bluuebunny/crossref_metadata_2025 huggingface dataset:
WE‐G‐500‐05: Red Spectral Shift of Cherenkov Emission with Applications in Image‐Guided and Intensity‐Modulated Radiation Therapy
Authors: Zlateva Y , El Naqa I , Quitoriano N
Published: July 2013

Purpose: This work aims to validate the potential application of Cherenkov emission (CE) in radiotherapy by a spectral shift to the optical window of biological tissue in order to increase CE detection during radiotherapy. Methods: 18 MeV and 18 MV clinical electron and photon beams are used for the experiments. The CE detector consists of a multi‐mode fiber optic cable (numerical aperture = 0.22), positioned out of the beam and connected to a spectrometer incorporating a front‐illuminated CCD array. In order to evaluate the dose versus CE correlation, depth and profile scans were acquired at the angle of maximum emission. A Monte Carlo CE simulator, designed with the Geant4 simulation toolkit, was used to validate the correlation. A spectral shift was achieved with CdSe/ZnS core/shell nanoparticles (NPs) emitting at 650 nm. Measurements were acquired with a water tank, in order to test the signal's capacity to stimulate NP photoluminescence, and at varying depths in a tissue‐simulating phantom composed of water, Intralipid and bovine blood. Results: A strong correlation between dose and CE is evident with a Spearman correlation coefficient of 0.99 or higher for simulated data. Water tank results confirm that CE by radiotherapy beams sufficiently stimulates NP photoluminescence. A considerable signal increase was observed near 650 nm with NPs placed at depths up to 1 cm in the tissue‐simulating phantom. Conclusion: These results indicate that development of spectral shifting techniques that enhance tissue transmission and detection of CE during radiotherapy will be beneficial for online tumor imaging and localization, since CE is intrinsic to the beam and non‐ionizing, and for intensity modulation based on tumor microenvironment information, such as oxygenation, contained within the spectral distribution. This setup and methodology will be used to investigate different beam qualities and wavelength‐shifting schemes, and fine‐tune the phantom optical properties. NSERC ‐ Discovery Grant
WE‐G‐500‐07: A Novel Method for Dosimetry Calculation Utilizing PET‐CT in Patients Treated with Radioembolization
Authors: Fourkal E , Veltchev I , Lin M , Johnson M , Ma C , Doss M , Yu J , Meyer J
Published: July 2013

Purpose: Radioembolization is an emerging therapeutic option for the treatment of metastatic and primary tumors of the liver. Simple formulas are currently utilized in order to prescribe the appropriate activity. While the activity infused to each patient is known, limited data is available concerning the delivered dose. This makes correlation of response to dose impossible. The aim of this study is to establish a method for calculation of dose delivered to intrahepatic targets. Methods: Six patients were enrolled in this study as part of one approved clinical trial. All patients underwent radioembolization for primary or metastatic liver tumor(s). Each patient underwent a post‐treatment PET‐CT for the quantification of activity of Yttrium‐90 labeled microspheres. The absorbed dose is calculated as the convolution of the pre‐calculated dose kernel with the PET‐measured activity. Results: Four patients underwent treatment with SIR‐Spheres for liver metastases from colorectal cancer and 2 patients were treated with Therasphere for hepatocellular cancer. A total of 11 target tumors were contoured on post‐treatment PET‐CT scans for dosimetric evaluation. Mean prescription activity was 1.51 GBq (range: 0.58 to 3.29 GBq). The resulting mean maximum measured dose to targets was 167 Gy (range: 71 to 311 Gy). Mean minimum dose to 70% of target (D70) was 54 Gy (range: 29 to 83 Gy). Mean minimum dose to 90% of target (D90) was 36 Gy (range: 13 to 58 Gy). The mean maximum dose for Therasphere and SIR‐spheres was 266 Gy versus 111 Gy. Conclusion: This pilot project demonstrates that dose can be calculated in patients after radioembolization, utilizing PET‐CT measured activity. Although this represents preliminary work with a small number of patients, the doses to patients treated with Therasphere may be larger. This process could lay the foundation for more sophisticated dose prescription methods in the future.
WE‐G‐WAB‐02: BEST IN PHYSICS (JOINT IMAGING‐THERAPY)‐MR and Linac Magnetic Field Mutual Decoupling for An MRI‐Guided Radiation Therapy System
Authors: Stanescu T , Tadic T , Marle J , Winter J , Petropoulos L , Sweitzer M , Jaffray D
Published: July 2013

Purpose: An MR‐guided RT system, including a 6X TrueBeam Varian linac and a 1.5T IMRIS MRI scanner‐on‐rails, is under installation at our institution. Numerical simulations and experiments were performed to quantify and test the magnetic field mutual decoupling between the MR and linac/couch ensemble. Methods: Linear programming and finite element methods were used to perform numerical simulations for the MR‐linac/couch system. First, the 1.5 T MR field was reconstructed and matched to data from manufacturer. The next step was to perform magnetic force simulations and measurements for a simplified assembly, replicating all key ferromagnetic sub‐structures of the couch (wrists, breaks, etc.). To guide the experiment, CAD data of the couch/linac (Varian) was used for various simulation scenarios, i.e. MR distance‐to‐couch/linac and gantry rotation. Measurements were performed for a) magnetic forces acting on various couch components, b)magnetic fringe field mapping (gauss probe) in the vicinity of linac and couch (e.g. wrists, breaks), c)MR imaging field homogeneity. The final set of simulations was for the entire MR‐linac vault, including ferromagnetic structural components obtained from architectural drawings of the facility (e.g. doors, beams, r‐bar). Results: The simulated MR fringe field was in good agreement with the measurements (within 1 gauss). The magnetic pull forces on the couch components were computed for arbitrary directions to match the experimental setups. For all scenarios the simulated and measured data was in good agreement. The couch operated optimally in the MR field. The fringe field at the linac head was less than 20 gauss. For all intended imaging locations, the MR full shimming was achieved, compensating for the presence of the linac/couch. Conclusion: The MR, linac, and couch ensemble was modeled via numerical simulations. The independent and optimal operation of all components was experimentally validated. The MR‐linac system can be integrated and operate with no special modifications. J. Winter and L. Petropoulos are employees of IMRIS. J. Marle and M. Sweitzer are employees of Varian Medical Systems. IMRIS and Varian are companies co‐developing the MRgRT system. Funding for the MRgRT project at Princess Margaret Hospital was provided by the Canadian Foundation for Innovation.
WE‐G‐WAB‐03: 4D Composite MR Image Distortion Field: Quantification and Applications for MRI‐Guided Radiotherapy
Authors: Stanescu T , Tadic T , Jaffray D
Published: July 2013

Purpose: To study the interplay effects of MR image distortion fields for moving/deformable anatomy and their implications for MR‐guided radiotherapy. Methods: The composite distortion field is given by the combination of two independent components a) the system‐related distortions due to B0 inhomogeneities and imaging gradients nonlinearities and b) the tissue magnetic susceptibility‐induced effects. Field a) is static while b) is dynamic, modifying as a function of tissue interface characteristics (shape, location, type of neighboring tissues). The field experienced by a moving or deformable (over time) target/organ can be very complex as entity changes location and shape. Field a) was measured with a linearity object in conjunction with a harmonic analysis. Field b) was computed numerically using a finite difference method. Both fields were quantified for moving/deformable anatomy using multiple image data bins derived from 4D CT and 4D MRI for several lung and liver cases. Results: The 4D composite field tools and methodology was validated in phantoms. The independent and combined distortion fields were quantified for several lung and liver patients. For a lung case with a target travelling 2.4 cm in the superior‐inferior direction, the susceptibility distortions for the inhale and exhale phases were 3.84 and 2.5 ppm, respectively. The system distortion difference for the two phases was about 1 mm. The maximum 4D composite distortion between the inhale and exhale was approximately 1.5 mm. For liver, the target experienced negligible distortions (embedded in soft‐tissue) and the diaphragm showed a local distortion larger than 1 mm. Conclusion: The combined effect due to various types of MR image distortions were quantified for organ motion/deformation. Geometric distortion introduced by the 4D composite field may be significant for certain scenarios. However, this can be mitigated by a careful selection of imaging and by implementing robust methods for image correction.
WE‐G‐141‐02: Impact of Imaging Quality On Interfraction Patient Setup Errors and Autocorrelation in Image Guided Head‐And‐Neck Radiotherapy
Authors: Qi S , Hu A
Published: July 2013

Purpose: To assess the impact of imaging quality of three commonly used on‐board image‐guidance systems on interfraction patient setup errors for head‐and‐neck (H&N) cancer irradiation. Serial correlation patterns and trend of the setup errors over the treatment course were investigated via time‐series analysis. Methods: 117 H&N patients imaged with different IGRT modalities: 29 kilovoltage cone beam CT (KVCBCT, Synergy, Elekta Inc.), 35 megavoltage fan beam CT (MVFBCT, TomoTherapy, Accuray Inc.), and 53 megavoltage cone beam CT (MVCBCT, MVision, Siemens Inc.) were studied retrospectively. The daily displacements in mediolateral (ML), craniocaudal (CC) and anteroposterior (AP) dimensions were investigated. The CTV‐to‐PTV margins were calculated using 1.96σp, where σp was random error for the pooled standard deviations in each cohort. Time series analysis was performed to further investigate possible autocorrelations and time trend of the setup errors serially for each patient during the course of the treatment. Results: Based on a total of 3302 pretreatment CT scans, including 632 KVCBCT, 974 MVFBCT and 1696 MVCBCT scans, the maximal random errors in three translational directions for H&N cases were 1.6 (KVCBCT), 1.7 (MVFBCT) and 2.1 mm (MVCBCT), the calculated CTV‐to‐PTV margins that would encompass 95% of the cohort were 3.1, 3.3 and 4.2 mm respectively. Approximately 24%, 29% and 52% of the shifts were auto correlated in one of the three translational directions with KVCBCT, MVFBCT and MVCBCT, approximately half of the autocorrelations were found in AP direction. Conclusion: CTV‐to‐PTV margin for H&N IMRT may be a function of the imaging modality if determined based on IGRT data. Superior image quality in KVCBCT, compared to MVFBCT and MVCBCT, results in smaller random errors and smaller CTV‐to‐PTV margins. Coincidentally, modality with superior image quality also resulted in less autocorrelation serially in setup errors. IGRT modalities with better image quality are encouraged in clinical practice.
WE‐G‐141‐04: Use of a Dynamic KV X‐Ray Collimator for Reduced‐Dose Fluoroscopic Fiducial Tracking
Authors: Pearson E , Belcher A , Grelewicz Z , Wiersma R , Pelizzari C
Published: July 2013

Purpose: Intra‐fraction organ motion can be problematic for highly conformal radiation therapy techniques. Fluoroscopy with the on‐board imager available on many linear accelerators can provide real time tumor position information however potentially high radiation dose can be a concern. The target may only account for a few percent of the image area, thus using a dynamic collimation device to restrict the field of view to an appropriate region‐of‐interest and follow its motions the imaging dose can be substantially reduced. Methods: A prototype collimator has been developed which can be mounted to the kV x‐ray source of a Varian Trilogy linear accelerator in place of the standard bowtie filter. The collimator tracking was tested using realistic motion profiles derived from patient data and a 3‐axis motion stage with a CT spot as the target fiducial. The clinical system does not currently provide real time access to the image data so the collimator was fed the fiducial position from the motion control system with controllable lag to simulate image‐processing time. Results: In the acquired fluoroscopic data the collimator blade edges were identified using a hough line transform on the thresholded gradient magnitude image and the fiducial position was identified with a template matching technique. Over four minutes of continuous tracking with intermittent burst of fluoroscopic imaging the fiducial was never more than 1.7mm from the aperture center and the root mean square distance was 0.8mm. Conclusion: The dynamic collimator exhibits accuracy that could enable highly conformal fluoroscopic imaging to provide real‐time target position information with greatly reduced imaging dose. This work was funded, in part, by Varian Medical Systems, Palo Alto CA. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of any supporting organizations.
WE‐G‐141‐05: The Image Quality of Ion Computed Tomography at Clinical Imaging Dose Levels
Authors: Hansen DC , Sorensen TS , Seco J
Published: July 2013

Purpose: To investigate the resolution and image quality that can be obtained from ion computed tomography (CT) at clinical dose levels (∼10mSv) and number of scanning angles. Methods: A 20cm cylindrial phantom was designed in the Monte Carlo code Geant4 with the explicit purpose of obtaining the edge‐spread function (ESF) and contrast levels. An ion‐CT scan of the phantom was simulated for 230MeV, 330MeV protons and 430MeV/u carbon‐ions. These were chosen as the represent the limits of currently available technology. Each simulation was repeated for a varying total number of scanning angles, ranging from 20–360. The target dose was set to 10mSv, similar to what is given in a standard x‐ray CT scan. The conversion from physical to biological dose was done following the publications of ICRP. The CT images were reconstructed using cubic spline curves for the proton trajectories. In addition to the ion‐CT images, the 230MeV proton images were also reconstructed using prior knowledge obtained from a simulated CT of the phantom.Resolution was defined as the σ of the Gaussian ESF. Results: In general the combined proton and x‐ray scans yielded the highest resolution (0.4mm) followed by 330MeV protons (0.8mm) and 230 MeV protons (1.0mm), with carbon ions giving the lowest resolution (1.1mm).When compared with the ground truth, the combined x‐ray and proton image yielded the best Result, followed by 230MeV protons, 330MeV protons and carbon ions yielding the worst Result. Conclusion: Both 230MeV and 330MeV protons have distinct advantages for ion‐CT, and for clinical scans the energy should be decided on a case by case basis. Carbon‐ions, while in theory superior, cannot be recommended for patient scans, due to the high dose level. The addition x‐ray CT prior knowledge into the reconstruction of ion‐CT yields a better image than either ion‐CT or x‐ray CT alone. This work was supported by the Danish Center for Interventional Research in Radiation Oncology (CIRRO).
TH‐A‐141‐11: Standardization of DQE(f) Measurements at MV Energies
Authors: Abel E , Meyer A , Sun M , Sawkey D , Fahrig R , Morf D , Star‐Lack J
Published: July 2013

Purpose: The detective quantum efficiency (DQE(f)) is a critical metric to evaluate detector performance. For diagnostic imagers operating at keV energies, there are clear standards from the International Electrotechnical Commission (IEC) using patient equivalent filters to appropriately harden the beam. Such standards do not exist for MeV imaging. Through a systematic investigation comparing simulated and measured DQE(f) we have demonstrated both the need and plausibility for such a standard at MV energies. Methods: The DQE(f) of a Varian AS1000 EPID imaged at 6 MeV by a Varian TrueBeam linac was both measured and simulated. The spectra necessary were extracted from the TrueBeam phase space files available from MyVarian.com website, then hardened through 20 cm of water. From this an equivalent thickness of aluminum was calculated, and the DQE(f) simulated and measured. The Geant4 simulation replicated the Varian AS1000 portal imager geometry, with optical blur in the Gd2O2S scintillator modeled using isotropic scattering. For the measurement of DQE, a narrow, angled slit was formed using two parallel tungsten blocks placed immediately before the imager from which the Fujita over‐sampled MTF was calculated. The NPS was generated from an ensemble of gain and offset corrected flood fields per the IEC guidelines. Results: Simulation indicates a 40% attenuation of the un‐flattened 6 MeV spectrum by 20 cm of water, the equivalent aluminum thickness of which is 85 mm. Simulation and measurement show, with 10% agreement, as much as a 70% decrease in DQE(f) for the hardened beamConclusion: We have demonstrated, through simulation and experiment, as much as a 70% reduction in DQE for a 6 MeV un‐flattened beam through 20 cm of water. We believe it plausible, therefore, to establish a measurement standard similar to that used for keV imaging to fairly evaluate imagers under realistic imaging conditions. This research was funded in part by Varian Medical Systems, Inc. and by NIH grant R01 CA138426
TH‐A‐103‐01: Investigation of Factors Affecting the Accuracy of a Dual Energy Quantitative CT Method for Estimating Regional Bone Marrow Cellularity
Authors: Shenoy A , Goodsitt M , Shen J , Schipper M , Dewaraja Y
Published: July 2013

Purpose: To evaluate a 3‐equation 3‐unknown Dual Energy Quantitative CT technique for determining region specific variations in bone marrow cellularity for improved red marrow dose estimation in radionuclide therapy. Methods: The DEQCT method was applied to 80/140kVp images of patient simulating lumbar phantoms of three different sizes (small, medium, large). External calibration rods of bone, marrow and fat simulating material were placed beneath the phantoms. Similar internal calibration inserts were placed at a vertebral location within the phantoms. Six test inserts of known volume fractions of bone, fat and red marrow (Red marrow range:0.15–0.74) were also scanned. External to internal calibration correction factors were derived. The effects of phantom size, slice thickness, dose, spongiosa region segmentation(1,4(2×2),9(3×3)) and calibration method on the accuracy of the calculated red marrow volume fractions (cellularity) were evaluated. The method was also applied to six patients. Results: For both the small and medium sized phantoms using internal calibration, RMS errors(RMSEs) in the estimated volume fractions of red marrow were 0.03, 0.04 and 0.05 for partitioning of a 23.4mmx23.4mm ROI into 1 region, 4 regions and 9 regions, respectively. Corresponding results for a large sized body phantom were significantly poorer (0.06,0.09,0.10). Reducing slice thickness to 2.5mm(vs. 5mm) increased RMSEs of the marrow volume fractions by 0.01 to 0.06. Similar increases in RMSEs were obtained for reduction in dose(mAs). Results for external calibrations exhibited large errors that were reduced to those of internal calibration by application of external to internal correction factors. Application of these factors to the 6 patient lumbar vertebrae yielded cellularity values that differed by 29% on average from biopsy values at the iliac crest, potentially due to regional variability. Conclusion: Phantom studies were promising, but show poorer results for large bodies, thinner slices, lower doses and smaller sub‐regions. Future validation studies in human cadaver vertebrae are needed. National Institutes of Health (NIH) under Grant 2RO1 EB001994
TH‐A‐103‐02: Initial Accuracy Assessment of a Post‐Processing DECT Method for Linear Attenuation Coefficient Estimation On a Commercial Fan Beam CT Scanner
Authors: Evans J , Whiting B , O' Sullivan J , Politte D , Klahr P , Yu Y , Williamson J
Published: July 2013

Purpose: A post‐processing dual‐energy CT (pDECT) method for non‐invasive estimation of photon cross‐section information has been experimentally implemented on a commercial CT scanner and an initial accuracy assessment performed in an idealized phantom geometry. Methods: Eight test materials of known composition were used to compare pDECT‐estimated linear attenuation coefficients to NIST reference values over the energy range of 10‐1,000keV. CT sinogram data was acquired on a commercial fan beam CT scanner at 90 and 140kVp. Improvement from increased spectral separation was also investigated by additional filtration of the high‐energy beam with 0.5mm of tin. pDECT accuracy was compared using images reconstructed with a conventional filtered backprojection (FBP) algorithm and an in‐house statistical iterative reconstruction algorithm, Alternating Minimization (AM), since AM has been shown to reconstruct images with less random and systematic error than FBP. Results: Mean accuracy of pDECT‐estimated linear attenuation coefficients, assessed by averaging pixels (N>149) within the test material, were found to be within 1% of reference for energies between 30‐1,000 keV, with errors rising to 3––6% at 10 keV. However, the sensitivity of the pDECT process to reconstructed image uncertainties leads to large pixel‐to‐pixel variations of estimated attenuation coefficients; in some cases a coefficient of variation on the order of 20% is seen. The AM algorithm noise advantage leads to attenuation coefficient standard deviations roughly 40% to 60% less than FBP. Measurements using increased spectral separation returned similar mean accuracy and slightly less pixel‐to‐pixel variation even when scanning with 25% of the unfiltered dose. Conclusion: The advantage of the AM algorithm over FBP in this idealized pDECT scenario is less random uncertainty. Increased spectral separation better conditions the pDECT problem. Future work intends to investigate pDECT accuracy in more complex phantom geometries where AM reconstruction may confer more advantage due to its more physically accurate forward model. This work was supported in part from grants (R01 CA 75371 and R01 CA149305, J. Williamson, Principal Investigator) awarded by the National Institutes of Health and a grant funded by Varian Medical Systems.
TH‐A‐103‐03: Cascaded Systems Analysis of a Silicon‐Strip Photon Counting CT System
Authors: Xu J , Zbijewski W , Gang G , Fredenberg E , Taguchi K , Carrino J , Stayman J , Siewerdsen J
Published: July 2013

Purpose: To develop an analytical cascaded systems analysis (CSA) model for investigating the limits of signal and noise performance [noise‐power spectrum (NPS), modulation transfer function (MTF), and detective quantum efficiency (DQE)] of photon‐counting detectors (PCDs). Methods: Previously developed models for signal and noise propagation in flat‐panel detectors were extended to include physical effects in the imaging chain specific to the performance of PCDs, including the effect of Compton scatter in the detector, time and energy dependent effects of pulse pileup, and signal threshold. A CT imaging bench incorporating a Si‐strip PCD was constructed to test and validate the theoretical model and provide a plaform for evaluation of image quality in photon counting CT. The linearity of detector response was measured across a range of techniques (80 kVp, 0.03– 0.15 mAs), and a phantom consisting of simulated bone and soft tissue was used to evaluate the contrast to noise ratio (CNR) in CT reconstructions. The model was used to investigate limits in DQE as a function of imaging conditions and detector characteristics. Results: The signal response of the imaging system was highly linear with exposure (mR) across the operating range of the x‐ray source as predicted by the model, suggesting a lack of pulse pileup effects under conditions of interest. The soft‐tissue CNR ranged from 1.3 (at 2 mA) to 2.7 (at 10 mA), suggesting quantum‐limited performance and the potential advantage of zero electronic noise for low‐dose imaging. The model predicts zero‐frequency DQE ∼0.2–0.6 over the range 40–90 kVp and demonstrates an optimal range of signal threshold above the electronic noise but below the level of true counts. Conclusion: CSA modeling can be extended to include characteristics unique to PCDs and provide a basis for understanding the limits of imaging performance in photon counting CT. National Institutes of Health Grant No. 2R01‐CA‐112163‐02. Some authors affiliated with Phillips Healthcare.
TH‐A‐103‐04: Microcalcification and Mass Detection with a Photon‐Counting Si Detector for Low Dose Breast Computed Tomography
Authors: Ding H , Cho H , Masaki F , Barber W , Iwanczyk J , Molloi S
Published: July 2013

Purpose: To investigate the feasibility of microcalcification and soft tissue lesion detection using an energy‐resolved photon‐counting detector based on Si strips for breast spectral computed tomography (CT). Methods: Simulations and experiments were performed with a prototype spectral CT system equipped with a Si‐based photon‐counting detector with energy discrimination capability and 0.1 mm pixel pitch. Simulations were performed according to the realistic experimental setup. The attenuation coefficient was computed from the linear combination of the individual linear attenuation coefficient of compositional elements. The detector was simulated with intrinsic pixel pitch along with 4×4 pixel binning for microcalcification and soft tissue lesion detection, respectively. In the experiment, two cylindrical phantoms of 13 mm in diameter were constructed from polymethyl methacrylate (PMMA) and polyethylene (PE) plastic for microcalcification and soft tissue lesion detection, respectively. Five cylindrical holes were bored onto the PMMA phantom and filled with resin inserts, which contained five groups of calcium hydroxyapatite (HA) granules, ranging in sizes from 0.090 to 0.210 mm. Three holes of diameters of 1, 2, and 3 mm were bored into the PE phantom and filled with water, which represents the contrast between soft tissue lesion and adipose tissue background. The phantoms were imaged at 65 kVp with an Entrance Skin Air Kerma (ESAK) of 3 mGy. Images were also reconstructed at reduced dose levels by under sampling the full sinogram. Results: The simulated phantom images agree well with those from the experiment. microcalcifications of approximately 0.1 mm can be observed with the investigated prototype spectral CT system at ESAK of 1 mGy. The contrast‐to‐noise ratio of water in PE background was improved with 4×4 binning. Conclusion: The proposed Si strip detector is expected to offer superior image quality with the capability to detect microcalcifications and masses for low dose breast imaging.
TH‐A‐103‐06: Radiation Dose Reduction with Optimal Energy Weighting for a Prototype Energy‐Resolved Photon‐Counting Si Strip Detector
Authors: Ng A Lam , Ding H , Cho H , Barber W , Iwanczyk J , Molloi S
Published: July 2013

Purpose: To develop simulation and experimental methods for optimal energy weighting to reduce dose and enhance contrast for the detection of microcalcification, soft‐tissue lesions and iodine‐contrast objects using a prototype energy‐resolved photon‐counting Si strip CT detector. Methods: A prototype energy‐resolved photon‐counting Si strip CT detector was investigated for optimal energy weighting. The Si strip detector has 256 pixels with 100 μm pitch. It was placed in an edge‐on geometry which results in an effective attenuation length of 0.6 mm. A tungsten anode X‐ray spectrum of 65 kVp with 2 mm Al filter was used. Glandular and Adipose equivalent phantoms were constructed using PMMA and polyethylene. Water, calcium hydroxyapatite and iodine with different concentrations were used to simulate soft tissue lesion, microcalcification and contrast material in breast tissue. A simulation package was generated to reconstruct images from charge‐integrated and energy‐resolving photon‐counting detectors. Energy weighting factors based on projection and image based methods were used in the four energy bins to enhance contrast. Different energy thresholds were investigated based on iodine K‐edge, equal counts per bin, and equally spaced bins. CNR was computed to compare image quality between methods. Results: Simulation results indicate that using an optimized energy weighting image reconstruction improves CNR for water (30%–40%), for hydroxyapatite (30% –50%), and for iodine (35%–40%) in comparison of those images from charge‐integrated reconstruction. Conclusion: The implementation of energy weighting methods in image reconstruction for the prototype energy‐resolved photon‐counting Si strip CT detector can improve CNR or reduce radiation dose. This technique can potentially be used for detection of microcalcification, soft‐tissue lesions and iodine‐contrast object in a spectral breast CT system.
TH‐A‐103‐11: Breast Density Measurement with Cone‐Beam CT and MRI: A Postmortem Study
Authors: Ding H , Johnson T , Lin M , Su L , Molloi S
Published: July 2013

Purpose: To investigate the feasibility of breast density quantification using a cone‐beam computed tomography (CBCT) and a magnetic resonance imaging (MRI) system with a postmortem study. Methods: Twenty pairs of postmortem breasts were imaged with a bench‐top CBCT system based on a CsI flat panel detector and an Aurora 1.5T dedicated Breast MRI system. The CT images were acquired at 50 kVp with a mean glandular dose (MGD) of approximately 3 mGy. A T1‐weighted gradient echo technique was used in MRI scans to maximize the contrast between the adipose and glandular tissues. Image processing procedures were implemented to segment the glandular tissue from adipose tissue. For CBCT images, standard histogram thresholdings based on visual assessment and automated segmentation based on a basic fuzzy c‐means (FCM) algorithm were used. For MRI images, FCM‐based clustering and the coherent local intensity clustering (CLIC) method, which estimated and removed the bias field, were used. Finally, the breast densities measured with the two systems were compared to the definitive tissue compositions obtained from chemical analysis. The linear correlation coefficient, Pearson r, was used to evaluate the two imaging modalities and the investigated image segmentation methods. Results: Both histogram thresholdings and FCM techniques used for CBCT images show excellent correlations with respect to the fibroglandular ratio (FGR) from chemical analysis, with r values of 0.964 and 0.980, respectively. The correlations of the MRI measurement were slightly worse than those from CBCT. The CLIC method successfully removed the intensity inhomogeneity induced by the bias field, and improved the r value from 0.864 to 0.915. Conclusion: CBCT can be used to accurately quantify the breast density with automated clustering algorithms. It is anticipated that CBCT may outperform breast MRI in breast density quantification due to high spatial resolution and the absence of the bias field.
TH‐A‐116‐01: A Statistical Approach to Quantification and Visualization of Setup and Range Uncertainties for Proton Plan Verification
Authors: Park P , Cheung J , Zhu X , Liu W , Sahoo N , Court L , Mohan R , Li H , Dong L
Published: July 2013

Purpose: Proton therapy is more sensitive to both setup and beam range uncertainties than the conventional photon therapy due to the finite range of protons. For the purpose of evaluation of plan robustness in the face of such uncertainties, we developed a method to quantify and visualize the uncertainties in proton plans using comprehensive statistical analysis. The application of the proposed method is shown for selected patients in various clinical sites. Methods: Clinically approved 20 lung, 10 prostate, and a brain cases were retrospectively analyzed. For a selected volume of interest, probability density function (PDF) describing variation in point dose value for every tissue element under setup and range uncertainties was computed based on over 600 dose calculations per plan. The expected dose‐volume histogram (E‐DVH) was calculated by weighting the probability of each tissue element receiving a specified dose level. Similarly, standard deviation of the E‐DVH was calculated (SD‐DVH). Furthermore for each volume element, their probability of receiving greater dose than the dose constraint was visualized as color‐wash, mapped onto the patients CT images for spatial visualization and assessment. Results: The E‐DVH value of target coverage under the uncertainties was consistently lower than that of the nominal value, with a mean difference of−1.1% (−0.9% for breath‐hold), −0.3%, and −2.2% for lung, prostate, and a brain cases respectively. The organs that were most sensitive to the uncertainties were esophagus and spinal cord for lung, rectum for prostate, and brain stem for brain cancer. Conclusion: A clinically feasibly plan robustness analysis tool based on statistical simulation has been developed for proton therapy plans. The passively scattered beam lung plans and scanning beam prostate plans appears to be robust in terms of target coverage. However, structures that are small in volume or located near the target area showed greater sensitivity to uncertainties. This project is supported by grant P01CA021239 from the National Cancer Institute.
TH‐A‐116‐05: A Robust Direct Aperture Optimization Approach for Left‐Sided Breast IMRT
Authors: Mahmoudzadeh H , Chan T , Purdie T
Published: July 2013

Purpose: To test the feasibility of a robust direct aperture optimization (DAO) model for cardiac sparing IMRT planning of left‐sided breast cancer patients. Methods: We extended a previously developed robust optimization method to obtain a DAO approach for tangential breast IMRT that is robust against uncertainty in the patient's breathing pattern. The robust DAO model contains additional constraints on the segment shapes and weights in order to reduce the complexity of delivery and ensure that high weight segments cover the whole breast volume. A 4DCT dataset was used to test the model, and 100 simulated breathing patterns were generated. Results were compared with the previously developed robust fluence map optimization (FMO) model as well as the clinical treatment methods for breast cancer (inhale breath‐hold with active breathing control (ABC) and free breathing). Results: On average, the robust DAO approach reduced the mean heart dose by 15.1% compared to the conventional IMRT method for breast cancer (with free breathing) using an equal number of segments. The maximum dose to 10cc of the heart was reduced by 19.7%. Compared to the previous robust FMO method, the robust DAO method delivered less than 1% higher dose to the heart and 1% lower dose to the CTV on average. The dose‐volume criteria for the heart and CTV were met in all of the breathing scenarios. The robust DAO approach resulted in a 20.8% reduction in the number of cases that would have required ABC treatment in comparison with the clinical method with free breathing. Conclusions: The proposed DAO approach results in lower heart dose compared to the conventional method with free breathing and shows low sensitivity to breathing pattern uncertainty. The robust DAO method is able to derive simpler IMRT segments during free breathing and can potentially reduce the need for breath‐hold techniques. This research was supported in part by the Ontario Ministry of Economic Development and Innovation, the Canadian Breast Cancer Foundation ‐ Ontario Region, and the Ontario Consortium for Adaptive Interventions in Radiation Oncology (OCAIRO) funded by the Ontario Research Fund (ORF).
TH‐A‐116‐06: Near‐Global Beam Angle Optimization for Intensity Modulated Proton Therapy
Authors: Chen W , Craft D , Unkelbach J , Bortfeld T
Published: July 2013

Purpose: To solve the beam angle optimization (BAO) problem of IMPT to provable near‐global optimality. Global BAO, the selection of a given small number of beams from a large set of candidate beams, remains computationally hard due to the large scale and combinatorial nature of the problem. Exhaustive search is computationally prohibited. Stochastic optimization and local search have been used but confidence in global optimality cannot be established. Methods: We use a fast projection solver for the multi‐criteria robust IMPT optimization. Equally spaced 36 beams are optimized initially to generate a global optimal solution. The 36‐beam plan is then reduced to a 3‐beam plan by iteratively removing the beam with least average intensity and re‐optimizing subjected to 3% relaxation on the best objective value. The true optimal 3‐beam plan is also found by exhaustive search as a benchmark. Results: We apply the method to a chordoma case and an orbital rhabdomyosarcoma case. The optimal 3‐beam plan is only 3–5% worse than the optimal 36‐beam plan in terms of the objective value. The 3‐beam plan efficiently reduced from the 36‐beam optimal plan is almost as good as the optimal 3‐beam plan. There exist a large number of nearly optimal 3‐beam combinations. For these two cases our method finds near‐global optimal plan using 3 beams in a time scale of minutes. Exhaustive search would take on the order of days. Conclusion: We find that IMPT plans using 3 or 4 beams are only marginally improved by adding more beams. By exploiting the high degeneracy of the solution space, we present an efficient column reducing framework for global IMPT BAO. It first finds an optimal many‐beam plan and then uses the column reducing approach to produce a plan with a small number of beams at a user‐selected tolerance from the global optimal plan. This work was supported in part by NCI Grant P01 CA21239 Proton Radiation Therapy Research and NCI Grant R01 CA103904‐01A1 Multi‐criteria IMRT Optimization.
TH‐A‐116‐07: Validation of Patient Contours for Head and Neck Treatments Using Population‐Based Metrics
Authors: Kavanaugh J , Wooten H , Green O Pechenaya , DeWees T , Li H , Mutic S , Altman M
Published: July 2013

Purpose: To develop a statistical and anatomical population‐based model that can be used to validate the accuracy and integrity of head and neck normal tissue structures of individual patients for use in preplanning and/or online adaptive radiation therapy. Methods: Normal tissue contours from 29 patients treated for head and neck cancers were used in development of the model. For each patient, DICOM plan and structure files were exported from the treatment planning system to an in‐house developed software program which calculated anatomic metrics for volume, shape, and intra‐structure distances for all structures. A statistical analysis of these metrics produced population specific rules that were used within the software program to evaluate the accuracy of head and neck contours for subsequent patients. The contour assessment program included only metrics for which the standard deviation was less than a heuristically determined limit of 15% of the mean for that metric. To verify the softwares utility, 42 common contouring errors were intentionally introduced within 9 specific structures for 9 different patients. These errors included incorrect laterality, position, size and shape, inclusion of small isolated pixels, deleted segments, and empty structures. The evaluation of all 9 head and neck structure sets was blinded to the nature and number of the generated errors. Results: The contour accuracy and integrity program correctly identified 40 of 42 generated errors. Small modifications to the structures shape and volume were the most difficult to correctly identify; however the program correctly identified all positional and laterality errors, deleted/isolated segments, small pixels, and deleted contours. Conclusion: Rules developed from a statistical analysis of anatomic population‐based metrics can provide much of the necessary information to correctly and efficiently evaluate the accuracy and integrity of a unique patient contour structure set for IMRT preplanning or for an online adaptive radiation therapy protocol.
TH‐A‐116‐08: Guaranteed Epsilon‐Optimal Treatment Plans with Minimum Number of Beams for Stereotactic Body Radiation Therapy
Authors: Yarmand H , Craft D
Published: July 2013

Purpose: To efficiently find quality‐guaranteed treatment plans with the minimum number of beams for stereotactic body radiation therapy (SBRT). Methods: We formulate the beam angle optimization problem (BAO) as a mixed integer programming problem (MIP) whose solution gives the optimal beam orientation as well as optimal beam intensity. We first allow use of all candidate beams and find the “ideal plan” with the objective of minimizing dose to organs at risk (OARs) while keeping the dose to the tumor and other structures in specified ranges. Then we resolve the MIP with the objective of minimizing the number of beams while remaining in a predetermined epsilon‐optimality of the ideal plan with respect to the dose to OARs. Since the treatment plan optimization is a multicriteria optimization problem, the planner can navigate the ideal dose distribution Pareto surface and select a plan of desired target coverage versus OARs sparing, and then use the proposed technique to reduce the number of beams while guaranteeing epsilon‐optimality. To reduce the computation time, we use a family of heuristic cuts, known as ((neighbor cuts)) while preserving epsilon‐optimality. For the numerical experiments a liver case with 34 and a lung case with 36 coplanar beams were considered. Results: The ideal plan uses an impractically large number of beams. The proposed technique reduced the number of beams to the range of practical application (5 to 9 beams) while remaining in the epsilon‐optimal range of 1% to 5% optimality gap. Adding the neighbor cuts reduced the computation time considerably. Conclusion: The proposed method can be integrated into a general algorithm for fast navigation of the ideal dose distribution Pareto surface and finding the treatment plan with the minimum number of beams, which corresponds to the delivery time, in epsilon‐optimality range of the desired ideal plan. The project was supported by the Federal Share of program income earned by Massachusetts; General Hospital on C06 CA059267, Proton Therapy Research and Treatment Center and also partially by RaySearch laboratories.
TH‐A‐116‐09: A Novel Prior‐Knowledge‐Based Optimization Algorithm for Automatic Treatment Planning and Adaptive Radiotherapy Re‐Planning
Authors: Zarepisheh M , Long T , Li N , Romeijn E , Jia X , Jiang S
Published: July 2013

Purpose: To develop a novel algorithm that takes existing prior‐knowledge information into account in optimization to facilitate automatic treatment planning and adaptive radiotherapy (ART) re‐planning. Methods: We developed an algorithm to automatically create a treatment plan guided by the DVH curves of a reference plan that contains information on the clinician approved dose‐volume trade‐offs among different targets/organs and within the same organ. This method has applications in automatic treatment planning and ART re‐planning. In ART, the reference plan is the initial plan for the same patient, while for automatic treatment planning the reference plan is selected (based on patient similarity) from a library of clinically approved and delivered plans of previously treated patients with similar medical conditions. The proposed algorithm employs a voxel‐based optimization model and approximates the large voxel‐based Pareto surface iteratively. The voxel weights are iteratively adjusted to approach a plan that is similar to the reference plan in terms of the DVHs. If the reference plan is feasible but not Pareto optimal, the algorithm generates a Pareto optimal plan consistent with the reference DVHs. If the reference plan is too restricting, the algorithm generates a Pareto plan with DVHs close to the reference ones. Results: The algorithm was tested using a series of patient cases and found to be able to automatically adjust the voxel‐weighting factors automatically in order to generate a Pareto plan with DVHs similar to the reference plan. The algorithm has been implemented on GPU for high efficiency. Conclusion: A novel prior‐knowledge‐based optimization algorithm has been developed that uses the prior knowledge in optimization process to automatically adjust the voxel weights and generate a clinical optimal plan at high efficiency. It is found that the new algorithm can significantly improve the plan quality and planning efficiency in ART re‐planning and automatic treatment planning.
TH‐A‐116‐10: A Dosimetric Comparison Between Volumetric Modulated Arc and 3D Treatment Plans for Synchronous Bilateral Breast Patients Treated with Simultaneous Integrated Boost
Authors: Levin D , Shekel E , Epstein D , Tova Y , Zalmanov‐Faermann S , Pfeffer R
Published: July 2013

Purpose: To evaluate dosimetric differences between standard tangential and volumetric modulated arc (VMAT) plans for synchronous bilateral breast patients treated with simultaneous integrated boost (SIB). Methods: For 10 bilateral breast patients we created separate left and right tangential plans and a single VMAT plan encompassing both breasts and tumor beds as target volumes. Patients were prescribed to 50 and 60 Gy in 2 and 2.4 Gy daily fractions to breasts and tumor beds respectively. For 3D plans each breast was planned independently. A sum of the independent plans was generated to ensure no dose overlaps, and for comparison with the VMAT plan. VMAT plans were normalized to 95% dose covering 95% breast volume. Optimization objectives for lungs, heart, and spine were based on QUANTEC data. Conformity index (CI) was calculated for all plans. Data were analyzed using the Mann‐Whitney Rank‐Sum test. Results: Tumor bed and breast coverage was similar for 3D and VMAT plans. However, CI for VMAT was significantly better (p<0.001) for both breast and tumor bed. All organ at risk (OAR) metrics (Mean Lung Dose, V20, V5, maximal heart and spine doses) were significantly better (p<0.001) in the 3D plans. Conclusion: Conformity in VMAT plans was superior to 3D plans, indicating that in 3D plans larger volumes of non‐target tissue received high doses. In addition, VMAT treatment setup is much simpler than for 3D plans, where treatment is delivered sequentially to each breast, necessitating separate setups for each side. Thus, VMAT treatment is faster, and much easier for patients. However, 3D plans are overwhelmingly superior to VMAT in terms of doses to OARs. Thus, possibly because the breast is a peripheral organ, it appears to be a unique site, where OARs are better spared by a ‘simple’ tangential plan rather than a more sophisticated VMAT technique.
TH‐B‐105‐01: Accuracy and Precision in Cranial Radiosurgery
Authors: Bichay T , Dieterich S , Schlesinger D
Published: July 2013

Cranial radiosurgery has developed as a highly accurate treatment procedure for relatively small lesions. There is much discussion in the literature as to what degree of accuracy is possible when the treatment system is evaluated as a whole; including cranial fixation, localization, imaging, registration, contouring, dose calculation, etc. This is now commonly referred to as the end‐to‐end (E2E) test. The now‐dated AAPM report 54 (1995) suggested that the total uncertainty can reach 2–3 mm. However, many newer publications suggest a much tighter tolerance is achievable. This discussion will focus on the strengths, and weaknesses of well‐established cranial radiosurgery systems including Gamma Knife and CyberKnife, as well as linac‐based systems. The typical, and maximum accuracy of each system will be presented including benefits of mask immobilization and invasive ring fixation. The intent of these presentations is to outline the current state of cranial radiosurgery, and review relevant technologies that have improved our treatment accuracy. Limitations that still persist at this point will also be outlined and reviewed.

Learning Objectives:

1. Advantages of various radiosurgery technologies.

2. Limitations of various radiosurgery technologies.

3. Main hurdles in achieving sub‐millimeter treatment accuracy.

4. Overall treatment accuracy assessed by E2E testing.