bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐858: Expanding the Search Space of Multiobjective IMRT Optimization
Authors: Holdsworth C , Phillips M
Published: November 2011

Purpose: To evaluate how more flexibility in the search space affects performance in multiobjective IMRT optimization. Method: Conventional inverse planning combines multiple objectives using a weighted sum. Typical structure‐specific objectives are characterized by a specific reference dose. Optimization with variable weights explores a region of feasible plan space and yields the Pareto front for that region. Fixed references doses and structure‐specific objectives can limit the region explored. Effects of varying weights, reference doses and segmentation of structures were explored. A multiobjective evolutionary algorithm (MOEA) was used to optimize plans for an example prostate case. Both the objective weights and reference doses were optimized in the MOEA and results were compared to optimizing only one parameter. In a second experiment, each voxel throughout structures were allowed different weights and reference doses. Plans were evaluated using decision objectives (OAR mean dose and target variance) which were distinct from the optimized parameters. Results: By optimizing both weighting and reference doses that define the penalty function for each structure, a set of IMRT plans was found that was superior to plans generated by only optimizing weights or by only using optimizing reference dose. Furthermore, when the search space was expanded so that every voxel in every structure was allowed its own reference dose and weight, plans were found that were superior to the best set of plans generated using structure‐specific parameters. Conclusion: Algorithms that donˈt allow all input parameters to be optimized or consider a voxel‐specific search space will not approach the best possible set of plans given the physical limitations of IMRT treatment when judged by clinically relevant and possibly non‐convex metrics. The set of plans generated using a limited search space can potentially be improved in all decision criteria by increasing the scope of the search space.
SU‐E‐T‐864: Beam Orientation Optimization Using Ant Colony Optimization in Intensity Modulated Radiation Therapy
Authors: Pei X , Cao R , Zheng H , Wang J , Wu Y
Published: November 2011

Purpose: In intensity modulated radiation therapy (IMRT) treatment planning, gantry angles are usually preselected on the basis of experience and intuition. Therefore, getting an appropriate beam configuration needs a very long time. Based on the present situation, the paper puts forward beam orientation optimization using ant colony optimization. Methods: Using ant colony optimization to select the optimal beam configurations, after getting the beam configuration using Conjugate Gradient (CG) algorithm to optimize the intensity profiles. Based on the information of the effect of pencil beam‐let, the algorithm could accelerate to find the global optimal solution. Results: In order to verify the feasibility of the presented algorithm, a simulated and clinical case was tested. Comparison of Dose‐Volume histogram and isodose line between target area and organ at risk, the results showed that the plan quality was improved after optimizing beam configurations. Conclusions: The optimization approach can make treatment planning meet clinical requirements more efficiently, so it has extensive application perspective.

Supported by the National Natural Science Foundation under grant No.30900386 and the Anhui Provincial Natural Science Foundation under grant No. 090413095 and 11040606Q55.
SU‐E‐T‐867: Improving the IMRT Plan Quality by Minimizing the Discrepancy Between the Optimized and Segmented Intensity Maps
Authors: Li Yongjie , Tan Youheng
Published: November 2011

Purpose: In step‐and‐shoot IMRT, beam intensity maps are first optimized, followed by a leaf‐sequencing to translate them into deliverable segments. The differences between the optimized and segmented intensity maps may degrade the deliverable dose distribution. Aiming to decrease this degradation, we present a method to minimize the discrepancy between the optimized and segmented intensity maps. Methods: The general idea of our algorithm is to minimize the sum of absolute differences (SOAD) between the optimized and sequenced intensity maps, similar to the work of Daliang Cao et al (Med. Phys. 2006, 33(4): 859–867). The improvement of our method includes: (1) the initial segment shapes and weights are produced based on the optimized intensity maps with a fast Xia and Verheyˈs leaf sequencing algorithm; (2) the segment weights and shapes are iteratively optimized with a modified gradient conjugate algorithm and genetic algorithm; and (3) the MLC‐dependent leaf transmission and penumbra are both accounted for. Results: For a simulated case involving the left and right lungs and an organ at risk (OAR) surrounded by a C‐shaped planning target volume (PTV), five beams were used. For the five optimized beam intensity maps, the segmented maps were computed using the proposed algorithm. The average computation time was about 17 sec per beam. Both of the dose distributions and dose volume histograms (DVHs) showed the improvements given by our method: more uniform dose distributions for PTV of the both cases, and lower doses for OARs of the clinical case, than that when the MLC‐dependent leaf transmission and penumbra were ignored. Also, the needed numbers of segment and monitor unit were reduced. Conclusions: The proposed method could produce a better segmentation result by improving the dose distributions and reducing the segment number.

Funding support: This work was supported by the Program for New Century Excellent Talents in University of China (NCET‐07‐0151) and the Natural Science Foundations of China (61075109, 30730036).
SU‐E‐T‐873: Dosimetric Difference Between Direct Dose Mapping and Energy/Mass Transfer Mapping for 4D Dose Accumulation
Authors: Li H , Zhong H , Kim J , Nurushev T , Glide‐Hurst C , Gulam M , Chetty I J
Published: November 2011

Purpose: To investigate the dosimetric difference between two dose warping methods for 4D dose accumulation: direct dose mapping and energy/mass transfer mapping. Method and Materials:Ten lung cancer patients with varied tumor motion amplitude (between 0.4 and 1.9 cm) and 10 phase 4DCT acquisition were selected for the study. Optimized treatment plans for ITV plus 3 mm margin were created on the 50% phase images (CT50) and applied to other phases for dose calculation with AAA. The other phase doses were warped to CT50 using deformable image registration and two mapping algorithms. With direct dose mapping, voxel doses in source image were tri‐linearly interpolated to image grid and then transferred to the destination image grid, and interpolated back to dose grid. With energy/mass mapping, the dose on source image was converted to energy first and then transferred to the destination along with the mass. The energy and mass to dose voxel were calculated by summing up the contributions from the image grids that intersect with it, allowing accurate calculation of voxel dose by dividing energy by mass. The mapped individual phase doses were finally accumulated with weighting proportional to beam‐on time spent on each phase. Results: With 2.5 mm dose grid, the difference in mean PTV dose between the two dose mapping methods was −0.8%∼1.0% (0.0+/−0.5% of mean+/−SD), and for ITV it was −0.4%∼0.8% (0.0+/−0.3%). However, for the regions extending 2 cm superiorly/inferiorly outsides the PTV surface a mean dose difference 8% (0.2+/−2.7%) was found. Coarser dose grid sizes caused the differences to be greater. Conclusion: PTV mean dose differences between the two dose mapping methods are generally small in uniform dose regions; minimum dose differences were much greater. Dose/density gradients cause the differences to be significantly larger. Increasing the dose grid resolution caused greater differences between the two methods.
SU‐E‐T‐883: Clinical Application of Monte Carlo for SBRT: Mediastinal Lung
Authors: Grimm J , LaCouture T , Asbell S , Kramer N , Pahlajani N , Chen Y , Xu Q , Croce R , Xue J
Published: November 2011

Purpose: Treatment planning systems have provided Monte Carlo dose calculations for several years but many physicians are still hesitant to use them clinically due to lack of data. Therefore we recalculated 200 Ray Tracing treatment plans using Monte Carlo with heterogeneity corrections and compared to SBRT dose tolerance limits. Methods: From among these 200 CyberKnife cases, 25 mediastinal lung cases are presented in this study. An extensive literature review obtained 105 published SBRT dose tolerance limits for the mediastinal critical structures aorta, bronchi, esophagus, heart, and trachea. These limits were partitioned into high‐risk and low‐risk categories. The DVH Evaluator software tool was used to generate DVH Risk Maps for these critical structures, which superimpose a) published dose tolerance limits b) unified high‐risk and low‐risk trends and c) published adverse event doses, onto Monte Carlo patient data to assess risk of adverse events. Results: Recalculated treatment plan data is within the expected range of published SBRT dose tolerance limits, providing optimism for clinical use. None of the patients experienced any Grade 3 or higher adverse events. The low‐risk dose tolerance limits were exceeded 22 times in these cases with no severe adverse event, thus helping to validate their safety. Conclusions: The range of doses calculated by Monte Carlo for our historical patient data is compatible with published SBRT dose tolerance limits. SBRT dose tolerance limits should be fine‐tuned by Monte Carlo dose calculations in long‐term statistical followup studies.

Disclosure: The first author has developed the DVH Evaluator software.
MO‐D‐BRC‐01: Dosimetric Evaluation of a Novel Cone‐Beam CT (CBCT) Estimation Method Using Prior Information for Lung Cancer Patient Treatment
Authors: Ren L , Jin J , Movsas B , Yin F , Chetty I
Published: November 2011

Purpose: To evaluate the dosimetric accuracy of a novel CBCT estimation method using prior information for use in adaptive radiotherapy (ART)‐based treatment of lung cancer patients. Methods: Patient planning CT or previous dayˈs CBCT images are used as the prior images. The new CBCT images are considered as a deformation of the prior images. The deformation field is solved by meeting the data fidelity constraint and minimizing the deformation energy of the field. The new CBCT images are then obtained by deforming the prior images according to the deformation field. Data for ten lung cancer patients were tested for estimation using 57 projections acquired over 60° and 360° scan angles. The new CBCT reconstructed from fully sampled projections were used as the ground truth. Geometric accuracy of the estimation was evaluated using tumor volume percentage error. Dosimetric accuracy was evaluated by comparing the PTV dose distribution in each set of images. Results: The tumor volume percentage error was 13.3% and 4.3% in CBCT estimated using 57 projections over 60° and 360° scan angles, respectively. PTV min., mean and max. doses in estimated CBCT agree with those in true CBCT within 2% and 1% for 60° and 360° scan angles, respectively. The percentage volume of PTV covered by prescription dose in estimated CBCT agrees with that in true CBCT within 4% and 1% for 60° and 360° scan angles, respectively. The isodose lines in the estimated CBCT images also match well with those in the true CBCT images. Conclusions: The proposed method is able to provide accurate guidance for plan adaptation in ART treatment of lung cancer patients using 57 projections acquired over 360° scan angle. This implies a 12‐fold reduction in imaging dose relative to conventional CBCT for similar image quality for target delineation and dose calculation in ART.
MO‐D‐BRC‐05: Reducing Megavoltage Cone Beam CT Imaging Dose without Loss of Spatial Resolution Using a Sintered Pixelated Array System
Authors: Breitbach E , Maltz J , Gangadharan B , Bani‐Hashemi A , Anderson C , Bhatia S , Stiles J , Edwards D , Flynn R
Published: November 2011

Purpose: To demonstrate that megavoltage cone beam CT (MVCBCT) image quality can be significantly improved without increasing imaging dose or reducing spatial resolution for both head‐ and pelvis‐sized volumes. The improvement is achieved with the combination of an imaging beam line (IBL) with a low atomic number electron target and a novel sintered pixelated array (SPA) detector. Methods: Three Siemens Oncor linear accelerators were equipped with an IBL+SPA system, an IBL system with a conventional Kodak Lanex Fast B scintillator (IBL+LFB), and a 6 MV treatment beam line system with an LFB (TBL+LFB). Head‐ and pelvis‐sized phantom images were acquired with all three systems at imaging doses ranging from 2‐60 cGy. Contrast to noise ratio (CNR) and modulation transfer function (MTF) were calculated from the phantom images. Head and neck, prostate, and lung cancer patients were imaged with the three imaging systems at doses ranging from 2–15 cGy. Results: For head‐ and pelvis‐sized phantom images acquired at 5 cGy or above, the CNR average percentage increases for imaging system upgrades from TBL+LFB to IBL+LFB to IBL+SPA were 52% (p < 1E‐7) and 42% (p < 1E‐6), respectively. The MTFs do not change with imaging system by statistically significant percentages. Soft tissue contrast is generally more easily differentiated on IBL+SPA images than TBL+LFB and IBL+LFB in the patient images. Conclusions: Since CNR scales with the square root of imaging dose, each step in the TBL+LFB to IBL+LFB to IBL+SPA upgrade halves the imaging dose required to obtain a given CNR. No statistically significant change in spatial resolution was observed with any upgrade, suggesting that the pixelation of the SPA prevents a loss in spatial resolution. IBL+SPA patient image quality was always better than that of the TBL+LFB system and as good as or better than that of the IBL+LFB system.

Sponsored partially by Siemens Oncology Care Systems.
MO‐F‐224‐01: Preparing for Diagnostic ACR Accreditation
Authors: Hernandez D
Published: November 2011

The American College of Radiology is a professional society and membership organization made up of radiologists, medical physicists, radiation oncologists and nuclear medicine physicians dedicated to making imaging safe and effective. The primary mission of the ACR is to serve patients and society by advancing the science of radiology, improving the quality of patient care, providing continuing education and conducting research for the future of radiology.

The ACR has a long history of developing and administrating accreditation programs. The purpose of the various accreditation programs is to set quality standards for practices and to help improve the quality of care given to patients. Accreditation is a peer review process designed to be educational and evaluates personnel qualifications, policies and procedures, equipment performance, effectiveness of quality control measures and the quality of clinical images.

The Diagnostic Modality Accreditation Program (DMAP) incorporates all of the ACR accreditation programs with the exception of mammography and radiation oncology. The current programs include Breast Magnetic Resonance Imaging, Breast Ultrasound, Computed Tomography, Magnetic Resonance Imaging, Nuclear Medicine, PET, Stereotactic Breast Biopsy and Ultrasound. While mammography accreditation is required by the FDA under MQSA, facilities seeking ACR accreditation in the other diagnostic modalities have done so voluntarily but for a variety of reasons including the MIPAA mandate requiring the providers of advanced diagnostic services that bill under part B of the Medicare Physician Fee Schedule to be accredited by January 1, 2012 in order to receive technical component reimbursement from Medicare.

ACR accreditation is a team process. The medical physicist plays a critical role in the process. A basic understanding of the overall accreditation process is crucial to the achievement of accreditation.

Educational Objectives

1. Understand the basics of the ACR diagnostic modality accreditation process

2. Understand the important roles of the medical physicist in the accreditation process
MO‐F‐BRA‐03: Secondary Neutron Dose Measurements Using CR‐39 Detectors in Photon and Proton Radiotherapy
Authors: Hälg R A , Besserer J , Mayer S , Schneider U
Published: November 2011

Purpose: It is assumed that modern radiation treatment techniques such as intensity‐modulated radiotherapy (IMRT), volumetric‐modulated arc therapy (VMAT) and proton radiotherapy are increasing the cancer cure rates and simultaneously reducing side effects. But there are also concerns about radiation‐induced malignancies caused by secondary neutron radiation, in particular, for younger patients. The aim of this study was to measure secondary neutron dose for a radiotherapy patient during typical treatments. Methods: In order to evaluate the neutron dose for a patient, an anthropomorphic Alderson‐Rando phantom was used. The measurements were performed using CR‐39 etched track detectors, placed inside and on the surface of the phantom, giving a three dimensional dose distribution and the organ doses. The neutron dose was compared with respect to treatment technique, therapy machine and radiation quality. The CR‐39 detectors were calibrated using an Am‐Be neutron source and an ICRU cubic phantom. The series of measurements included photon irradiations delivered by treatment machines of the manufacturers Varian, Elekta and Siemens and the treatment techniques 3D‐conformal, IMRT and VMAT. The proton irradiations were performed using active spot scanning and passive scattering techniques. Results: The doses were determined in terms of equivalent dose. In addition, effective dose was calculated using organ specific weighting factors of the ICRP. The effective doses for the complete course of treatment were 3.8 mSv, 3.0 mSv, 3.4 mSv, 0.7 mSv, 2.8 mSv, 20.3 mSv and 26.8 mSv for Varian 3D‐conformal, Varian IMRT, Varian VMAT, Elekta IMRT, Siemens IMRT, spot scanning protons and passively scattered protons, respectively. Conclusions: The differences in neutron dose between the different treatment techniques for photons were not large. However, Elekta machines produce considerably lower neutron dose than Varian and Siemens machines. For proton treatments, neutron dose is higher in general and apparently higher for scattered compared to spot scanned protons.
MO‐F‐BRC‐01: 3D Target Trajectory Reconstruction Using CBCT Projection Images
Authors: Cho B , Ahn W , Yoon S , Kim J , Poulsen P , Keall P
Published: November 2011

Purpose: With wide use of CBCT in IGRT, extracting the 3D target trajectory from CBCT projection images would be useful to manage intra‐ fraction motion for motion inclusive, gated and tumor tracking delivery methods. The purpose of this work was to develop a target trajectory estimation method from CBCT by employing inter‐dimensional correlation between each direction of target motion, and demonstrate its feasibility with a liver SBRT case. Methods: To estimate the target trajectory from CBCT projection images we utilize an inter‐dimensional correlation between each direction of target motion: LR(t)=a_LR×SI(t)+b_LR, AP(t)=a_AP×SI(t)+b_AP. The parameters of the simple linear correlation model were determined with least‐squares estimation by minimizing the discrepancy between the measured and model‐estimated projection positions on the imager coordinates. The method was demonstrated using a CBCT projection images obtained during liver SBRT treatment for 3D/3D matching purpose using On‐board imager of a Varian iX machine. From each projection image of the CBCT, positions of an implanted gold seed were extracted using Varian research tool (RPM‐Fluoro). The estimated trajectory was compared with the marker trajectory obtained from 10‐binned respiratory phase 4DCT. Results: In general the estimated trajectory followed well the reference trajectory from 4DCT in the case. Mean root‐ mean‐square error of the model estimation was 0.67mm in lateral direction and 0.00mm in longitudinal direction on the imager coordinates. Conclusions: A simple 3D marker trajectory estimation method using CBCT projection images was developed. Preliminary results show that it could be used to accurately localize a moving tumor.
MO‐F‐BRC‐04: Is the External Point/marker a Good Surrogate for the Breast Surface Motion during the Left Breast DIBH Treatment?
Authors: Tang X , Lian J , Chang S , Xu Z , Halle J , Jones E , Marks L
Published: November 2011

Purpose: The ideal way of treating left breast DIBH is through direct breast surface matching. Many DIBH techniques, however, rely on a surrogate—external point/marker motion. We try to quantify the predictability of the breast surface motion using external point/marker. Methods: AlignRT Beam Hold system is applied to perform real‐time surface matching and the external point/marker tracking simultaneously. The skin rendering of breath hold CT scan is served as the reference. During the treatment, the patient surface is monitored and registered to the reference to calculate the corresponding distance (S(t)). Radiation beam is turned on when this distance is within a preselected threshold. The external point/marker tracking is implemented by tracking the vertical amplitude of a point in the center part of the left breast skin. The real‐time distance (P(t)) of the selected point to the corresponding reference point is calculated. A model is built to predict S(t) using P(t). Statistical and computational complexity analyses are conducted. Results: 5 patients are included in this study. Based on our statistical analysis, S(t) can be modeled as a proportioned P(f). For each patient, the ratio is calculated for the first treatment day and applied on the subsequent days. The difference between the prediction and the true S(t) is calculated. The average standard deviations of the difference over all the treatment days are 1.69–2.85mm for different patient, which corresponds to 3.8%–14.6% error rate for 3mm threshold. The biggest standard deviation is from the patient with the largest breast. The computational complexity of the fast ICP based surface matching algorithm is O(N), and of the point/marker tracking is O(1). Conclusions: The breast surface motion and external point/marker tracking result do have strong correlation. Surface matching is more accurate at the price of higher computational complexity.
MO‐F‐214‐03: Calibration of a Reference Standard Ionization Chamber in 6‐ and 18‐ MV Photon Beams Using a Sealed Water Calorimeter at Room Temperature
Authors: Bateman F , Chen‐Mayer H , Tosh R
Published: November 2011

Purpose: To investigate the feasibility of using room‐temperature water calorimetry for the calibration of ionization chambers in high‐energy x‐ray radiotherapy beams. Methods: 6‐ and 18‐MV photon beams provided by a Clinac 2100C were used to compare dose measurements from a sealed, Domen‐type water calorimeter operated at room temperature and an Exradin A12 ionization chamber, calibrated in Co‐60. Measurements were conducted at 10‐cm depth in a 27‐liter, cubic water phantom, with SSD = 100 cm. For both energies, the Clinac was configured to deliver a 10‐cm × 10‐cm beam at the water surface with a nominal dose rate at dmax of 2.4 Gy/min, arranged as a sequence of 1‐minute exposures separated by 1‐ minute rest intervals that would typically proceed for one hour per run. Beam stability was monitored by a PTW pancake chamber of 10‐cm diameter. Calorimeter response, analyzed via mid‐point extrapolation and Fourier analysis, was scrutinized for evidence of natural convection. A further test for convection within the calorimeter was done by gauging its linearity as the nominal dose rate of an 18‐MV beam at 10‐cm depth was varied from 0.64 to 3.2 Gy/min. Results: Average dose measurements obtained from nearly a dozen calorimeter runs at each of 6‐ and 18‐MV were determined with a precision of better than ±0.15% (1 std) and agree with chamber measurements, obtained via kQ factors obtained from TG‐51, within experimental uncertainty. No evidence of convection was observed in the calorimeter data, and its response over the 5x variation in dose rate was found to be linear with standard error on the slope of <0.15%. Standard correction factors will be assessed. Conclusions: Direct calibration of ionization chambers via room‐temperature water calorimetry appears feasible. Further studies of calorimeter response at 4‐deg C, where convection is extinguished, are anticipated.
MO‐F‐110‐06: Developing Deformable 3D Dosimeters: Radiochromic Gels in Latex Balloons
Authors: Walsh A , Battista J , Jordan K
Published: November 2011

Purpose: To develop radiochromic hydrogels contained in latex balloons as a model for 3D dosimetry in deformable tissue. Methods: Films of latex solution were thermally cured to form thin sheets of transparent rubber. Spheroid, elastic dosimeters were made by filling adaptor tubes sealed at one end by circular rubber sheets with radiochromic hydrogel. These dosimeters were evaluated for optical and mechanical properties and dosimetric performance. Dosimeters were scanned with a fast 3D optical cone beam computed tomography system and dose distributions were reconstructed with isotropic 0.25 mm sized voxels. Gels were irradiated in a water tank to a uniform 3 Gy dose or by an array of 5 mm square beamlets in a grid pattern, with the dosimeter in either relaxed or deformed state. Four percent gelatin gels were employed in the study and dosimeter rigidity was adjustable by filling balloons with more gel. Dosimeters were maintained at a temperature of 20° C throughout the experiments. Deformed gels were allowed to return to the relaxed state for 30 minutes while the radiochromic reaction completed prior to CT scanning. Results: Ferrous ‐ xylenol orange ‐ gelatin hydrogels in latex balloons exhibited similar auto‐oxidation rates and dose sensitivities to samples contained in rigid plastic vessels. The thin‐walled latex balloons provided excellent optical transmission performance allowing accurate dosimetry to within 1 mm of the balloon wall. Uniform irradiations resulted in uniform dose reconstructions for both the relaxed and deformed dosimeters. Deformed gel irradiated with the beamlet array and scanned in relaxed state demonstrated ability of dosimeter to record 3D dose deformation patterns. Conclusions: The elastic properties of transparent films and hydrogels can be combined to produce deformable 3D radiochromic dosimeters compatible with optical CT scanners. These dosimeters can be used to validate multi‐ fraction dose accumulation algorithms based on image warping algorithms.

A. Walsh supported by grant: Optimization of Radiation Therapy: Uncertainty Analysis and Strategies for Improvement. Canadian Institutes of Health Research #200403MOP‐128435‐BME‐CEEA‐11379
MO‐F‐211‐04: Fast Phase Contrast MR Angiography with Simplified Skipped Phase Encoding and Edge Deghosting with Array Coil Enhancement (S‐ SPEED‐ACE)
Authors: Chang Z , Xiang Q , Yin F
Published: November 2011

Purpose: The aim of this work is to investigate the feasibility of accelerating phase contrast MR angiography by the fast imaging method of Simplified Skipped Phase Encoding and Edge Deghosting with Array Coil Enhancement (S‐SPEED‐ACE). Methods: The parallel imaging method of Skipped Phase Encoding and Edge Deghosting with Array Coil Enhancement (SPEED‐ACE) has been simplified with enhanced acceleration for imaging sparse objects and is termed S‐SPEED‐ACE. As demonstrated previously with a computer simulated study, S‐SPEED‐ACE partially samples k‐space with multiple coils in parallel and yields a deghosted image based on a least‐square‐error solution. Without differential filtering and full central k‐space sampling, S‐SPEED‐ACE uses more straightforward reconstruction and achieves further scan time reduction as compared with SPEED‐ACE. In this work, S‐SPEED‐ACE is further developed to accelerate real human 3D phase‐contrast MR angiography. The proposed method was tested with a 3D head PC‐MRA scan, which was acquired using a clinical 1.5T scanner on a healthy volunteer Results: Images are reconstructed by S‐SPEED‐ACE to achieve an undersampling factor of up to 8.3 with four receiver coils. The reconstructed images generally have comparable quality as that of the reference images reconstructed from full k‐space data. Maximum‐intensity‐projection (MIP) images generated from the reconstructed images also demonstrated to be consistent as those from the reference images. Conclusions: By taking advantage of signal sparsity naturally existing in the data, SPEED‐ACE was simplified and its efficiency was improved. In this work, the previously proposed parallel imaging method named S‐SPEED‐ACE has been further developed to accelerate a 3D head PC‐MRA scan.
MO‐F‐211‐08: Development and Application of Novel Hyperpolarized 3He Magnetic Resonance Imaging Segmentation to Measure Acute Treatment Effects in Pulmonary Disease
Authors: Kirby M , Heydarian M , Wheatley A , Etemad‐Rezai R , McCormack DG , Parraga G
Published: November 2011

Purpose: To develop a semi‐automated segmentation method for generating novel helium‐3 (3He) magnetic resonance imaging (MRI) functional information from 3He and proton (1H) MRI, determine the relationship between measurements obtained by manual and semi‐automated segmentation, and apply this method in chronic obstructive pulmonary disease (COPD) patients to evaluate the lung functional effects of bronchodilator therapy. Methods: Fourteen COPD ex‐smokers provided written informed consent and underwent hyperpolarized 3He and 1H MRI, before and 25±2 minutes after administration of salbutamol. Semi‐ automated segmentation was accomplished in three steps: 1) hierarchical K‐ means clustering of 3He MR images into 5 clusters where cluster 1 (C1) represented regions of ventilation defect and background, and cluster 2 to cluster 5 (C2–C5) represented gradations of signal intensity/ventilation, 2) 1H MRI segmentation using seeded region‐growing algorithm, and 3) registration of 1H and 3He MRI to differentiate the ventilation defects from the background. 3He VDP was generated using ventilation defect volume (VDV or C1) normalized to the thoracic cavity volume. 3He VV was generated by summing the remaining ventilation clusters (C2–C5). Results: Manual and semi‐automated segmentation were significantly correlated for 3He VDV (r=.95, p<.0001) and VV (r=.94, p<.0001). The time for manual segmentation was approximately 60 to 90 minutes per subject dataset, whereas the time for semi‐automated segmentation to complete a dataset was between 4 and 8 minutes. Following bronchodilator therapy, there was a significant decrease in VDP and 3He C2 (p=.01), and significant increases in C3 (p=.03), C4 (p<.0001) and C5 (p=.02). Conclusions: We developed a method for generating novel quantitative 3He MRI functional information that enables sensitive detection of regional lung functional improvements that occur following inhaled therapy in COPD patients. This has implications for the broader translation of functional MRI in clinical therapy studies including those for smoking‐related lung cancer. Research funding support from CIHR Operating grants MOP 97748 and Team Grant CIF 97687.
MO‐F‐220‐02: Imaging Educational Course ‐ Ultrasound Miniaturization
Authors: Smith S
Published: November 2011

Ultrasound is used more often than any other medical imaging modality because compared to the alternatives it is fast, safe, portable, and inexpensive. Moreover ultrasound has evolved symbiotically with advances in integrated electronics to allow increasingly higher image quality, along with enhanced functional information, like color flow, perfusion, tissue‐strain, and elasticity imaging. It has now become possible to incorporate in compact instruments capabilities that were only available in full size consoles a decade ago.

Imagers consist of a sensor that both transmits and receives ultrasound, an electronic beamformer to create focused beams of energy, a mid processor to convert these into grey scale images or functional information, and a display module for output. User interface and control elements complete the units. Three key technologies drive ultrasound miniaturization. First, integrated electronics that allow more processing per volume and per watt are important for both the analog front end as well as the digital beamforming and mid processing units. Second, advances in CPU, and now GPUs, along with simpler software platforms allow many of the functions that used to be done in dedicated hardware to be done in software. Third, the migration of beamforming into the probe shrinks the cable diameter and distributes processing and power consumption.

Several examples from recent progress will be used to illustrate these points: the introduction of laptop, tablet, and hand‐held imagers; automatic image optimization to simplify the user experience, and automatic physiological parameter estimates. Recent studies show that in some cases the diagnostic utility of small ultrasound imagers matches results from the larger and more costly consoles. Ultrasound imaging will be adopted in more point‐of‐care and rural settings. Spurred by technological innovations, the ability of ultrasound to address quality, access, and cost issues will likely increase its utilization worldwide.

Research supported by General Electric Corporation

Learning Objectives:

1. Be able to describe the operation of the major parts of an ultrasound imager.

2. Appreciate recent advances in probes and microelectronics.

3. Understand how engineering specifications impact the diagnostic workflow.
MO‐G‐BRA‐04: Advantages of Implementation of a Self‐Regulating Thermobrachy Seed for Solid Tumors
Authors: Gautam B , Shvydka D , Feldmeier J , Subramanian M , Parsai E
Published: November 2011

Introduction: The recently proposed thermobrachy seed combines a sealed radioactive source with a ferromagnetic core serving as a source for self‐ regulating hyperthermia when placed in an alternating electromagnetic field. This seed has the potential to address some shortcomings of other methods suggested for delivery of radiotherapy and hyperthermia, such as, as an inadequate temperature control due to complex invasive thermometry and feedback loops, temperature non‐uniformity, and prolonged time delay between hyperthermia and radiation treatments. Materials and Method: The thermobrachy seed consists of Ni‐Cu ferromagnetic alloy core coated with carbon containing I‐125 source and encapsulated in titanium capsule. The external dimension of the seed is the same as the BEST Model 2301 I125. In our modeling study, seeds are located in the central region of cylindrical water phantom. An alternating magnetic field was applied using an externally wrapped current carrying coil. The temperature distribution within the phantom due to the induction heating of the ferromagnetic seed was obtained using a finite element analysis method. This distribution was compared to radiation dose distribution from treatment planning system. Results: The thermoseed surface temperature rises rapidly and stays constant around the Curie temperature of the ferromagnetic material used. The temperature self‐regulation eliminates the need for invasive thermometry and simplifies the treatment procedure. The phantom volume reaching the therapeutic temperature range increases with increase of frequency or magnetic field strength. The temperature coverage over the target volume can be optimized by changing the magnetic field variables (intensity and frequency). Furthermore the tuning of the variables allows us achieving isothermal surfaces exactly similar to the radiation isodoses. Conclusion: The result shows that one can obtain the optimal isothermal distribution on the target volume to match with the prescribed radiation isodose distribution for the seed configuration by changing frequency and intensity of the alternating applied magnetic field.
MO‐G‐BRA‐06: A Primary Standard for HDR Brachytherapy Calibrations
Authors: McCaffrey JP , Tessier F , Downton B
Published: November 2011

Purpose: A new Canadian primary standard for high dose rate (HDR) brachytherapy calibrations has been developed, offering improved accuracy through the use of a Pb wedge to determine the scatter component ksc at each measurement position. Methods: The new HDR standard calibrates, 192,Ir brachytherapy sources in terms of air‐kerma strength (SK). An HDR brachytherapy calibration rig was manufactured and commissioned. This rig utilizes a low (0.5 m) aluminum table to which is mounted a motorized single‐axis linear stage. An afterloader feeds an, 192,Ir source vertically up through a fixed 1 m tall PMMA tube column attached to the table. A second 1 m PMMA tube column is attached to a movable carriage on the linear stage and supports a spherical graphite ionization chamber. A series of 10 measurements are made with the source‐to‐detector distance varying from 10 cm to 55 cm in 5 cm increments. The largest correction factor, ksc (scatter from the stage, walls, floor and ceiling) is determined using a Pb‐wedge technique to block the direct beam and allow a direct measurement of scatter at each distance. The small distance correction is then a simple matter to determine, allowing a more accurate determination of the source strength based on distance squared. Results: : Measurements with a Pb‐wedge technique for determining ksc demonstrated that the scatter component for each position varied, in contradiction to the constant‐scatter supposition of the alternative 7‐distance measurement calculation. These measurements were supported by EGSnrc Monte Carlo simulations. The disagreement between these two techniques for determining ksc represent an (SK) difference of approximately 0.4%. Conclusions: The new HDR brachytherapy calibration capability demonstrates higher accuracy through a more accurate scatter correction. This standard will participate in a formal international comparison for HDR brachytherapy, tentatively scheduled for 2011.
MO‐G‐BRC‐01: Comparison of the Risk of Second Malignant Neoplasm in a Developed Country versus a Developing Country for a 13‐Year‐Old Girl Receiving Craniospinal Irradiation
Authors: Taddei P J , Jalbout W , Khater N , Geara F B , Mirkovic D , Howell R M , Zhang R , Giebeler A , Mahajan A , Newhauser W D
Published: November 2011

Purpose: Childhood cancer is a global health problem that affects nations of every socioeconomic status. The incidence of treatment‐induced second malignant neoplasms (SMNs) for these children is high and will increase with years of follow up. Advanced radiotherapy techniques may reduce the risk of SMN, but these techniques are not available in developing countries. The purpose of this study was to compare the predicted SMN risk for a 13‐ year‐old girl who received craniospinal irradiation (CSI) in a developed country versus that if she had been treated in a developing country. Methods: Treatment plans were created for the girl on the basis of the standards of care in each country, comprised of 4 proton fields in the developed country and 4 6‐MV photon fields in the developing country. Mean organ equivalent dose, HT, values from primary radiation fields were calculated using commercial treatment planning systems in the clinics of the respective countries while HT from stray radiation were determined based on Monte Carlo simulations in the case of proton therapy and on thermoluminescent dosimeter measurements in an anthropomorphic phantom in the case of photon therapy. An organ‐, age‐, and sex‐specific risk model was applied to predict the risk of SMN incidence for each standard of care. Results: The predicted risk of SMN incidence was almost a factor of two higher for the standard of care of a developing country using photons versus that of a developed country using protons. The absolute risks were predominated by second thyroid, lung, and other solid cancers. Conclusion: Our findings suggest that SMN incidence of children undergoing CSI in developing countries may be improved if they are treated with advanced radiotherapy techniques currently available only in developed countries.
MO‐G‐BRC‐05: Impact of Secondary Particles on Linear Energy Transfer for Biological Treatment Planning in Proton Therapy
Authors: Grassberger C , Paganetti H
Published: November 2011

Purpose: To assess the contribution of secondary particles to pencil and passively scattered proton beams, in particular to determine the impact on linear energy transfer (LET), a central quantity in biological treatment planning. Methods: Proton Monte Carlo simulations were performed directly in patient geometries considering all primary and secondary particles, including recoils stemming from inelastic nuclear interactions. Simulations were conducted for a prostate and a head&neck case to investigate the impact of different initial proton energies. Results: Examination of the results reveals that secondary protons exhibit LET‐ values up to a factor 10 higher than those of the primary protons in the same region. This causes significant changes to the dose‐averaged LET when including secondary protons, from 50% along the central axis of the beam to >200% in the penumbra. Furthermore the LET‐maximum increases from 12 to 15 MeV/mm. The contribution of recoils (A>3) was observed to be 1.2% in the entrance region of the prostate case, increasing slightly compared to the same beam in a water phantom. The degree of biological damage inflicted by recoils remains hard to quantify, but can be discussed on account of detailed energy spectra of the preeminent particles. Conclusions: These results indicate that secondary protons have a significant influence on LET and should therefore be incorporated into RBE calculations for proton therapy, as well as into radiobiological experiments. The contribution of heavier secondary particles is complex, yet may have an impact on precise measurements of the radiobiological effectiveness (RBE) of protons, especially in the plateau region of high‐energy beams. In addition the LET distributions presented demonstrate the impact of inhomogeneities on the LET and the subtle changes between the LET distributions of passively scattered and actively scanned beams. The latter observation suggests higher RBE variations in active scanning than in passively scattered fields.

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.
TU‐A‐BRB‐08: Incorporation of Spatial Information into IMRT Plan Optimization
Authors: Uribe‐Sanchez A , Jia X , Men C , Jiang S
Published: November 2011

Purpose: In plan optimization of intensity‐modulated radiation therapy (IMRT), all voxels inside the same structure are treated equally. In realty, the spatial location of hot/cold spots does have some clinical. One example is that hot spots, if cannot be removed, are more preferred to be located at the center, rather than the edge, of the tumor. Our goal is to develop a method that while preserving the plan quality represented by the DVH curves, reallocates hot and cold spots to more clinically preferred locations, by incorporating voxelˈs spatial information into the optimization model. Methods and Materials: In traditional optimization models, usually a convex objective function penalizes the deviation of the dose received by each voxel from its prescribed (if it is part of the target) or threshold dose (if it is part of a critical structure). In our model, instead of assuming an equal weight for all voxels inside a structure, we associate weights as a function of their physical position in the structure. By this modification, our model incorporates voxel‐based penalty weights for overdosing (penalizing the presence of hot spots) and underdosing (penalizing the presence of cold spots) depending of the location in the structure. The resulting convex optimization problem is solved by implementing a gradient algorithm with Armijo search. Results: We tested our model in four prostate cancer cases and we compare the characteristics of resulting hot/cold spots to those resulting from a correspondingly model without incorporating spatial information. It is observed that without sacrificing the plan quality represented by the DVH curves, our model relocates the hot/cold spots to more desired positions. Conclusions: We have proposed a new optimization model that, by incorporating voxelˈs spatial location information, generates similar DVH curves to those from traditional model, while capable of redistributing hot/cold spots to more desirable locations.
TU‐A‐BRB‐11: 4D Optimization for Respiratory Phase‐Dependent IMRT Treatment Planning
Authors: Suh Y , Keall P
Published: November 2011

Purpose: To develop 4D optimization for respiratory phase‐dependent IMRT treatment planning with dynamic MLC motion tracking, including MLC leaf motion constraints, which takes respiration‐induced anatomic motion into account and is robust to the variations of fractional time spent in respiratory phases within a 4D CT planning scan. Method and Materials: A tool for 4D treatment‐planning optimization that integrates a commercially available treatment‐planning system and a general‐purpose optimization system was developed and applied to the 4D CT planning scans of two lung cancer patients. The optimization variables were MLC leaf positions as a function of monitor units and respiratory phase. The objective function was the deformable dose‐summed 4D treatment plan score. MLC leaf motion was constrained by the maximum leaf velocity between control points in terms of monitor units and between phases and between leaves for tumor motion parallel and perpendicular to the leaf travel direction, respectively, in terms of phases. Without loss of generality, due to the computational burden, 4D optimization was performed with two phases, end inhale and exhale. For comparison and a starting point for the 4D optimization, 3D optimization was performed on each of the phases. Results: 4D treatment plan score improved during 4D optimization by 34% for Patient 1 and 50% for Patient 2, indicating 4D optimization generated a better 4D plan than the sum of individually optimized phase plans. The DVHs for each phase remained similar, indicating robustness of the 4D plan to respiratory variations expected during treatment delivery. Conclusions: 4D optimization for respiratory phase‐dependent treatment planning with dynamic MLC motion tracking improved the 4D plan score by 42% on average compared with 3D optimization. 4D treatment plans generated changed from phase to phase to account for anatomic motion, but showed similar dose distributions in each phase.

Research supported by NIH/NCI R01‐93626 and Philips Healthcare.