bluuebunny/crossref_metadata_2025 huggingface dataset:
TH‐A‐213AB‐07: Beam Orientation Optimization in 4pi Space for Liver SBRT
Authors: Dong P , Long T , Lee P , Ruan D , Romeijn E , Low D , Sheng K
Published: June 2012

Purpose: Non‐coplanar beam geometry has been shown to improve plan conformity but manual selection of non‐coplanar beams is difficult. Previous beam orientation optimization algorithms did not include dose distribution explicitly in the objective function. Therefore, their ability to achieve high quality plans efficiently and automatically was limited. The purpose of this study is to implement a novel non‐coplanar 4pi optimization algorithm that overcomes these difficulties. Methods: A two stage inverse optimization was performed to select the beams and optimize fluences. For both stages, a pricing and column generation algorithm was used to iteratively optimize the inner product of the derivative of the objective function and the beam or beam aperture. The iterative procedure continued until a certain number of beams or beam apertures are selected or until the incremental inner product becomes negative. Ten liver stereotactic body radiation therapy (SBRT) patients previously treated with volumetric modulated arc therapy (VMAT) were replanned using the 4pi technique and compared with the actual delivered plan. 14 noncoplanar fields were selected by the 4pi algorithm to cover 95% of the PTV with the prescription dose. Results: With the same number of beams, the objective function value and the 50% isodose volume (V50) of the 4pi plan converge to a lesser value than the coplanar plan. For the liver cases, an average reduction of 14% in V50 was observed. The average liver dose and the volume of normal liver receiving 15 Gy or higher were reduced for 14% and 19%, respectively. Improved OAR sparing and target dose homogeneity were achieved simultaneously. Greater dosimetric gains were observed on lesions off patient center axis. Conclusions: Significantly improved dosimetry was achieved using the novel 4pi planning method compared with state‐of‐the‐art coplanar technique. 4pi technique is well suited for liver SBRT due to the organ laterality.
TH‐A‐213AB‐08: Robust Multi‐Criteria IMPT Optimization
Authors: Chen W , Unkelbach J , Trofimov A , Madden T , Kooy H , Bortfeld T , Craft D
Published: June 2012

Purpose: We present a method to include robustness in a multi‐criteria optimization (MCO) framework for intensity‐modulated proton therapy (IMPT). The approach allows one to simultaneously explore the trade‐off between different objectives including robustness and nominal plan quality. Methods: A database of plans each emphasizing different treatment planning objectives, is pre‐computed to approximate the Pareto surface. An IMPT treatment plan that strikes the best balance between the different objectives can be selected by navigating on the Pareto surface. We integrate robustness into MCO by adding robustified objectives and constraints. Uncertainties are modeled by pre‐calculated dose‐influence matrices for a nominal scenario and a number of pre‐defined error scenarios (shifted patient positions, proton beam undershoot and overshoot). A robustified objective represents the worst objective function value that can be realized for any of the error scenarios and thus provides a measure of plan robustness. The optimization method uses a linear projection solver and is capable of handling large problem sizes resulting from a fine dose grid resolution, many scenarios, and a large number of proton pencil beams. Results: A base‐of‐skull case is used to demonstrate that the robust optimization method reduces the sensitivity of the treatment plan to setup and range errors to a degree that is not achieved by a safety margin approach. A chordoma case is analyzed in more detail to demonstrate the involved trade‐offs between target underdose and brainstem sparing as well as robustness and nominal plan quality. The robust optimization for each Pareto optimal plan takes less than 5 min on a standard computer. Conclusions: The uncertainty pertinent to the IMPT procedure can be reduced during treatment planning by optimizing a database of plans that emphasize different treatment objectives, including robustness. The planner can then interactively explore all convex combinations of database plans to decide on the most‐preferred trade‐off.
TH‐A‐BRB‐02: Mechanistic Linear‐Quadratic‐Linear (LQL) Model for Large Doses Per Fraction
Authors: Guerrero M , Carlone M
Published: June 2012

The Linear‐Quadratic (LQ) model has been successfully used for at least two decades to quantify radiation effects at standard (1.2‐3Gy) doses. The validity of the standard LQ model for larger doses per fraction has been called into question by many authors, in part due to the continuous bending of the cell survival curve at large doses, contrary to results from in‐vitro experiments. In recent years, Stereotactic Body Radiation Therapy (SBRT) treatments with large fraction doses have become more common, making the modeling for large doses per fraction of significant clinical relevance. Many authors have proposed extensions of the LQ model to address the shape of the cell survival curve at larger doses by arbitrary adding a phenomenological parameter to the LQ model. In this work, we discuss a Linear‐Quadratic‐Linear (LQL) model with a mechanistic formulation. This LQL model has one additional parameter with respect to the standard LQ model and can be derived by mechanistic arguments. The additional parameter makes the survival curve linear at high doses and has been estimated using in‐vivo and in‐vitro data. The advantage of a mechanistic model is that it allows for modeling of radiation effects for any given dose‐ rate and fractionation scheme. As an example, we present the LQL prediction for split dose experiments and low‐dose‐rate (LDR) prostate brachytherapy. The LQL model is equivalent to other mechanistic models like the Lethal‐Potentially‐Lethal model (LPL) but it retains the well‐known parameters from the standard LQ model. As a clinical application, using estimated values of the additional parameter, we investigate the impact on the biologically effective dose (BED) for clinical SBRT treatment schedules of various cancer sites.

Learning Objectives:

1. Understand the limitations of the standard LQ model for large doses per fraction.

2. Understand the mechanistic formulation of the proposed LQL.

3. Understand how the LQL can be used to calculate the BED for SBRT radiation treatment schedules and compare them with standard fractionation.
TH‐A‐BRA‐10: The Role of Treatment Plan Adaptation during the Course of Proton Radiotherapy for Patients with Head and Neck Cancer
Authors: Shusharina N , Chan A , Adams J , Chen G , Sharp G
Published: June 2012

Purpose: To evaluate the advantages of adaptive replanning associated with proton radiotherapy treatment of patients with head and neck cancer. Methods: Six patients with tumors in the base of skull were treated with proton radiotherapy. Two CTs of the same patient, planning and repeat, were first registered rigidly in relation to bony structures of the skull to remove the setup error. A deformable B‐spline based registration was performed between the two CTs. The resulting deformation field was used to automatically transfer structure contours from the planning‐CT to the repeat‐CT. The original treatment plan was applied to the repeat‐CT and the dose distribution was recalculated. The differences between new and original dose distributions were evaluated by comparing isodose lines and by performing distance‐to‐agreement dose distribution analysis. The automatically transferred contours were used to compute dose‐volume histograms for the repeat‐CT and these were compared to the original DVHs. Results: Autotransferred contours were investigated by a physician and found satisfactory for the most critical structures. The targets were delineated manually. Anatomic changes caused by patient response to the treatment were primarily due to tumor shrinkage in the nasal cavity and in the paranasal sinuses. As a result the areas around target became highly heterogeneous with respect to density distribution. In 2 cases these heterogeneities resulted in hot spots within the tumor larger than 1 cm3 with a dose increase of 6%. In one case a cold spot with a dose decrease of 7% was observed. Conclusions: Evaluation of six cases shows no significant dosimetric changes that can favor adaptive replanning at the mid‐course of proton radiotherapy of head and neck cancer. Analysis of a larger patient set is needed to establish robust patterns of tissue deformation and dosimetric changes, with a focus on variations of the dose distribution in the presence of heterogeneities.
TH‐A‐217BCD‐02: BEST IN PHYSICS (IMAGING) ‐ Spectral Distortion Correction for Breast Tissue Composition Measurement Using Spectral CT
Authors: Zhao B , Ding H , Molloi S
Published: June 2012

Purpose: To investigate breast tissue compositions using a photon‐counting spectral Computed Tomography (CT) after spectral distortion correction. Methods: A total of 37 postmortem breast samples were scanned with a spectral CT based on a Cadmium‐Zinc‐Telluride (CZT) photon‐counting detector with five energy thresholds. All scans were acquired at beam energy of 100 kVp and a tube current of 1 mA. The energy thresholds of the CZT detector were set at 22, 42, 53, 65 and 90 keV. An image‐based spectral distortion correction method was used to eliminate the pulse pileup and other artifacts. Image‐based dual energy decomposition was performed with a three‐material calibration phantom to quantify postmortem breast tissue into water, lipid and protein contents. Results from dual energy decomposition were compared to chemical analysis. The Root Mean Square (RMS) errors for all samples were compared for images with and without spectral distortion correction. Results: The average RMS percentage errors of water, lipid, and protein over 37 breast samples, compared to data from chemical analysis, were 5.7% and 13.3% for images reconstructed with and without spectral distortion corrections, respectively. Conclusions: The results of this study suggest that the spectral distortion correction reduced the RMS percentage error of breast tissue composition by a factor greater than 2. With the proposed spectral distortion correction method, photon‐ counting detectors can be used for accurate quantitative tissue decomposition studies in spectral CT applications.
TH‐A‐217BCD‐04: Accuracy of Dual‐Energy CT Photon Cross‐Section Mapping Using a Non‐Separable Two Parameter Cross‐Section Model
Authors: Han D , Sampson A , Politte D , O'Sullivan J , Siebers J , Williamson J
Published: June 2012

Purpose: To quantify the achievable accuracy in estimating photon radiological quantities in the 20‐1000 keV range from idealized dual‐energy CT measurements using a modified non‐separable two‐parameter cross section model. Methods: The parametric fit model (PFM) [Med Phys 33:4115 2006] is modified based on method of Torikoshi et al [Phys Med Biol 48:673 2003], yielding a modified parametric fit model (mPFM) to describe attenuation coefficients by the sum of individual interaction cross‐sections. Cross‐sections are estimated as products of power functions of energy (E) and atomic number (Z), the Klein‐Nishina cross section, and slowly varying tables of correction factors to account for residual errors, F(E,Z) and G(E,Z), correcting the photoelectric and scattering processes respectively. For mixtures and compounds, each pair of dual energy measurements yields a pair of non‐separable nonlinear equations in two unknowns, effective atomic number (Z*) and electron density (ρe*) that can be solved iteratively. For mixtures describing the range of biological tissue compositions, the accuracy of the mPFM was compared to a previously described [Med Phys 33:4115 2006] basis vector model (BVM). Results: The mean percent absolute error for mPFM of linear attenuation coefficient, photoelectric interaction coefficient, and energy absorption coefficient of five mixtures and compounds are 0.03%–0.17%, 1.10%–6.03%, and 0.2%– 1.19%, compared to 0.02%–0.15%, 0.44%–11.32%, and 0.11%–1.57% for BVM, compared 0.6%–2.2%, 10.8%–22.4% and 5%–10% for PFM over the 20‐1000 keV range. mPFM reduces maximum errors to 0.15% compared to 4.49% for BVM for fluorine‐based tissue. Conclusions: mPFM improves accuracy of photon cross‐section compared to PFM. BVM shows greater accuracy than mPFM for all tested tissue except for Teflon. mPFM also has the potential to extend DECT to computation of charged particle radiological quantities. Supported by Grant R01‐CA149305 and P01‐CA‐ 11602
TH‐A‐217BCD‐07: Validation of Breast Density Measurement Using Dual Energy Mammography: A Post Mortem Study
Authors: Ducote J L , Ding H , Klopfer M , Molloi S
Published: June 2012

Purpose: To validate the use of dual energy mammography for the measurement of breast density in 40 post mortem ex vivo breast tissue samples. Methods: Twenty pairs of post mortem breast samples (n=40) were imaged with a full field digital mammography system. Low energy images were acquired at 28 kVp with a 50 μm rhodium filter at 60 mAs. High energy images were acquired at 49 kVp with a 300 μm copper filter at 30 mAs. Dual energy decomposition of the low and high energy images yielded individual pixel measurements of glandular and adipose equivalent material thicknesses. The decomposition was based on a previous calibration with glandular and adipose equivalent phantoms. Breast density (BD) was calculated as the ratio of glandular thickness over the sum total thickness. Each post mortem sample was then analyzed by chemical analysis based on desiccation, trituration and fat extraction to determine the weight and subsequent percent composition of water, lipid and lean (i.e. protein + mineral) contents. The fibroglandular ratio (FGR), a surrogate quantity to breast density, was calculated from the chemical analysis data as the ratio of the total volume of water and lean contents over the sum of all three contents. Results: Linear relationships between breast density and the water, lipid, and lean contents were all highly correlated. BD and FGR data were related by BD = 1.9 FGR ‐ 50 (r>0.99). Conclusions: The high correlation between breast composition data determined from images and data determined from chemical analysis serve to validate the use of dual energy mammography as a measurement technique. It is thought that more quantitative means of measuring breast density, like the one proposed here, will help to further enhance its use and utility as a marker and risk factor for the development of breast cancer.
TH‐A‐217A‐01: Image Gently and Image Wisely in Nuclear Medicine
Authors: Fahey F
Published: June 2012

There has been recent interest in the radiation dose delivered to the public as a result of medical imaging in both children and adults. Reports in the public press as well as hearings held by our public servants have concerned themselves with the need to be judicious about the use of medial imaging that involves ionizing radiation and to consider ways to reduce the radiation dose to the public without compromising the quality of the diagnostic information provided to our clinical collaborators. For these reasons, it is imperative that health professionals understand the nature of the health risks associated with ionizing radiation, the dosimetry in the context on medical imaging and ways that may be considered for the reduction of radiation dose. This presentation will cover these topics in the context nuclear medicine imaging performed in both children and adults. The basis for radiation risk estimation will be discussed as well as estimates of risk in the dose range associated with clinical nuclear medicine. Ways to communicate this risk to our patients and their families will also be described. The dosimetric models used in nuclear medicine, factors that affect dose and methods that may be used to reduce dose will be presented. The dosimetry of computed tomography in the context of hybrid imaging will also be described. Lastly, efforts and plans of both the Image Gently (in children) and Image Wisely (in adults) campaigns with respect to nuclear medicine will be discussed including the recently published North American Consensus Guidelines for Children and Adolescents developed by the Image Gently Group.

Learning Objectives:

1. List 2 factors that affect radiation dose in nuclear medicine

2. Discuss 3 approaches that may lead to dose reduction in nuclear medicine

3. Discuss the advantages and disadvantages of 3 ways of conversing about the radiation from nuclear medicine with their patients
TH‐C‐211‐01: How to Be a Journal Referee: A Panel Discussion
Authors: Becker S , Boone J , Orton C , Starkschall G , Thomadsen B
Published: June 2012

Being a referee for a journal article is an important yet daunting task. It is especially worrisome for someone who has little reviewing experience. It is important to remember is that it is an honor and perhaps a duty, but not a burden. Someone considers you to be an expert in the subject and it is with that mindset that you should approach the task.

Each part of the review process has its own difficulties:

• Grammar and Language: How much of this do you need to do? Does poor grasp of the English language warrant rejection? Is it effective or excessively verbose?

• Science: Is this project novel? How do you determine that? How similar can it be to other previously published manuscripts?

• References: Are the proper studies referenced? How much does this affect the review?

• Methods: How do you determine if the methods are sound? How much can you suggest without trying to be a collaborator?

• Results: Are graphs and tables being utilized wisely? Are there an excessive number of them?

All of these issues are important part of the review process and need to be considered. But how do you weight them in your final recommendation and communicate them to the authors and editors? This panel of senor medical physics, which has extensive experience in refereeing, will help guide the audience through the entire review process. These experts will share their experiences and methods to help new referees best prepare for the opportunity of reviewing manuscripts.

Learning Objectives:

1. Learn the role of the referee and what is expected of them.

2. Learn how to perform a review.

3. Learn how to communicate a review with authors and editors.
TH‐C‐213AB‐01: Spatial and Dose‐Response Analysis of Fibrotic Lung Changes After Stereotactic Body Radiation Therapy
Authors: Vinogradskiy Y , Diot Q , Kavanagh B , Gaspar L , Miften M
Published: June 2012

Purpose: To evaluate fibrotic lung changes in patients that underwent stereotactic body radiation therapy (SBRT) for lung cancer. Specifically, we contoured volumes visually observed to have fibrotic CT changes, performed a dose‐response analysis, and studied the spatial location and movement of the fibrotic volumes. Methods: Our study cohort included 51 SBRT patients and 231 follow up CT scans. The follow up CT scans were binned into intervals of 3, 6, 12, 18, 24, 30, 36, 42, and 48 months post therapy. Areas of visible fibrotic changes were contoured on follow up CTs. Deformable registration was used to align each follow up CT to the pre‐treatment CT scan. Spatial analysis was performed by determining the centroid of the gross tumor volume (GTV) and each fibrosis volume and analyzing the location and motion of the fibrosis volume relative to the GTV. A dose‐response analysis was performed by binning each voxel in the fibrosis volume into 1 0Gy dose‐bins and analyzing the CT numbers in each dose‐bin with respect to dose and time after treatment. Results: The distribution of fibrosis volumes was 51%, 39%, and 10% in the superior, middle, and inferior regions of the lung respectively. The average radial travel between the fibrosis volume centroid and GTV centroid was 2.6±0.2cm. The volume of the fibrosis region was greatest at 3 months (47cc) and decreased over time to 18cc at 46 months. Dose‐response data revealed increasing CT numbers as a function of dose (with no decline at higher doses) and increasing CT number as a function of time. Conclusions: The highest concentration of fibrosis volumes was located in the upper third of the lung. Future work will explore the relationship between fibrosis location, fibrosis volume, and dose. Our results show consolidation of the fibrotic volume and increase of CT number with dose and time.
TH‐C‐213AB‐05: Correlating Areas of Radiation Toxicity with Relative Biological Effectiveness‐Weighted Dose Distributions in Proton Radiotherapy
Authors: Peeler C , Mirkovic D , Grosshans D , Titt U , Mohan R
Published: June 2012

Purpose: To determine if biologically effective proton dose calculated using a relative biological effectiveness (RBE) model with multiple parameters may be used to explain areas of radiation induced brain necrosis indicated on follow‐up imaging studies for a patient treated with postoperative intensity modulated proton radiotherapy for malignant meningioma. Methods: In this study, treatment plan information is extracted from a clinical treatment planning system (TPS) and used to recalculate physical proton dose and linear energy transfer (LET) distributions using Monte Carlo simulations. An RBE model which accounts for tissue specific parameters, LET, biological endpoint, and dose per treatment fraction is applied on a voxel‐by‐voxel basis to compute a biological effective dose distribution. This dose distribution is subsequently compared to follow‐up imaging studies which have indicated the presence of radiation induced necrosis in the treated region of the patient being analyzed. Corresponding areas of toxicity and dose/RBE are compared to determine the governing relationship. Results: Areas of increased biological effective dose compared to the original dose distribution calculated by the clinical TPS are found to correspond to areas of radiation induced necrosis identified on follow‐up imaging. Biologically effective dose is found to be increased over TPS dose by as much as 8% of the prescribed dose for the patient. Conclusions: Dose distributions which account for the variable nature of proton RBE may provide insight into toxicity risk for patients treated with proton radiotherapy.
TH‐C‐213AB‐07: Extending the Webb‐Nahum Tumour Control Probability Model for Breast Conserving Therapy
Authors: Chen W , Gilhuijs K , Stroom J , Sonke J‐J
Published: June 2012

Purpose: Modeling tumour control probability (TCP) in breast conserving therapy (BCT) with the classic Webb‐Nahum radiotherapy model cannot explain the outcomes of treatments without radiotherapy (65% TCP at 10 years) from the Fifth Cycle data of the Early Breast Cancer Trialists' Collaborative Group (EBCTCG). The TCP model was extended to solve this inconsistency. Methods: We proposed a statistical model originating from the Webb‐Nahum model, but also considered tumour cell quantity as a heterogeneous variable and introduced a new heterogeneity factor: indolent‐cell fraction (ICF). With assumed Beta distribution, this ICF factor describes the proportion of tumour cells remaining after primary surgery which do not cause local recurrence even if radiotherapy is omitted. The residual microscopic tumour cell quantity was estimated using detailed digital analysis of the microscopic slides from breast‐cancer resection specimens (60 patients, 1818 slides). The ICF factor together with an approximation of radiosensitivity was estimated using Shor‐Zhurbenko optimization to fit data from the EORTC boost‐vs‐no‐boost trial to the new model. Uncertainties were incorporated using bootstrapping. The analysis was performed separately for younger (age = 50 years) and older patients. Finally, the radiotherapy dose‐TCP responses of two groups were compared. Results: The estimated number of residual microscopic tumour cells after surgery was significantly larger in younger patients (9.1e7) than in older patients (1.6e7, P<0.001). Using ICF increased the modeling accuracy at 0Gy for younger and older patients by 40% and 29%, respectively. The estimated tumour radiosensitivity was larger in younger patients than in older patients (0.060Gy‐1 vs 0.054Gy‐1, P<0.001), but the Beta parameter of ICF for younger patients was smaller (1.4e7 vs 1.8e7), which means older patients have a larger fraction of indolent tumour cells (P<0.001). This combination accurately described the TCP differences between these groups. Conclusions: The accuracy of TCP modeling in BCT was improved with our model. This research was supported by the Center for Translational Molecular Medicine (BreastCARE).
TH‐C‐BRB‐01: Credentialing Results from a Spine Anthropomorphic Phantom
Authors: Molineu A , Alvarez P , Hernandez N , Yin F , Followill D
Published: June 2012

Purpose: To use an anthropomorphic spine phantom for dosimetric credentialing purposes in National Cancer Institute sponsored clinical trials. Methods: An anthropomorphic spine phantom that consists of lungs, a heart, an esophagus, a spinal cord, vertebrae and an abutting planning target volume (PTV) was sent to institutions interested in becoming credentialed for a radiosurgery protocol for spine metastasis. The phantom contained film in 2 planes and TLD in the PTV. Institutions were asked to fill the phantom with water, image the phantom, create an IMRT plan for the spinal SBRT, perform image‐guidance for the target localization, and to irradiate the phantom. They were also asked to perform the type of patient specific measurements that they would for an actual patient. TLD inside the PTV are required to be within ±7%. 85% of the analyzed area of film was required to pass a gamma analysis of ±5%/3 mm. Results: 125 phantom treatment plans from 88 institutions were analyzed since 2009. Only 83 (66%) of the phantom irradiations passed the acceptance criteria. Of the 42 failures, 14 of the irradiations failed both the TLD and film criteria, 27 failed only the film criteria and 1 failed only the TLD criteria. Six institutions passed the criteria after including the couch in the calculation. The most represented planning systems, machine manufacturers and algorithm types have the ability to pass the phantom. Both pencil beam and superposition/convolution type algorithms can adequately account for the bone heterogeneity present in this phantom. Conclusion: The phantom test is a critical credentialing tool to secure the high quality protocol study. Treatment couch attenuation could affect tumor dose delivery accuracy, especially in instances where the majority of the beams pass through the couch. Supported by PHS grants CA10953 and CA81647 (NCI, DHHS). The investigation was supported by PHS grants CA10953 and CA81647 (NCI, DHHS).
TH‐C‐BRB‐03: External Beam Pulsed Low Dose Radiotherapy Using Volumetric Modulated Arc Therapy: Planning, Delivery and Verification
Authors: Tyagi N , Yang K , Sandhu R , Yan D
Published: June 2012

Purpose: To evaluate the mechanical stability and dosimetric accuracy of planning and delivering pulsed low‐dose radiotherapy (PLRT) using volumetric modulated arc therapy (VMAT) on Elekta linacs Methods: Ten patients previously treated for Glioblastoma Multiforme were replanned using PLRT VMAT to deliver ten 0.2 Gy pulses separated by 3 min intervals with an effective dose rate of 0.067 Gy/min. VMAT parameters such as leaf speed and arc length were optimized to deliver 2 Gy/fraction to a total of 60 Gy to the target volume in ten sub‐fractions or pulses. Plan quality was assessed using conformity and homogeneity indices. Absolute dose distribution for individual pulses was measured using the Arccheck cylindrical diode array. Individual pulses were analyzed for reproducibility and stability using machine log file saved in clinical mode. Machine characteristics at low monitor units and low dose rate were also investigated. Results: An optimal arc length of 140 – 160 degree and a leaf speed of 0.18 − 0.25 cm/degree were sufficient to provide stable delivery and equivalent plan coverage to IMRT. The average time and dose rate required to deliver a single 0.2 Gy pulse was 39.5 ± 2.3 seconds and 49 ± 32.3 cGy/min. Average reduction in MUs for the PLRT plan compared to IMRT for PTV was 16.0% (Range: −5.5% to 36.1%). Significant improvement was seen in maximum doses to all sensitive structures when planned with VMAT PLRT. The average absolute dose gamma passing rate for the 10 pulses combined and 2 Gy plan were 91.6 ± 2.5% and 97.3 ± 1.2%. Cumulative monitor units, dose rate, gantry angles and leaf positions evaluated using machine log files were within 2% for all pulses. Flatness and symmetry were within Elekta specifications. Conclusions: Elekta linacs are capable of delivering reproducible and stable PLRT plans. Prospective clinical study employing PLRT is currently in process.
TH‐C‐BRB‐04: Dose‐Painting VMAT and Patient Dose QA: Showcase of Whole Brain Radiotherapy with Hippocampal‐Sparing and Simultaneous Boost to Brain Metastases
Authors: Chan M , Zhang P , Yang J , Xiong J , Li J , Burman C , Parhar P , Schupak K , Mageras G
Published: June 2012

Purpose: Radiotherapy techniques for sparing the hippocampus during cranial irradiation are emerging. This work is to study dose‐painting with volumetric modulated arc therapy (VMAT) in the setting of oligo brain metastases and the clinically relevant dose QA. Methods: A patient having 3 brain metastases was re‐planned using 2‐arc 6 MV photons with in‐house VMAT treatment planning system (TPS). The clinical goal is to dose paint the whole brain and brain metastases to 32 Gy and 63 Gy in 15 fractions, respectively while keeping the mean dose to the hippocampus less than 6 Gy. The VMAT dose QA was carried out using ArcCHECK (AC) and 3DVH (Sun Nuclear Corp) on a Varian Trilogy LINAC. The AC Planned‐Dose‐Perturbation (ACPDP) algorithm was employed to estimate the actual patient dose distribution, at the same grid resolution as that from the TPS calculation. The patient's DICOM RT dose/plan/structures/CT images were loaded into the software. The measured doses and TPS calculated doses for relevant structures were compared. Their differences represented systematic errors induced from the combination of TPS dose calculation algorithm and beam‐delivery. Results: Clinical planning criteria were all satisfied. A 99.5% 3D matching rate for 2%/2mm DD/DTA was observed between the ACPDP reconstruction and TPS‐calculated dose. Despite the excellent agreement, the relative dose differences for patient's critical structures between measurements and TPS are: 0.8%, mean brain lesions; 0.7%, mean whole brain; 9% (<3% of prescription), mean L_hippocampus; 9% (<3% of prescription), mean R hippocampus; 0.1%, max cord; 1.8%, max chiasm; 3.7%, max brainstem; 0.5%, max L_OpticN; 1.9%, max R OpticN, 0.2%. Conclusions: For the application of dose painting VMAT planning for patients with oligo brain metastases, the patient relevant dose QA result has demonstrated the capability and the accuracy of our VMAT technique, allowing for 3‐level dose‐painting to spare critical structures with acceptable target coverage and homogeneity.
TH‐C‐BRB‐05: Monte Carlo Simulations for Quality Assurance of Varian TrueBeam 6MV FFF RapidArc SBRT Treatments
Authors: Teke T , Duzenli C , Mestrovic A , Hyde D , Milette M , Popescu I , Gete E
Published: June 2012

Purpose: To establish feasibility of performing quality assurance for Flattening Filter Free (FFF) RapidArc stereotactic body radiotherapy treatments (SBRT) on a TrueBeam LINAC using Monte Carlo simulations. Methods: Phase‐space files for TrueBeam FFF photon beams were made available by Varian in IAEA‐compliant format. Monte Carlo simulations were performed using BEAMnrc and DOSXYZnrc to validate the 6MV FFF phase space files for use in this study. The phase space data provided by Varian is in cylindrical geometry and required conversion into a format that was compatible with BEAMnrc prior to use. To establish validity of the phase space data, dose calculations in a water phantom for fields ranging from 3×3 cm̂2 to 40 × 40 cm̂2 were performed using DOSXYZnrc. Percent depth doses (PDDs), transverse profiles and output factors were calculated and compared with measurements. Monte Carlo simulations of 6MV FFF SBRT RapidArc plans were performed using a 2mm̂3 voxel size and compared with both ion chamber measurement and Eclipse Treatment Planning System (TPS) dose calculations. 3D gamma analysis (3%,3mm) comparing Monte Carlo and TPS results was performed. Results: Monte Carlo simulations and measured values agreed within 1% and 1.5% for PDDs and profiles respectively for all fields. The agreement between measured and calculated output factors was within 1 % including for highly asymmetric fields. These results indicate that the 6MV FFF phase space data is sufficiently accurate for use in quality assurance in radiation therapy. For the 6 MV FFF RapidArc plans the agreement between MC and both measured and TPS dose calculations was within 2%. Over 95% of the points passed the 3D Gamma test Conclusions: We have demonstrated the feasibility of performing patient specific quality assurance for 6 MV FFF SBRT RapidArc treatments using Monte Carlo simulations for a TrueBeam linac. This project is funded by Varian Medical Systems
TH‐C‐BRB‐11: Comparability of Four Different Planar IMRT QA Methods
Authors: Keeling V , Ahmad S , Jin H
Published: June 2012

Purpose: To investigate comparability of four different planar IMRT QA Methods: Patient Gantry Angle Composite (PGAC), Single Gantry Angle Composite (SGAC), Field by Field (FBF), and Patient Gantry Angle Composite using film (FPGAC). Methods: IMRT verification plans were generated by Varian Eclipse TPS for twelve heavily modulated head and neck (H&N) and twelve moderately modulated Prostate treatment plans. Sun Nuclear MapCHECK2 (2D diode array with detector spacing of 7.07 mm for SGAC, PGAC, and FBF) and GAFCHROMIC EBT2 film (0.34 mm/pixel for FPGAC) respectively sandwiched with 10 cm thick water equivalent phantom were used for the IMRT plan verification. All measurements were analyzed with Sun Nuclear ‘Patient’ software using the gamma criteria (MapCHECK2/Film) of 1%‐1mm/2%‐1mm (G1), 2%‐2mm/3%‐2mm (G2), and 3%‐3mm/4%‐3mm (G3) considering known 1% inherent dosimetric uncertainty of MapCHECK2. The Student's t‐test was used to compare passing rates with ‘Absolute Dose’ comparison and 10% threshold. Results: The average passing rates (H&N/Prostate) were 98.9/99.7% (PGAC), 99.6/100.0% (SGAC), 99.5/100.0% (FBF), and 92.0/95.8% (FPGAC) with the G3 criteria. Three MapCHECK2 QAs had statistically comparable passing rates for all comparisons (p‐value>0.05), while they did not show any significant difference based on the plan modulation. FPGAC showed systematically lower passing rates than the MapCHECK2 methods and statistically distinguished the degrees of modulation with all gamma criteria (p‐values of H&N against Prostate: 0.037 (G1), 0.012 (G2), and 0.016 (G3)). Conclusions: Three IMRT QA methods using MapCHECK2 are found to be comparably used for routine IMRT QA for both H&N and Prostate plans. This is against the belief that PGAC is more effective IMRT QA than FBF or SGAC. FPGAC showed significantly different passing rate based on the degree of plan modulation. These findings imply that the gamma passing rate heavily depends on the spatial resolution of detectors and there are intrinsic limitations of the gamma analysis.
TH‐C‐BRA‐03: Fast Iterative Cone Beam CT Reconstruction on GPGPU Using OpenCL
Authors: Zhou L , Chang J
Published: June 2012

Purpose: Algebraic reconstruction technique (ART) type algorithms produce superior image quality for CBCT and CT reconstructions over the popular filtered‐back‐projection based approaches but are too slow for real‐time clinical applications. The purpose of this study is to employ the emerging OpenCL architecture to accelerate simultaneous ART (SART) by parallelizing the most time‐consuming forward‐ and back‐projections using General‐Purpose‐Graphics‐Processing‐Unit (GPGPU). Methods: For each iteration, SART sequentially performs three ray‐driven projections (one forward‐ and two back‐projections) for each acquired projection image. To accelerate SART reconstruction, both forward projection and back‐projection kernels were scheduled on GPGPU using data parallelism to take full advantage of compute units on GPGPU. The single‐work‐item‐for‐single‐ray technique was employed as parallelization mechanism. We conducted numerical experiments to test OpenCL‐based implementation on a Dell Precision T7500 workstation with two quad‐core CPUs and one Nvidia Tesla C2050 GPGPU. Poly‐energetic projection data (512×512) for the Mohan 4 MV energy spectrum were simulated each degree for 360 gantry angles for a head‐and‐neck digital phantom and were fed into the SART algorithms for CBCT reconstruction of 256×256×256 volume. To accelerate poly‐energetic projection computation, we partitioned the workloads using task parallelism and data parallelism and scheduled them in a parallel computing ecosystem consisting of CPU and GPGPU using OpenCL only. Results: The GPGPU computation time including the kernel launch time, kernel running time and data transfer time was 42 ms for forward‐projection and 95 ms for back‐projection. Each SART iteration took 101 s on GPGPU in comparison to 7195 s on a single‐threaded CPU. The proposed method achieved a ∼71 ‐times speedup. The relative difference of the reconstructed images between the CPU‐based and OpenCL/GPGPU‐based implementations was on the order of 0.00001 and virtually indistinguishable. Conclusions: We have successfully implemented the SART algorithm on GPGPU using OpenCL and significantly reduced the reconstruction time to a level that is almost suitable for real‐time clinical applications.
TH‐C‐BRA‐04: Endorectal Balloons in Post‐Prostatectomy: Do Gains in Stability Lead to More Predictable Dosimetry?
Authors: Jameson M , Leon J De , Windsor A , Cloak K , Holloway L , Vial P , Sidhom M , Metcalfe P
Published: June 2012

Purpose: To perform a comparative study assessing potential benefits of endorectal balloons (ERB) in post‐prostatectomy patients Methods: Ten retrospective post‐prostatectomy patients without ERB and ten prospective patients with the ERB in situ were treated with IMRT using kilo voltage cone beam computed tomography (kVCBCT) image guidance (IGRT). A kVCBCT dataset from each treatment week was registered to the planning dataset, recontoured and the original plan recalculated on the kvCBCTs to recreate actual anatomical conditions during treatment. The imaging, structure and dose data were imported into Matlab. In house applications analysed geometric variation in the two groups and calculated cumulative equivalent uniform dose (EUD). Results: Analysis of the kVCBCT datasets showed that the ERB improved target volume stabilization from millimetre to sub‐millimetre reproducibility. The variation in bladder location in the non ERB cohort was 1.5 ± 0.9 cm compared to 0.6 ± 0.4 in the ERB group. While the superior rectum location variation was 1.1 ± 0.8 cm and 0.3 ± 0.3 cm for non ERB and ERB respectively. The mean difference in cumulative EUD and planned EUD for the clinical target volume (CTV) was 0.6 ± 0.04 Gy for non ERB and 0.4 ±0.1 Gy for ERB. The largest normal tissue differences were seen for the bladder with an increase of 4.4 ± 0.1 Gy for non ERB and 1.2 ± 0.02 Gy for ERB in the cumulative compared with the planned EUD. Differences in delivered superior rectal EUD was improved in the ERB cohort with a difference of−0.2 ± 0.04 Gy verse 3.0 ± 0.3 Gy in the non ERB group. Conclusions: The ERB cohort was found to have more stable and reproducible anatomy when compared to the non ERB cohort. Planned and cumulative EUD differences were greatest for the bladder and rectum.
TH‐C‐BRA‐05: High Quality Four Dimensional Cone‐Beam CT by Deforming Prior Planning CT
Authors: Wang J , Gu X , Solberg T
Published: June 2012

Purpose: To enhance the image quality of four dimensional cone beam computed tomography (4D‐CBCT) of lung cancer patients through using information from the initial planning CT. Method and Materials: 4D‐CBCT is obtained by deforming the planning CT. The estimation of the deformation vector fields (DVF) to deform the planning CT is formulated as a unconstrained optimization problem. The objective function to be minimized is the sum of the squared difference between the forward projection of the deformed planning CT and the measured 4D‐CBCT projection. A non‐linear conjugate gradient (NLGC) method is used to solve the DVF. As the number of the variables in the DVF is much greater than the number of measurements, the solution to such a highly ill‐posed problem is very sensitive to the initial parameters during the optimization process. We propose to estimate the DVF using demons registration between planning CT and 4D‐CBCT reconstructed by total variation minimization as the initial parameters in the optimization process. A 4D nonuniform rotational B‐spline based cardiac‐torso (NCAT) phantom and a patient 4D‐CBCT are used to evaluate the algorithm. Results: Image quality of 4D‐CBCT is substantially improved by using the proposed strategy in both NCAT phantom and patient studies. For the NCAT phantom, the relative error within lung in 4D‐CBCT reconstructed using the standard FDK from 20 projection views is 43.3%; the relative error is 21.5% in images reconstructed by total variation minimization, and further reduced to 12.5% in images reconstructed using the proposed strategy. Conclusion: Using the proposed strategy, high quality 4D‐CBCT can be obtained by deforming an initial planning CT . Results from the phantom evaluation study demonstrates that 20 projections are sufficient to reconstruct 4D‐CBCT of high accuracy. This work was supported in part by Cancer Prevention and Research Institute of Texas (CPRIT RP110562‐p2).
TH‐C‐BRA‐06: Measurement Based Source Model for On‐Board Imager Monte Carlo Dosimetry Simulations
Authors: McMillan K , Michailian A , McNitt‐Gray M , Ruan D
Published: June 2012

Purpose: The purpose of this study is to investigate and develop a systematic approach to quantify the dose impact of the radiographic and cone‐beam computed tomography (CBCT) modes of operation of the onboard imager (OBI) on radiotherapy treatment machines, independent of manufacturer's data specifications. Methods: In order to facilitate dose calculations using Monte Carlo (MC) methods, we have developed an equivalent source model for OBI systems consisting of an equivalent spectrum module incorporative of intrinsic filtration and an equivalent filter module characterizing the added bowtie filter. The equivalent spectrum was generated by iteratively updating the source spectrum estimate until the calculated beam behavior matched the measured half‐value layer and quarter‐value layer values. The equivalent filter was established by obtaining a series of exposure readings along the bowtie profile and calculating the equivalent path lengths of a particular filter material. The equivalent spectrum module was validated by assessing the consistency of the estimation results obtained from half‐ and full‐ bowtie measurements. The end‐to‐end system was validated by comparing dose distributions from MC simulations based on an equivalent spectrum and an equivalent filter with dose distributions from film measurements for both the full‐ and half‐bowtie scenario. Results: Results were generated using a 95 kVp beam. Agreement was observed between the equivalent source spectrums derived from measurements using either full‐ or half‐ bowtie filters. This consistency indirectly validates the consistency and accuracy of the equivalent source approach, as the experimental data are mutually independent, with source and internal filtration being the intrinsic common factor. A root‐mean‐square error of approximately 3% was observed between the MC and film measurement dose distributions for both bowtie filters. Conclusions: We have developed a source model that generates valid input for OBI MC dosimetry simulations. The method uses relatively simple measurement data, instead of manufacturer's specifications.
TH‐C‐BRA‐09: Robust 3D Lung Tumor Tracking Using Maximum Likelihood Estimation
Authors: Wang X , Kearney V , Yan H , Jia X , Cervino L , Jiang S
Published: June 2012

Purpose: To develop a robust 3D lung tumor tracking method based on a single x‐ray projection using maximum likelihood estimation. Methods: 4DCT images at different phases are registered to yield motion vector fields and a PCA based lung motion model is built. An acquired CBCT projection is compared with the simulated projections under different metrics to yield the correct PCA coefficients and hence the corresponding motion vector fields, which can be used to derive the 3D tumor location at this projection. The metrics we considered include normalized cross correlation and normalized mutual information between the gradient images of the acquired CBCT and that of the simulated 4DCT projection, and the position of diaphragm. A maximum likelihood estimation method is developed to combine the tracking results from all the metrics and integrate the constraints regarding tumor motion smoothness and wave form, leading to robust tumor locations. The method is tested on real patient data. Results: It is found that normalized cross correlation, normalized mutual information, and diaphragm location can be used to determine the PCA coefficients and the tumor position, although the efficacy of any single metric depends on the gantry angles and breathing phases. However, with the combination of different metrics, the tumor tracking accuracy is greatly improved. The resulting mean 3D error is found to be 1.3mm. Conclusions: A robust 3D tumor tracking method is developed based on maximum likelihood estimation to derive tumor location corresponding to an x‐ray projection for lung cancer patient. This work is supported by Varian Medical Systems through a Master Research Agreement.
TH‐C‐BRA‐10: An Open‐Source 2D/3D‐Image‐Registration Algorithm: Cranial Image Guided Radiotherapy
Authors: Warmerdam G , Steininger P , Neuner M , Sharp G , Winey B
Published: June 2012

Purpose: To determine the robustness and accuracy of an open source 2D/3D GPU accelerated image registration algorithm in the context of cranial image guided radiotherapy. Methods: The open source 2D/3D image registration algorithm, Reg23, has been released under the GNU license. The algorithm utilizes an iterative digitally reconstructed radiograph (DRR) approach to the image registration problem. The DRR generator is accelerated on a GPU to rapidly iterate the optimization process. Multiple cost functions are supported and were analyzed. Robustness was determined by comparing a baseline set of orthogonal kV images of a cranial phantom with a predetermined isocenter to the planned isocenter in the CT image set and introducing more than 6000 combinations of rotations and translation to the position of the isocenter in the CT. Accuracy and time efficiency of various cost function were analyzed for the virtual patient shifts. Furthermore, a set of 43 experimental orthogonal images were acquired with a linac mounted kV imaging system of predetermined physical shifts which were compared to the results of the Reg23 algorithm. Results: The Reg23 algorithm was found to be accurate to 0.04±0.02mm for the virtual isocenter shifts and 0.23±0.40mm for real images compared to the CBCT registration results. Time to solution could be reduced from >70 s to < 40 s without a significant change in the algorithm accuracy depending upon the cost function employed. Conclusions: The Reg23 algorithm is robust and sensitive to sub‐mm variations of virtual shifts of the isocenter position. The Normalized Cross Correlation (NCC) cost function was determined to be most accurate and fastest for cranial image registration. For real experimental data, the Gradent Difference (GD) cost function was most accurate and both GD and NCC delivered results accurate to within 0.5 mm and 0.4° when compared to CBCT/CT registrations.