4DMRI Image Acquisition
Authors: Hu Y , Caruthers S , Low D , Parikh P , Mutic S
Published: June 2012

Purpose: To develop a practical triggering system based on pre‐selected respiratory amplitudes to guide prospectively the image acquisition of 4DMRI to track abdominal tumor motion for radiotherapy treatment planning. Methods: The proposed triggering scheme consists of a preparation stage and an acquisition stage. Immediately prior to MRI acquisition, the preparation stage monitors the respiration via an external respiratory belt. Based on the respiratory amplitude and status (i.e., inhalation↑ or exhalation↓), the respiration cycle is equally divided into N respiratory states. For example, in the 4‐state case, the 4 respiratory states are 5%↓″, 50%↑, 95%↑ and 50%↓. The 5%↓ and 95%↑ are used instead of 0% and 100% to improve the robustness of the triggering system. Each state is associated with a trigger which starts image acquisition for one and only one slice. A complete 4DMRI imageset requires N dynamic scans. In each dynamic scan, all slices are acquired once and each is in a specific respiratory state. In different dynamic scans, each slice is associated with different respiratory states to form a 4D dataset. For proof‐of‐principle, the triggering system was integrated into a T2‐weighted turbo spin echo sequence (TE=100ms, TR=7122ms) on a clinical 1.5T MRI (Philips Achieva) scanner. The 4‐state case was tested on a healthy subject. Results: Plots of the physiology data recorded and exported from the scanner clearly verified that triggers occurred at the expected locations. Four T2‐weighted images from a representative slice recorded the liver motion corresponding to the 4 respiratory states. Conclusions: Our results confirmed that the newly‐developed system could trigger prospectively to guide 4DMRI image acquisition to achieve T2 weighting, which has a better tumor‐tissue contrast than those offered by previous 4DMRI techniques with T1 or T2/T1 weighting. This is a first report of a pure T2‐weighted 4DMRI to track respiration induced abdominal motion.
WE‐G‐213CD‐06: Implementation of Real‐Time Tumor Tracking Using Robotic Couch
Authors: Buzurovic I , Yu Y , Podder T
Published: June 2012

Purpose: The purpose of this study was to present a novel method for real‐ time tumor tracking using a commercially available robotic treatment couch, and to evaluate tumor tracking accuracy. Methods: Commercially available robotic couches are capable of positioning patients with high level of accuracy; however, currently there is no provision for compensating tumor motion using these systems. Elekta's existing commercial couch (PreciseTM Table) was used without changing its design. To establish the real‐time couch motion for tracking, a novel control system was developed and implemented. The tabletop could be moved in horizontal plane (laterally and longitudinally) using two Maxon‐24V motors with gearbox combination. Vertical motion was obtained using robust 70V‐Rockwell Automation motor. For vertical motor position sensing, we used Model 755A‐Accu‐ Coder encoder. Two Baumer‐ITD_01_4mm shaft encoders were used for the lateral and longitudinal motions of the couch. Motors were connected to the Advance Motion Controls (AMC) amplifiers: for the vertical motion, motor AMC‐20A20‐INV amplifier was used, and two AMC‐Z6A8 amplifiers were applied for the lateral and longitudinal couch motions. The Galil DMC‐4133 controller was connected to standard PC computer using USB port. The system had two independent power supplies: Galil PSR‐12‐ 24‐12A, 24vdc power supply with diodes for controller and 24vdc motors and amplifiers, and Galil‐PS300W72 72vdc power supply for vertical motion. Control algorithms were developed for position and velocity adjustment. Results: The system was tested for real‐time tracking in the range of 50mm in all 3 directions (superior‐inferior, lateral, anterior‐ posterior). Accuracies were 0.15, 0.20, and 0.18mm, respectively. Repeatability of the desired motion was within ± 0.2mm. Conclusions: Experimental results of couch tracking show feasibility of real‐time tumor tracking with high level of accuracy (within sub‐millimeter range). This tracking technique potentially offers a simple and effective method to minimize healthy tissues irradiation.Acknowledgement: Study supported by Elekta,Ltd. Study supported by Elekta, Ltd.
WE‐G‐217BCD‐08: Image Quality Effects of Dynamic Iodine Concentrations for Contrast‐Enhanced Cone‐Beam CT
Authors: Jones B , Altunbas C , Kavanagh B , Miften M
Published: June 2012

Purpose: To study the image effects of the time‐wise dynamic aspect of intravenous contrast agents to enable contrast‐enhanced cone‐beam CT (CE‐ CBCT) localization of liver lesions for stereotactic body radiation therapy (SBRT). Methods: A model was developed to study dynamic IV contrast agents using static phantoms and to derive optimum parameters for CE‐ CBCT imaging. Ten samples containing iodine at 0–5 mg/mL were prepared in cylindrical tubes, corresponding roughly to 0–100 HU as measured by 120 kV helical CT imaging. Each sample was imaged separately in a tissue‐ equivalent phantom, yielding ten datasets (roughly 650 projections each) corresponding to these static CBCT images. To reconstruct images of dynamic contrast concentrations, the CBCT 2D projections were re‐ assembled to match the expected amount of contrast at different points in time. This model was applied to published hepatic contrast enhancement curves, and optimum imaging and contrast injection parameters were derived. Results: A signal‐to‐noise ratio (SNR) decrease of 25%–75% in dynamic CE‐CBCT images from ideal CT of samples with a 20–100 HU difference from water was observed in the un‐optimized scans. This demonstrates the difficulty of CE‐CBCT, and was noticed even in geometries that minimize or eliminate x‐ray scatter, detector glare, and motion. Using our model, we found parameters for iodine injection, CBCT scanning, and injection/scanning timing which optimize contrast enhancement, and a 100% SNR increase with respect to the un‐optimized scans was achieved. Conclusions: The effect of IV contrast is severely degraded in CBCT, and optimization of image and timing parameters is crucial for improved CE‐CBCT imaging for target localization. CBCT has very low temporal resolution, and the pharmacokinetics of IV contrast must be carefully considered in order to apply this technique to localize liver lesions for SBRT. This model will be used to establish the feasibility of CE‐ CBCT for routine localization of liver lesions.
WE‐G‐217BCD‐10: Shading Correction in Image Domain for Cone‐Beam CT Without Prior Information
Authors: Fan Q , Niu T , Zhu L
Published: June 2012

Purpose: Cone‐beam CT (CBCT) images contain severe shading artifacts mostly due to scatter. Many algorithms have been proposed to alleviate this problem by data correction on projections. Sophisticated methods have also been designed to further improve the image quality when prior patient information is available. In this work, we develop a novel algorithm for shading correction directly on CBCT images without any prior information. Methods: In CBCT, projection errors (mostly scatter) result in dominant low‐frequency shading artifacts in image domain. Due to circular scan geometry, these artifacts often show global or local radial patterns. We first convert the uncorrected images into the polar coordinate system. Median filtering and polynomial fitting are applied on the transformed images to accurately estimate the low‐frequency bias field of shading angle‐by‐angle and slice‐by‐slice. The estimated bias field is then converted back to the Cartesian coordinate system, followed by 3D low‐pass filtering to eliminate possible high‐frequency components. The shading‐corrected images are finally obtained as the uncorrected volume divided by the shading bias field. Results: The proposed algorithm has been evaluated on CBCT images of a pelvis patient and a head patient. Within regions of interest, the average CBCT error is reduced from around 250 HU to 42 and 38 HU, and the spatial non‐uniformity error is reduced from above 17.5% to 2.1% and 1.7% for the pelvis and the head patients, respectively. As our method suppresses only low‐frequency shading artifacts, patient anatomy and contrast are well retained in the corrected images of both cases. Conclusions: We propose an effective shading correction algorithm on CBCT images, with several advantages compared to existing approaches. The method has a high efficiency since it is deterministic and directly operates on the reconstructed images. It also requires no prior information, which facilitates its clinical use as a standard image correction solution.
WE‐G‐217BCD‐11: A New Phantom to Study Combined Effects of In‐Plane (x,y) and Z Axis Resolution for 3‐D Imaging
Authors: Goodenough D , Levy J , Kristinsson S , Fredriksson J , Olafsdottir H , Healy A
Published: June 2012

Purpose: The aim of this work is to develop phantoms that can be used to sample the radial and 3D properties of a CT image, including in‐plane (x,y) and z‐axis information. The Phantom is amenable to mathematical analysis of the x, y, and z axis resolution properties separately and combined. Methods: A periodic pattern of a pair of opposed (30°) angled ramps is configured to produce a waveform profile across the CT image. A perfect CT image (with no loss of resolution) of the test object would produce a consistent geometric pattern of the intersection of a line with the pair of angled ramps. However, due to the finite resolution (x, y and z), the CT waveform profile will not yield the perfect profile; rather it will be influenced by slice thickness, and in‐plane resolution (PSF, MTF), as well as noise limitations, and other sources of non‐uniformity such as beam hardening etc. Various characteristics of the waveform profile including, amplitude, frequency, and slope (rate of climb) of the peaks, can be studied using mathematical analysis such as the Fourier transform. It will be shown how these performance characteristics are encoded in the wave pattern. Results: The waveform profiles are visually examined and mathematically analyzed, to demonstrate the effect of Slice Thickness (z axis) and changes of In‐Plane (x,y) Resolution and non‐uniformity across the image field; moreover, the harmonic analysis of the waveform is used to predict, either the in‐plane resolution (MTF), or the z‐axis MTF when one of the two is already known. Conclusions: The Wave pattern phantom offers a way to consider 3‐D imaging characteristics of a CT scanner by scanning a single repetitive test object that encodes both in‐plane resolution and z‐axis resolution and also offers a way to study non‐uniformity effects throughout the CT plane (volume). DJG is a consultant to The Phantom Laboratory and Image OWL, Salem, NY. Funding of other authors is supplied by Image OWL Salem, NY.
WE‐G‐217A‐02: RF Heating of DBS Lead with T/R Head Coil versus T/R Body Coil
Authors: Lin C , Bernstein M
Published: June 2012

Purpose: To evaluate the difference in RF heating of deep brain stimulator (DBS) leads during MRI procedures using head‐only transmitting coil versus whole‐body transmitting coil. Methods: A DBS lead was inserted into the center of a 3‐liter head phantom constructed and filled with gel prepared according the specifications in ATSM standard 2182. The portion of the lead outside of the head phantom was either coiled and fixed to phantom exterior (lead only), similar to that in DBS implanting surgery, or connected through an extension cable to an IPG, which is set to zero mA output and turned off (lead+Ext.+IPG). Imaging was performed on a 1.5T MRI scanner using either a T/R head coil or the built‐in T/R body coil. Two different scans: 3D MPRAGE (low SAR) and 3D TSE (high SAR) were performed. The temperature at the tip of the DBS lead was recorded every 10s using a fiber optical probe. Results: For lead only, the maximum temperature change (peak‐to‐peak) was less than 0.1 C with all scans or coils. For lead+Ext.+IPG, 0.4 C change was observed with T/R head coil and 0.95 C with T/R body coil for 3D MPRAGE (low SAR). However, 18.0 C and 35.3 C changes were observed, respectively, for 3D TSE (high SAR). Conclusions: If the DBS lead is NOT connected to the IPG, no substantial difference in RF heating between the use of a T/R head coil and body coil transmit was observed. The temperature changes were minimal even under high SAR. Such results suggests that DBS MRI may be performed safely with transmitting through body coil and receiving through phased array surface coils provided ONLY the lead is present which is often the case during intra‐operative procedures. Since further study is needed, extreme caution is advised before extrapolating these results to patient exams.
WE‐G‐217A‐03: Respiratory‐Related External/Internal Motion Based MR Image Reconstruction Using Dynamic Keyhole for Real‐Time Tumor Monitoring
Authors: Lee D , Pollock S , Keall P , Kim T
Published: June 2012

Purpose: It is aim to test the hypothesis that a dynamic keyhole MRI reconstruction technique using external/internal respiratory surrogate position reduces acquisition time while retaining image quality for real‐time tumor monitoring, compared to the conventional keyhole technique. Methods: 46 thoracic MRI studies with 13 healthy human subjects have been acquired using a 3T GE MRI. Acquired MR images were reconstructed using zero‐filling, conventional keyhole and the proposed respiratory motion based dynamic keyhole techniques; resultant images were then compared for image quality. Undersampled k‐space rate in the phase encoding direction was determined based on the difference between the original image and the reconstructed image. The position of abdominal muscles and diaphragm were used to determine any excess data that exists in the overlaid temporal data. In addition, the feasibility of the dynamic keyhole method was applied using lung tumor MR images. Results: The result from dynamic keyhole using respiratory motion demonstrated significant improvement compared with the zero‐filling and conventional keyhole methods. Firstly, the dynamic keyhole method using external respiratory motion had an overlaid average of 79.7% (204 lines) of 256 lines from 46 datasets, compared to 63.9% in zero‐filling and 74.3% in conventional keyhole. Secondly, dynamic keyhole method using internal diaphragm motion had an overlaid average of 84.5% (216 lines) of 256 lines from all datasets compared to 67.1% and 77.8% or zero‐filling and conventional keyhole, respectively. Lastly, dynamic keyhole has been validated with one dataset involving lung tumor MR images. Image blurring artifacts and inferior resolution were not present in the final MR images using dynamic keyhole. Conclusions: Dynamic keyhole method using respiratory external/internal surrogate motion has been proposed to reconstruct MR images without image artifacts and with superior resolution. This method is applicable to MR images targeting lungs and other organs affected by respiratory motion for real‐time tumor motion monitoring.
WE‐G‐217A‐08: Routine ACR SNR Measurement Failed to Detect 32‐Channel Head Coil Receiver Malfunction
Authors: Peng Q
Published: June 2012

Purpose: To study if malfunction of a receiver can be detected robustly using the simple ACR SNR measurement approach on a 32‐channel head coil. Methods: Standard ACR T1W images (11slice) were acquired with a commercial 32 channel head coil on a 3T Philips Achieva MR scanner following the ACR recommended setup. Raw data were saved and were used to reconstruct 32 image datasets, each with one coil channel turned off and signal were excluded from reconstruction. Routine simple SNR evaluation method was used to measure SNR for each dataset. Specifically, region of interest (ROI) analysis was performed on slice #7 for each dataset. Signal was the mean value of the pixel intensity measured using an ROI with area of 200 cm2 positioned at the center of phantom. Noise was the standard deviation derived from an ROI positioned in the background in a corner of the image. SNR was then calculated from signal divided by noise. For comparison purposes, we empirically chose 5% SNR drop compared to the full 32 channel dataset SNR as a significant SNR drop that is correlated a potential coil channel defect. Results: Among the 32 image datasets reconstructed each with one receiver turned off, only 4 showed SNR drop of more than 5% or more compared to the reference SNR obtained from the original dataset. Four other datasets had SNR drop between 0.1−5%. The rest (24 image sets) did not show any SNR drop. Therefore, SNR monitoring based on the large ROI approach as the routine ACR QC procedure failed detect receiver malfunction in this coil. Conclusions: More advanced and thorough coil evaluation methods, instead of the routine simple ACR SNR measurement method, have to be applied to evaluate the performance of the phased‐array head coil with 32 or more channels.
TH‐A‐BRCD‐01: Electron Radiotherapy: Past, Present, and Future
Authors: Antolak J , Hogstrom K
Published: June 2012

Purpose: Delivery constraints on monitor units (MUs) are not currently considered during optimization of intensity modulated proton therapy (IMPT) planning in our treatment planning system (TPS). This may result in differences between the delivered and ideally optimized dose distributions. The purpose of this study is to develop a deliverable optimization algorithm incorporating MU constraints (DOMU) in IMPT treatment planning and evaluate its clinical applicability. Methods: A novel objective formula was incorporated with the limited‐memory Broyden‐Fletcher‐Goldfarb‐Shannon (L‐BFGS) algorithm to directly optimize the deliverable IMPT plan with minimum MU constraint (0.005 MUs per spot). Treatment plans for a homogeneous phantom were generated for different spot spacings, using optimization without MU constraints and post‐processing (OMPP) and DOMU. A treatment plan for a head‐neck cancer patient prescribed to 63 Gy was also designed with DOMU using different spot spacings. For all plans, the dose‐volume‐histograms (DVHs) were analyzed to evaluate the benefits of the DOMU. Results: For the regular shaped homogeneous phantom, only the plan generated by OMPP using 5mm spacing showed pronounced dose distortion compared to ideally optimized dose. Plan using 7mm spacing still produced accepted target coverage. For the patient study, plans generated by OMPP using both 5mm and 7mm spacing displayed significant dose distortion and unacceptable target coverage. The dose covering 99% of the target volume was 60.2 Gy and 61.7 Gy for 5mm and 7mm spot spacing plans generated by DOMU, an increase of 18.8 Gy and 18.4 Gy compared with plans generated by OMPP. The parotid sparing of DOMU generated plans in 5 mm spot spacing was significantly better than that in 7 mm spot spacing indicating the importance of using smaller spacing to improve the IMPT quality. Conclusion: The DOMU algorithm, which incorporates the minimum MU constraints into the optimization, can reduce the dos distortion caused by post‐processing process and achieve the optimum delivered dose.
TH‐A‐211‐01: Deformable Registration in the Clinic: From Commissioning to Advanced Applications
Authors: Schreibmann E
Published: June 2012

With deformable registration presented by several companies as a tool to improve treatment accuracy, clinical users often question the advantages and disadvantages of adopting this sophisticated technology to daily practice. This course will detail our experience in using deformable registration in the clinic. It will cover topics such as commissioning a system of deformable registration, patient‐specific QA to identify errors in the displacement field, common applications, anticipated changes in the clinical workflow and subsequent time savings. Emphasis will be placed on explaining the technology's mathematics and applications in layman terms to illustrate how various parameters can be chosen to fit various applications or can produce significant errors if improperly selected.

Learning Objectives:

1. Detail the installation and commissioning

2. Present how parameter selection influence results

3. Show clinical examples and practical conclusions drawn from the daily use of deformable registration.
TH‐A‐213AB‐01: A Novel Method to Accelerate Optimization by Employing Approximate Dose Values
Authors: Spaans J , Malhotra H , Ringland M , Nazareth D
Published: June 2012

Purpose: To introduce a novel method that increases the efficiency of beam and beam let weight and fluence optimization, by employing approximate dose values in the objective function. Methods: A least squares objective function was used, based on clinical dose‐volume objectives. In the initial stage of optimization, the method utilizes dose values determined by the additive dose approximation (ADA) to evaluate the objective function. These dose values are based on pre‐computed dose indices, along with a novel correction scheme. During this stage, no calculation of the true dose is required, resulting in a significant reduction in computation time, since the manipulation of large dose matrices is avoided. The resulting optimized parameters are subsequently used as the starting point in the final optimization step. This method was evaluated on prostate bed, esophageal, and brain patients with varying numbers of treatment fields, by comparing the CPU time required with that of a standard optimization method. The downhill simplex algorithm was used for the evaluations. Results: Nine prostate bed patients with three different beam configurations were used: 4 field 3D‐CRT plans, 16 field 3D‐CRT plans and a 64‐beamlet IMRT‐like plan. These demonstrated an average reduction in optimization time of 33%, 54%, and 46%, respectively. Two 3D‐CRT esophageal patients were evaluated, one with 14 fields and one with 16 fields, resulting in an average improvement of 29%. Finally, a 3D‐CRT 16 field brain patient was studied and exhibited a 57% decrease in CPU time. Conclusions: The proposed method, when used in conjunction with existing optimization algorithms, dramatically decreases the number of standard objective function evaluations and, as a result, computation time. This technique may be applied to any type of optimization algorithm and set of clinical variables requiring pre‐computed dose matrices and an objective function involving dose‐volume objectives (eg. multi‐criteria optimization).
TH‐A‐213AB‐02: Isodose Curve Manipulation for Interactive Dose Shaping
Authors: Kamerling C , Ziegenhein P , Heinrich H , Oelfke U
Published: June 2012

Purpose: The interactive dose shaping (IDS) planning paradigm aims to perform interactive local dose adaptations of an IMRT plan without compromising already established valuable dose features in real‐time. In this work we introduce an interactive isodose curve manipulation tool which enables local modifications of a dose distribution by intuitively dragging an isodose curve. Methods: We developed an in‐house IMRT TPS framework consisting of a 3D GUI for dose manipulation and visualization. Dose calculations are performed with an adapted pencil beam algorithm, enabling ultra‐fast dose calculation for local fluence map changes. Starting point of our planning approach is a homogeneous, conformal dose of the target volume at the prescribed dose level. This initial dose distribution will be interactively modified by our isodose curve manipulation tool, which induces a dose modification as a direct change of dose in one or more voxels. This modification is incrementally obtained by fluence adjustments. These will naturally lead to unintended and unwanted dose features outside the vicinity of modified dose. A subsequent recovery step identifies these voxels and aims to recover their original dose by selecting and modifying an orthogonal set of fluence amplitudes that does not alter the initially achieved dose modification. Results: We evaluated the proposed tool by adapting the dose distribution of a prostate case (9 beams, coplanar). The initial homogeneous, conformal dose distribution led to a severe over dosage of the rectum. By manipulation of isodose curves it was possible to reduce the dose in the rectum to clinically acceptable values. Dose modification for one dragged point on the isodose curve took about 1 second, subsequent recovery of 25 points took 2 seconds. This time includes approximately 300 local fluence updates per beam as well as 300 dose calculations. Conclusions: We proposed, implemented and successfully tested an isodose curve manipulation tool.
TH‐A‐213AB‐06: Investigation and Evaluation of Two‐Point Step‐Size Gradient Methods for IMRT/VMAT Treatment Plan Optimization
Authors: Men C , Han X , Willcut V
Published: June 2012

Purpose: To investigate and evaluate fast two‐point step‐size gradient methods for IMRT/VMAT treatment plan optimization with nonlinear convex models. Methods: Commonly applied optimization models for IMRT/VMAT planning are nonlinear and convex, including many quadratic physical models and general nonlinear biological models. The solutions are typically found through gradient‐based algorithms. Existing methods, however, suffer from the low efficiency of the line search process. In this work, we investigate the use of two‐point step‐size gradient methods for solving general nonlinear convex models in IMRT/VMAT planning. With such algorithms, the line search step can be avoided or highly reduced, and significant speedup can be obtained. As a specified form of nonlinear convex models, the quadratic models are particularly investigated and the best form of the gradient method without line search is found. Results: Five clinical prostate and five clinical head‐and‐neck cancer cases were tested for both IMRT and VMAT planning. For general nonlinear convex models, the Modified Two‐Point Step‐Size Gradient Method was found to have the best efficiency: for all test cases, approximately 5 times speedup was obtained with similar convergence properties compared to the traditional line search method. For quadratic models, the original Barzilai‐Borwein method offered the best performance: for all test cases, approximately 10 times speedup was obtained with better convergence properties (i.e., better treatment quality). For instance, for typical prostate cancer cases with a penalty‐based quadratic model, it takes ∼4 seconds to generate optimized 9‐beam fluence maps compared to ∼40 seconds using traditional line search method on a PC. Conclusions: This work provides a guideline to speedup IMRT/VMAT treatment plan optimization for general nonlinear convex models without loss of treatment quality. The current experiments were all run on CPU. Based on our previous experience, additional 10–30 times speedup can be expected with GPU–based implementation.