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To explore implications of various plan normalizations when implementing a linear Boltzmann transport equation solver dose calculation algorithm (LBTE) for lung stereotactic body radiation therapy (SBRT). Eighty-seven plans originally calculated with a convolution-superposition algorithm (CS) were recalculated with LBTE and normalized in 3 ways: prescription covering 95% of planning target volume (PTV), 99% of internal target volume (ITV), and keeping the original planned PTV coverage. Effect on delivered dose after implementing the new algorithm was quantified using change in total monitor units for each renormalization strategy. Treatment planning system–reported changes in PTV, ITV, and organ at risk (OAR) doses were also quantified, along with the feasibility of LBTE plans to meet institutional OAR planning objectives. LBTE renormalization resulted in monitor unit increases of 7.0 ± 8.8%, 0.31 ± 5.8%, and 7.9 ± 8.6% when normalizing to the PTV D95%, ITV D99%, and planned coverage, respectively. When normalizing to PTV D95%, the LBTE reported increased PTV and ITV D1% (Gy) relative to CS (median, 3.4% and 3.2%, respectively), and normalizing to ITV D99% showed a median 1.9% decrease. For LBTE plans, reported OAR doses were increased when normalizing to PTV D95% or planned coverage (median chest wall V30 Gy [cc] increase of 0.85 and 1.7 cc, respectively) and normalizing to ITV D99% resulted in decreased dose (median chest wall V30 Gy [cc] decrease of 1.8 cc). LBTE plans normalized to PTV D95% showed inferior ability to meet the OAR objectives, but reoptimizing kept the objectives manageable while maintaining PTV coverage. When transitioning from CS to LBTE dose calculation for lung SBRT, maintaining a PTV coverage-based normalization generally results in increased dose delivered relative to CS and increased reported target and OAR dose. In cases where PTV normalization results in unacceptably high doses to targets or OARs, normalizing based on ITV coverage can be considered to maintain similar target dose as CS. [ABSTRACT FROM AUTHOR]

Purpose: To determine clinic-specific linear accelerator quality assurance (QA) TG-142 test frequencies, to maximize physicist time efficiency and patient treatment quality.Methods and Materials: A novel quantitative approach to failure mode and effect analysis is proposed. Nine linear accelerator-years of QA records provided data on failure occurrence rates. The severity of test failure was modeled by introducing corresponding errors into head and neck intensity modulated radiation therapy treatment plans. The relative risk of daily linear accelerator QA was calculated as a function of frequency of test performance.Results: Although the failure severity was greatest for daily imaging QA (imaging vs treatment isocenter and imaging positioning/repositioning), the failure occurrence rate was greatest for output and laser testing. The composite ranking results suggest that performing output and lasers tests daily, imaging versus treatment isocenter and imaging positioning/repositioning tests weekly, and optical distance indicator and jaws versus light field tests biweekly would be acceptable for non-stereotactic radiosurgery/stereotactic body radiation therapy linear accelerators.Conclusions: Failure mode and effect analysis is a useful tool to determine the relative importance of QA tests from TG-142. Because there are practical time limitations on how many QA tests can be performed, this analysis highlights which tests are the most important and suggests the frequency of testing based on each test's risk priority number. [ABSTRACT FROM AUTHOR]

Objective Our purpose was to address challenges associated with single-isocenter radiosurgery for multiple intracranial targets (SIRMIT) including increased sensitivity to rotational uncertainties (resulting from distance of the targets from isocenter) as well as potential for decreased plan quality from larger multileaf collimator width > 4 cm from isocenter. Methods and materials We evaluated the effect that a 6 degrees-of-freedom couch correction had on localization uncertainty for SIRMIT using thermoplastic mask immobilization. Required setup margin was determined from rotation of the skull and mask (setup kV cone beam computed tomography relative to planning computed tomography). Intraoperational margin was determined from skull rotation within the mask (difference between pre- and posttreatment cone beam computed tomography). We also investigated 4 isocenter placement strategies: volume centroid, centroid of equally weighted points (1 per target), centroid of points weighted by inverse of volume, and Eclipse’s built-in method. Results When no 6 degrees-of-freedom couch correction is performed after initial setup, a 0.35-mm margin is required per centimeter of target-isocenter separation to account for 95% of rotational uncertainties at initial setup. This margin is reduced to 0.10 mm/cm of target-isocenter separation to account for intraoperative rotational uncertainties when the initial setup uncertainty is eliminated via image guided 6 degrees-of-freedom couch correction. Analysis of 11 multitarget plans (37 targets) showed that conformity index and gradient index improved with decreasing distance from isocenter, this trend being more pronounced for targets < 1 mL. Alternative isocenters aimed at decreasing distance of small targets improved their gradient index, but resulted in poorer dose indices for large targets. Mean distance from isocenter was smallest for the centroid of equally weighted points (4.1 ± 1.6cm vs 4.2-4.5cm). Conclusions Rotational corrections via image guidance are necessary for SIRMIT with a thermoplastic mask for immobilization. There is a clear tradeoff between dosimetric quality of small and large targets that should be considered carefully when placing the isocenter. [ABSTRACT FROM AUTHOR]

Abstract: Purpose: The phase 2 component of Radiation Therapy Oncology Group (RTOG) 0631 assessed the feasibility and safety of spine radiosurgery (SRS) for localized spine metastases in a cooperative group setting. Methods and Materials: Patients with 1-3 spine metastasis with a Numerical Rating Pain Scale (NRPS) score ≥5 received 16 Gy single fraction SRS. The primary endpoint was SRS feasibility: image guidance radiation therapy (IGRT) targeting accuracy ≤2 mm, target volume coverage >90% of prescription dose, maintaining spinal cord dose constraints (10 Gy to ≤10% of the cord volume from 5-6 mm above to 5-6 mm below the target or absolute spinal cord volume <0.35 cc) and other normal tissue dose constraints. A feasibility success rate <70% was considered unacceptable for continuation of the phase 3 component. Based on the 1-sample exact binomial test with α = 0.10 (1-sided), 41 patients were required. Acute toxicity was assessed using the National Cancer Institute Common Toxicity Criteria for Adverse Events, version 3.0. Results: Sixty-five institutions were credentialed with spine phantom dosimetry and IGRT compliance. Forty-six patients were accrued, and 44 were eligible. There were 4 cervical, 21 thoracic, and 19 lumbar sites. Median NRPS was 7 at presentation. Final pretreatment rapid review was approved in 100%. Accuracy of image guided SRS targeting was in compliance with the protocol in 95%. The target coverage and spinal cord dose constraint were in accordance with the protocol requirements in 100% and 97%. Overall compliance for other normal tissue constraints was per protocol in 74%. There were no cases of grade 4-5 acute treatment-related toxicity. Conclusions: The phase 2 results demonstrate the feasibility and accurate use of SRS to treat spinal metastases, with rigorous quality control, in a cooperative group setting. The planned RTOG 0631 phase 3 component will proceed to compare pain relief and quality of life between SRS and external beam radiation therapy. [Copyright &y& Elsevier]

Abstract: The ExacTrac X-Ray 6D image-guided radiotherapy (IGRT) system will be described and its performance evaluated. The system is mainly an integration of 2 subsystems: (1) an infrared (IR)-based optical positioning system (ExacTrac) and (2) a radiographic kV x-ray imaging system (X-Ray 6D). The infrared system consists of 2 IR cameras, which are used to monitor reflective body markers placed on the patient''s skin to assist in patient initial setup, and an IR reflective reference star, which is attached to the treatment couch and can assist in couch movement with spatial resolution to better than 0.3 mm. The radiographic kV devices consist of 2 oblique x-ray imagers to obtain high-quality radiographs for patient position verification and adjustment. The position verification is made by fusing the radiographs with the simulation CT images using either 3 degree-of-freedom (3D) or 6 degree-of-freedom (6D) fusion algorithms. The position adjustment is performed using the infrared system according to the verification results. The reliability of the fusion algorithm will be described based on phantom and patient studies. The results indicated that the 6D fusion method is better compared to the 3D method if there are rotational deviations between the simulation and setup positions. Recently, the system has been augmented with the capabilities for image-guided positioning of targets in motion due to respiration and for gated treatment of those targets. The infrared markers provide a respiratory signal for tracking and gating of the treatment beam, with the x-ray system providing periodic confirmation of patient position relative to the gating window throughout the duration of the gated delivery. [Copyright &y& Elsevier]

Abstract: Purpose: An artificial intelligence (AI) guided method for parameter adjustment of inverse planning was implemented on a commercial inverse treatment planning system. For evaluation purpose, four typical clinical cases were tested and the results from both plans achieved by automated and manual methods were compared. Methods and materials: The procedure of parameter adjustment mainly consists of three major loops. Each loop is in charge of modifying parameters of one category, which is carried out by a specially customized fuzzy inference system. A physician prescribed multiple constraints for a selected volume were adopted to account for the tradeoff between prescription dose to the PTV and dose–volume constraints for critical organs. The searching process for an optimal parameter combination began with the first constraint, and proceeds to the next until a plan with acceptable dose was achieved. The initial setup of the plan parameters was the same for each case and was adjusted independently by both manual and automated methods. After the parameters of one category were updated, the intensity maps of all fields were re-optimized and the plan dose was subsequently re-calculated. When final plan arrived, the dose statistics were calculated from both plans and compared. Results: For planned target volume (PTV), the dose for 95% volume is up to 10% higher in plans using the automated method than those using the manual method. For critical organs, an average decrease of the plan dose was achieved. However, the automated method cannot improve the plan dose for some critical organs due to limitations of the inference rules currently employed. For normal tissue, there was no significant difference between plan doses achieved by either automated or manual method. Conclusion: With the application of AI-guided method, the basic parameter adjustment task can be accomplished automatically and a comparable plan dose was achieved in comparison with that achieved by the manual method. Future improvements to incorporate case-specific inference rules are essential to fully automate the inverse planning process. [Copyright &y& Elsevier]

Purpose: Cone-beam computed tomography (CBCT) images are currently used for positioning verification. However, it is yet unknown whether CBCT could be used in dose calculation for replanning in adaptive radiation therapy. This study investigates the dosimetric feasibility of CBCT-based treatment planning. Methods and Materials: Hounsfield unit (HU) values and profiles of Catphan, homogeneous/inhomogeneous phantoms, and various tissue regions of patients in CBCT images were compared to those in CT. The dosimetric consequence of the HU variation was investigated by comparing CBCT-based treatment plans to conventional CT-based plans for both phantoms and patients. Results: The maximum HU difference between CBCT and CT of Catphan was 34 HU in the Teflon. The differences in other materials were less than 10 HU. The profiles for the homogeneous phantoms in CBCT displayed reduced HU values up to 150 HU in the peripheral regions compared to those in CT. The scatter and artifacts in CBCT became severe surrounding inhomogeneous tissues with reduced HU values up to 200 HU. The MU/cGy differences were less than 1% for most phantom cases. The isodose distributions between CBCT-based and CT-based plans agreed very well. However, the discrepancy was larger when CBCT was scanned without a bowtie filter than with bowtie filter. Also, up to 3% dosimetric error was observed in the plans for the inhomogeneous phantom. In the patient studies, the discrepancies of isodose lines between CT-based and CBCT-based plans, both 3D and IMRT, were less than 2 mm. Again, larger discrepancy occurred for the lung cancer patients. Conclusion: This study demonstrated the feasibility of CBCT-based treatment planning. CBCT-based treatment plans were dosimetrically comparable to CT-based treatment plans. Dosimetric data in the inhomogeneous tissue regions should be carefully validated. [ABSTRACT FROM AUTHOR]

Purpose: To generate on-board digital tomosynthesis (DTS) and reference DTS images for three-dimensional image-guided radiation therapy (IGRT) as an alternative to conventional portal imaging or on-board cone-beam computed tomography (CBCT). Methods and Materials: Three clinical cases (prostate, head-and-neck, and liver) were selected to illustrate the capabilities of on-board DTS for IGRT. Corresponding reference DTS images were reconstructed from digitally reconstructed radiographs computed from planning CT image sets. The effect of scan angle on DTS slice thickness was examined by computing the mutual information between coincident CBCT and DTS images, as the DTS scan angle was varied from 0 degrees to 165 degrees . A breath-hold DTS acquisition strategy was implemented to remove respiratory motion artifacts. Results: Digital tomosynthesis slices appeared similar to coincident CBCT planes and yielded substantially more anatomic information than either kilovoltage or megavoltage radiographs. Breath-hold DTS acquisition improved soft-tissue visibility by suppressing respiratory motion. Conclusions: Improved bony and soft-tissue visibility in DTS images is likely to improve target localization compared with radiographic verification techniques and might allow for daily localization of a soft-tissue target. Breath-hold DTS is a potential alternative to on-board CBCT for sites prone to respiratory motion. [ABSTRACT FROM AUTHOR]

Oligometastases refer to metastases that are limited in number and location and are amenable to regional treatment. The majority of these metastases appear in the brain, lung, liver, and bone. Although the focus of interest in the past within radiation oncology has been on the treatment of intracranial metastases, there has been growing interest in extracranial sites such as the liver and lung. This is largely because of the rapid development of targeting techniques for oligometastases such as intensity-modulated and image-guided radiation therapy, which has made it possible to deliver single or a few fractions of high-dose radiation treatments, highly conformal to the target. The clinical decision to use radiation to treat oligometastases is based on both radiobiological and physics considerations. The radiobiological considerations involve improvement of treatment schema for time, dose, and volume. Areas of interests are hypofractionation, tumor and normal tissue tolerance, and hypoxia. The physics considerations for oligometastases treatment are focused mainly on ensuring treatment accuracy and precision. This article discusses the physics and imaging aspects involved in each step of the radiation treatment process for oligometastases, including target definition, treatment simulation, treatment planning, pretreatment target localization, radiation delivery, treatment verification, and treatment evaluation. [Copyright &y& Elsevier]

Purpose: To develop a spatiotemporal-constrained sorting technique for motion-robust 4 dimensional-magnetic resonance imaging.Methods and Materials: This sorting method implemented 2 new approaches for 4-dimensional imaging: (1) an optimized sparse k-space acquisition trajectory with self-gating signal derivation, and (2) a retrospective k-space sorting for reconstruction using a novel spatiotemporal-constrained strategy to minimize breathing variation-induced motion artifacts. Such sorting was regularized by a spatiotemporal index. Volumetric reconstruction was implemented iteratively with a secnd-order total generalized variation penalty. The proposed method was evaluated and compared with the conventional phase-sorting and amplitude-sorting methods in 2 studies. In a computer simulation study, 6 abdominal motion scenarios, including 2 cosine and 4 patient breathing motion patterns, were studied. Reconstruction accuracy was evaluated quantitatively in reference to the ground truth by average image relative error (IRE) in 10 phases and target Dice similarity coefficients (DSCs) in end-of-exhalation/inhalation phases. In addition, the proposed method was evaluated using a custom-made motion phantom. Reconstruction accuracy was evaluated by motion range measurement and image quality comparison in both fast and slow breathing motions.Results: In the simulation study, stitching motion artifacts in restricted images were lessened using the proposed method compared with those using the conventional methods. The average IRE and target DSC (end-of-exhalation/inhalation) were 0.031 and 0.95/0.94, respectively, suggesting better motion reconstruction accuracy than the phase-sorted method (IRE, 0.057; DSC, 0.89/0.89) and the amplitude-sorted method (IRE, 0.048; DSC, 0.91/0.88). In the phantom study, the moving target reconstructed by the proposed method demonstrated better rendering with less edge blurring. With fast breathing motion, the range measured using the proposed method was more accurate than that of the phase-sorted method and was comparable to the result of amplitude-sorted method and ground truths.Conclusions: Preliminary results suggested that the proposed sorting technique could reconstruct high-quality images and accurate motion estimation with reduced artifacts in 4 dimensional-magnetic resonance imaging. [ABSTRACT FROM AUTHOR]

Abstract: Purpose: To evaluate how well tumor motion measured prior to treatment based on 4-dimensional computer tomography (4DCT) reflects actual tumor motion during beam-on throughout the course of treatment. Methods and Materials: Twenty-three patients who had lung stereotactic body radiation therapy (SBRT) treatments were retrospectively selected. All patients had 4DCT simulation for treatment planning, from which tumor motion ranges were measured (R4DCT). Tumor motion was monitored during treatment using megavoltage (MV) imaging. Tumor motion trajectories were extracted from cine MV images and were used to determine mean and maximum tumor motion range (Mean RMV, Max RMV) throughout entire course of treatment. Comparison and correlations between mean and max RMV and R4DCT were calculated. Results: On average, an insignificant difference was found between mean RMV and R4DCT (P = .67, mean [±SD] difference = −0.7 [±1.6] mm); meanwhile a significant difference was found between Max RMV and R4DCT (P = .03, mean [± SD] difference = 1.9 [±1.6] mm). The difference between RMV and R4DCT was found inversely proportional to R4DCT (Y = −0.4X + 0.6, r = 0.76). Max RMV was greater than R4DCT in all patients; difference between the 2 showed no correlation with R4DCT (Y = −0.02X + 1.9, r = 0.05). Correlation between Mean RMV and R4DCT and between Max RMV and R4DCT can be expressed as Y = 0.7X (r = 0.88) and Y = 0.8X (r = 0.50), respectively. The same analysis performed on tumors that moved less than 5 mm from 4DCT revealed the following correlations: Y = 1.3X (r = 0.83) and Y = 1.7X (r = 0.49). Conclusions: Tumor motion measured from 4DCT approximates the overall average tumor motion range, but consistently underestimates the overall maximum tumor motion range. These findings may lead to a potential strategy for managing uncertainties of 4DCT in the application of lung SBRT. [Copyright &y& Elsevier]

Previous work demonstrated improved dosimetry of single isocenter volumetric modulated arc therapy (VMAT) of multiple intracranial targets when they are located ≤ 4 cm from isocenter because of narrower multileaf collimators (MLCs). In follow-up, we sought to determine if decreasing isocenter-target distance (d iso ) by using 2 to 3 isocenters would improve dosimetry for spatially dispersed targets. We also investigated the effect of a maximum dose constraint during VMAT optimization, and the dosimetric effect of the number of VMAT arcs used for a larger number of targets ( i.e. , 7 to 9). We identified radiosurgery cases that had multiple intracranial targets with d iso of at least 1 target > 5 cm. A single isocenter VMAT plan was created using a standardized 4-arc technique with 18 Gy per target. Each case was then replanned (1) using 2 to 3 isocenters, (2) including a maximum dose constraint per target, and in the case of 7 to 9 targets, (3) using 3 to 6 arcs. Dose evaluation included brain V 6 Gy and V 12 Gy , and conformity index (CI), gradient index (GI), and heterogeneity index (HI) per target. Two isocenters were sufficient to limit d iso to ≤ 4 cm and ≤ 5 cm for 11/15 and 13/15 cases, respectively; after replanning with 2 to 3 isocenters, d iso decreased from 5.8 ± 2.8 cm (2.3 14.9) to 2.5 ± 1.4 cm (0 5.2). All dose statistics improved on average, albeit modestly: V 6 Gy = 6.9 ± 7.1%, V 12 Gy = 0.9% ± 4.4%, CI = 2.6% ± 4.6%, GI = 0.9% ± 12.7%, and HI = 2.6% ± 5.2%; however, the number of arcs doubled and monitor units increase by nearly 2-fold. A maximum dose constraint had a negative effect on all dose indices, increasing V 12 Gy by 9.7 ± 6.9%. For ≥ 7 targets, increasing number of arcs to > 3 improved CI, V 12 Gy , and V 6 Gy . A single isocenter is likely sufficient for VMAT radiosurgery of multiple intracranial metastases. Optimal treatment plan quality is achieved when no constraint is placed on the maximum target dose; for cases with many targets at least 4 arcs are needed for optimal plan quality. [ABSTRACT FROM AUTHOR]

Abstract: We propose a method to quantify superior–inferior (SI) motion of a rigid target using the 3D contour from free-breathing CT (FBCT). The technique utilizes similarity between 2D contours (Jaccard Index) and a population based density function for probability of motion amplitude, and is applicable both when the static target shape is and is not known beforehand. Simulations and phantom measurements showed that motion reconstruction is often feasible, with decreasing accuracy as discrepancy is introduced between assumed and actual static shape. When no static shape is used the analysis is most robust for slow scanning speeds relative to the motion period. [Copyright &y& Elsevier]

Purpose: To investigate whether the effect of organ motion can be further reduced with the application of a cardiac gating technique, together with respiratory gating. Methods and Materials: Axial and coronal images through the heart and liver were continuously scanned with fast cine magnetic resonance imaging scans at three different gating settings: (1) without respiratory and cardiac gating; (2) with respiratory gating, but without cardiac gating; and (3) with both respiratory and cardiac gating. The effect of motion for either the heart or liver was analyzed with probability maps. Results: With the application of respiratory gating only, the marginal region on the probability map was reduced by 10.0% in the axial slice and 19.8% in the coronal slice for the heart. It was reduced by 5.2% in the axial slice and 20.8% in the coronal slice for the liver. With the application of cardiac gating together with respiratory gating, the marginal region on the probability map was reduced further. The reduction was 8.0% in the axial slice and 13.6% in the coronal slice for the heart and 5.9% in the axial slice and 7.0% in the coronal slice for the liver. Conclusion: The effect of organ motion can be further reduced with the application of cardiac gating together with respiratory gating. The potential application to treatment planning merits further investigation. [Copyright &y& Elsevier]

Purpose: Pancreas stereotactic body radiation therapy (SBRT) treatment planning requires planners to make sequential, time-consuming interactions with the treatment planning system to reach the optimal dose distribution. We sought to develop a reinforcement learning (RL)-based planning bot to systematically address complex tradeoffs and achieve high plan quality consistently and efficiently.Methods and Materials: The focus of pancreas SBRT planning is finding a balance between organ-at-risk sparing and planning target volume (PTV) coverage. Planners evaluate dose distributions and make planning adjustments to optimize PTV coverage while adhering to organ-at-risk dose constraints. We formulated such interactions between the planner and treatment planning system into a finite-horizon RL model. First, planning status features were evaluated based on human planners' experience and defined as planning states. Second, planning actions were defined to represent steps that planners would commonly implement to address different planning needs. Finally, we derived a reward system based on an objective function guided by physician-assigned constraints. The planning bot trained itself with 48 plans augmented from 16 previously treated patients, and generated plans for 24 cases in a separate validation set.Results: All 24 bot-generated plans achieved similar PTV coverages compared with clinical plans while satisfying all clinical planning constraints. Moreover, the knowledge learned by the bot could be visualized and interpreted as consistent with human planning knowledge, and the knowledge maps learned in separate training sessions were consistent, indicating reproducibility of the learning process.Conclusions: We developed a planning bot that generates high-quality treatment plans for pancreas SBRT. We demonstrated that the training phase of the bot is tractable and reproducible, and the knowledge acquired is interpretable. As a result, the RL planning bot can potentially be incorporated into the clinical workflow and reduce planning inefficiencies. [ABSTRACT FROM AUTHOR]

Linac based radiosurgery to multiple metastases is commonly planned with volumetric modulated arc therapy (VMAT) as it effectively achieves high conformality to complex target arrangements. However, as the number of targets increases, VMAT can struggle to block between targets, which can lead to highly modulated and/or nonconformal multi-leaf collimator (MLC) trajectories that unnecessarily irradiation of healthy tissue. In this study we introduce, describe, and evaluate a treatment planning technique called Conformal Arc Informed VMAT (CAVMAT), which aims to reduce the dose to healthy tissue while generating highly conformal treatment plans. CAVMAT is a hybrid technique which combines the conformal MLC trajectories of dynamic conformal arcs with the MLC modulation and versatility of inverse optimization. CAVMAT has 3 main steps. First, targets are assigned to subgroups to maximize MLC blocking between targets. Second, arc weights are optimized to achieve the desired target dose, while minimizing MU variation between arcs. Third, the optimized conformal arc plan serves as the starting point for limited inverse optimization to improve dose conformity to each target. Twenty multifocal VMAT cases were replanned with CAVMAT with 20Gy applied to each target. The total volume receiving 2.5 Gy c m 3 , 6 Gy c m 3 , 12 Gy c m 3 , and 16 Gy c m 3 , conformity index, treatment delivery time, and the total MU were used to compare the VMAT and CAVMAT plans. In addition, CAVMAT was compared to a broad range of planning strategies from various institutions (108 linear accelerator based plans, 14 plans using other modalities) for a 5-target case utilized in a recent plan challenge. For the linear accelerator-based plans, a plan complexity metric based on aperture opening area and perimeter, total monitor units (MU), and MU for a given aperture opening was utilized in the plan challenge scoring algorithm to compare the submitted plans to CAVMAT. After re-planning the 20 VMAT cases, CAVMAT reduced the average V 2.5Gy c m 3 by 25.25 ± 19.23%, V 6Gy c m 3 by 13.68 ± 18.97%, V 12Gy c m 3 by 11.40 ± 19.44%, and V 16Gy c m 3 by 6.38 ± 19.11%. CAVMAT improved conformity by 3.81 ± 7.57%, while maintaining comparable target dose. MU for the CAVMAT plans increased by 24.35 ± 24.66%, leading to an increased treatment time of 2 minutes. For the plan challenge case, CAVMAT was 1 of 12 linac based plans that met all plan challenge scoring criteria. Compared to the average submitted VMAT plan, CAVMAT increased the V 10% Gy[%] of healthy tissue (Brain-PTV) by roughly 3.42%, but in doing so was able to reduce the V 25% Gy[%] by roughly 3.73%, while also reducing V 50% Gy[%] , V 75% Gy[%], and V 100% Gy[%]. The CAVMAT technique successfully eliminated insufficient MLC blocking between targets prior to the inverse optimization, leading to less complex treatment plans and improved tissue sparing. Tissue sparing, improved conformity, and decreased plan complexity at the cost of slight increase in treatment delivery time indicates CAVMAT to be a promising method to treat brain metastases. [ABSTRACT FROM AUTHOR]

Purpose: To develop a tradeoff hyperplane model to facilitate tradeoff decision-making before inverse planning.Methods and Materials: We propose a model-based approach to determine the tradeoff hyperplanes that allow physicians to navigate the clinically viable space of plans with best achievable dose-volume parameters before planning. For a given case, a case reference set (CRS) is selected using a novel anatomic similarity metric from a large reference plan pool. Then, a regression model is built on the CRS to estimate the expected dose-volume histograms (DVHs) for the current case. This model also predicts the DVHs for all CRS cases and captures the variation from the corresponding DVHs in the clinical plans. Finally, these DVH variations are analyzed using the principal component analysis to determine the tradeoff hyperplane for the current case. To evaluate the effectiveness of the proposed approach, 244 head and neck cases were randomly partitioned into reference (214) and validation (30) sets. A tradeoff hyperplane was built for each validation case and evenly sampled for 12 tradeoff predictions. Each prediction yielded a tradeoff plan. The root-mean-square errors of the predicted and the realized plan DVHs were computed for prediction achievability evaluation.Results: The tradeoff hyperplane with 3 principal directions accounts for 57.8% ± 3.6% of variations in the validation cases, suggesting the hyperplanes capture a significant portion of the clinical tradeoff space. The average root-mean-square errors in 3 tradeoff directions are 5.23 ± 2.46, 5.20 ± 2.52, and 5.19 ± 2.49, compared with 4.96 ± 2.48 of the knowledge-based planning predictions, indicating that the tradeoff predictions are comparably achievable.Conclusions: Clinically relevant tradeoffs can be effectively extracted from existing plans and characterized by a tradeoff hyperplane model. The hyperplane allows physicians and planners to explore the best clinically achievable plans with different organ-at-risk sparing goals before inverse planning and is a natural extension of the current knowledge-based planning framework. [ABSTRACT FROM AUTHOR]

Purpose: Imaging parameters from 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) before and after chemoradiation therapy (CRT) for anal canal cancer correlate with clinical outcomes. This prospective, hypothesis-generating pilot study investigates the relationship between interim PET imaging during CRT for anal canal cancer and clinical outcome.Methods and Materials: From June 2012 to August 2015, 30 patients with anal canal cancer were enrolled in a prospective clinical study of PET prior to and during CRT after ∼30 Gy. PET parameters of the primary site included maximum standardized uptake value (SUVmax), metabolic tumor volume (MTV), and total lesion glycolysis (TLG). MTV and TLG were calculated based on 40% SUVmax (MTV40, TLG40) or SUV 2.5 (MTV2.5, TLG2.5) thresholds for pretreatment and interim images. Absolute and change in PET parameters were assessed for association with freedom from local and regional recurrence (FFLR) using single-predictor Cox regression models. Local and regional recurrence were primary and nodal (in-field) recurrences, respectively.Results: Twenty-three patients were eligible for analysis. Patients were excluded with nonsquamous cell histology, recurrent anal cancer, and incomplete studies due to treatment toxicity or patient choice. Median follow-up was 2.5 years. Pretreatment MTV40 (HR 1.4 [95% CI 1.02-2.05]), interim MTV2.5 (1.4 [1.04-1.89]), and interim TLG2.5 (1.1 [1.01-1.21]) were associated with FFLR.Conclusions: In this prospective pilot study, interim PET parameters were associated with FFLR. These results warrant further investigation assessing the value of interim PET as a biomarker of response in the treatment of patients with anal cancer. [ABSTRACT FROM AUTHOR]

Purpose: To use knowledge-based planning (KBP) as a method of producing high-quality, consistent, protocol-compliant treatment plans in a complex setting of spine stereotactic body radiation therapy on NRG Oncology Radiation Therapy Oncology Group (RTOG) 0631.Methods and Materials: An internally developed KBP model was applied to an external validation cohort of 22 anonymized cases submitted under NRG Oncology RTOG 0631. The original and KBP plans were compared via their protocol compliance, target conformity and gradient index, dose to critical structures, and dose to surrounding normal tissues.Results: The KBP model generated plans meeting all protocol objectives in a single optimization when tested on both internal and protocol-submitted NRG Oncology RTOG 0631 cases. Two submitted plans that were considered to have a protocol-unacceptable deviation were made protocol compliant through the use of the model. There were no statistically significant differences in protocol spinal cord metrics (D10% and D0.03cc) between the manually optimized plans and the KBP plans. The volume of planning target volume receiving prescription dose increased from 93.3% ± 3.2% to 98.3% ± 1.4% (P = .01) when using KBP. High-dose spillage to surrounding normal tissues (V105%) showed no significant differences (2.1 ± 7.3 cm3 for manual plans to 1.8 ± 0.6 cm3 with KBP), and dosimetric outliers with large amounts of spillage were eliminated through the use of KBP. Knowledge-based planning plans were also found to be significantly more consistent in several metrics, including target coverage and high dose outside of the target.Conclusion: Incorporation of KBP models into the clinical trial setting may have a profound impact on the quality of trial results, owing to the increase in consistency and standardization of planning, especially for treatment sites or techniques that are nonstandard. [ABSTRACT FROM AUTHOR]

Purpose: To provide a benchmark for seminal vesicle (SV) margin selection to account for intrafractional motion and to investigate the effectiveness of 2 motion surrogates in predicting intrafractional SV coverage.Methods and Materials: Fifteen prostate patients were studied. Each patient had 5 pairs (1 patient had 4 pairs) of pretreatment and posttreatment cone beam CTs (CBCTs). Each pair of CBCTs was registered on the basis of prostate fiducial markers. All pretreatment SVs were expanded with 1-, 2-, 3-, 4-, 5-, and 8-mm isotropic margins to form a series of planning target volumes, and their intrafractional coverage to the posttreatment SV determined the "ground truth" for exact coverage. Two motion surrogates, the center of mass (COM) and the border of contour, were evaluated by the use of Pearson product-moment correlation coefficient and exponential fitting for predicting SV underdosage. Action threshold of each surrogate was calculated. The margin for each surrogate was calculated according to a traditional margin recipe.Results: Ninety-five percent posttreatment SV coverage was achieved in 9%, 53%, 73%, 86%, 95%, and 97% of fractions with 1-, 2-, 3-, 4-, 5-, and 8-mm margins, respectively. The 5-mm margins provided 95% intrafractional SV coverage in over 90% of fractions. The correlation between the COM and border was weak, moderate, and strong in the left-right (L-R), anterior-posterior (A-P), and superior-inferior (S-I) directions, respectively. Exponential fitting gave the underdosage threshold of 4.5 and 7.0 mm for the COM and border. The Van Herk margin recipe recommended 0-, 0.5-, and 0.8-mm margins in the L-R, A-P, and S-I directions based on the COM, and 1.2-, 3.9-, and 2.5-mm margins based on the border.Conclusions: Five-millimeter isotropic margins for the SV constitute the minimum required to mitigate the intrafractional motion. Both the COM and the border are acceptable predictors for SV underdosage with 4.5- and 7.0-mm action threshold. Traditional margin based on the COM or border underestimates the margin. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study was to develop a techique to generate on-board volumetric cine-magnetic resonance imaging (VC-MRI) using patient prior images, motion modeling, and on-board 2-dimensional cine MRI.Methods and Materials: One phase of a 4-dimensional MRI acquired during patient simulation is used as patient prior images. Three major respiratory deformation patterns of the patient are extracted from 4-dimensional MRI based on principal-component analysis. The on-board VC-MRI at any instant is considered as a deformation of the prior MRI. The deformation field is represented as a linear combination of the 3 major deformation patterns. The coefficients of the deformation patterns are solved by the data fidelity constraint using the acquired on-board single 2-dimensional cine MRI. The method was evaluated using both digital extended-cardiac torso (XCAT) simulation of lung cancer patients and MRI data from 4 real liver cancer patients. The accuracy of the estimated VC-MRI was quantitatively evaluated using volume-percent-difference (VPD), center-of-mass-shift (COMS), and target tracking errors. Effects of acquisition orientation, region-of-interest (ROI) selection, patient breathing pattern change, and noise on the estimation accuracy were also evaluated.Results: Image subtraction of ground-truth with estimated on-board VC-MRI shows fewer differences than image subtraction of ground-truth with prior image. Agreement between normalized profiles in the estimated and ground-truth VC-MRI was achieved with less than 6% error for both XCAT and patient data. Among all XCAT scenarios, the VPD between ground-truth and estimated lesion volumes was, on average, 8.43 ± 1.52% and the COMS was, on average, 0.93 ± 0.58 mm across all time steps for estimation based on the ROI region in the sagittal cine images. Matching to ROI in the sagittal view achieved better accuracy when there was substantial breathing pattern change. The technique was robust against noise levels up to SNR = 20. For patient data, average tracking errors were less than 2 mm in all directions for all patients.Conclusions: Preliminary studies demonstrated the feasibility of generating real-time VC-MRI for on-board localization of moving targets in radiation therapy. [ABSTRACT FROM AUTHOR]

Purpose To identify an optimal margin about the gross target volume (GTV) for stereotactic radiosurgery (SRS) of brain metastases, minimizing toxicity and local recurrence. Methods and Materials Adult patients with 1 to 3 brain metastases less than 4 cm in greatest dimension, no previous brain radiation therapy, and Karnofsky performance status (KPS) above 70 were eligible for this institutional review board–approved trial. Individual lesions were randomized to 1- or 3- mm uniform expansion of the GTV defined on contrast-enhanced magnetic resonance imaging (MRI). The resulting planning target volume (PTV) was treated to 24, 18, or 15 Gy marginal dose for maximum PTV diameters less than 2, 2 to 2.9, and 3 to 3.9 cm, respectively, using a linear accelerator–based image-guided system. The primary endpoint was local recurrence (LR). Secondary endpoints included neurocognition Mini-Mental State Examination, Trail Making Test Parts A and B, quality of life (Functional Assessment of Cancer Therapy-Brain), radionecrosis (RN), need for salvage radiation therapy, distant failure (DF) in the brain, and overall survival (OS). Results Between February 2010 and November 2012, 49 patients with 80 brain metastases were treated. The median age was 61 years, the median KPS was 90, and the predominant histologies were non–small cell lung cancer (25 patients) and melanoma (8). Fifty-five, 19, and 6 lesions were treated to 24, 18, and 15 Gy, respectively. The PTV/GTV ratio, volume receiving 12 Gy or more, and minimum dose to PTV were significantly higher in the 3-mm group (all P <.01), and GTV was similar ( P =.76). At a median follow-up time of 32.2 months, 11 patients were alive, with median OS 10.6 months. LR was observed in only 3 lesions (2 in the 1 mm group, P =.51), with 6.7% LR 12 months after SRS. Biopsy-proven RN alone was observed in 6 lesions (5 in the 3-mm group, P =.10). The 12-month DF rate was 45.7%. Three months after SRS, no significant change in neurocognition or quality of life was observed. Conclusions SRS was well tolerated, with low rates of LR and RN in both cohorts. However, given the higher potential risk of RN with a 3-mm margin, a 1-mm GTV expansion is more appropriate. [ABSTRACT FROM AUTHOR]

Purpose To evaluate the relationship between liver tumor motion and diaphragm motion. Methods and Materials Fourteen patients with hepatocellular carcinoma (10 of 14) or liver metastases (4 of 14) undergoing radiation therapy were included in this study. All patients underwent single-slice cine–magnetic resonance imaging simulations across the center of the tumor in 3 orthogonal planes. Tumor and diaphragm motion trajectories in the superior–inferior (SI), anterior–posterior (AP), and medial–lateral (ML) directions were obtained using an in-house-developed normalized cross-correlation–based tracking technique. Agreement between the tumor and diaphragm motion was assessed by calculating phase difference percentage, intraclass correlation coefficient, and Bland-Altman analysis ( D i f f ). The distance between the tumor and tracked diaphragm area was analyzed to understand its impact on the correlation between the 2 motions. Results Of all patients, the mean (±standard deviation) phase difference percentage values were 7.1% ± 1.1%, 4.5% ± 0.5%, and 17.5% ± 4.5% in the SI, AP, and ML directions, respectively. The mean intraclass correlation coefficient values were 0.98 ± 0.02, 0.97 ± 0.02, and 0.08 ± 0.06 in the SI, AP, and ML directions, respectively. The mean Diff values were 2.8 ± 1.4 mm, 2.4 ± 1.1 mm, and 2.2 ± 0.5 mm in the SI, AP, and ML directions, respectively. Tumor and diaphragm motions had high concordance when the distance between the tumor and tracked diaphragm area was small. Conclusions This study showed that liver tumor motion had good correlation with diaphragm motion in the SI and AP directions, indicating diaphragm motion in the SI and AP directions could potentially be used as a reliable surrogate for liver tumor motion. [ABSTRACT FROM AUTHOR]

The purpose is to dosimetrically compare the following 3 delivery techniques: 3-dimensional conformal radiation therapy (3D-CRT), intensity-modulated arc therapy (IMRT), and volumetric-modulated arc therapy (V-MAT) in the treatment of accelerated partial-breast irradiation (APBI). Overall, 16 patients with T1/2N0 breast cancer were treated with 3D-CRT (multiple, noncoplanar photon fields) on the RTOG 0413 partial-breast trial. These cases were subsequently replanned using static gantry IMRT and V-MAT technology to understand dosimetric differences among these 3 techniques. Several dosimetric parameters were used in plan quality evaluation, including dose conformity index (CI) and dose-volume histogram analysis of normal tissue coverage. Quality assurance studies including gamma analysis were performed to compare the measured and calculated dose distributions. The IMRT and V-MAT plans gave more conformal target dose distributions than the 3D-CRT plans (p < 0.05 in CI). The volume of ipsilateral breast receiving 5 and 10Gy was significantly less using the V-MAT technique than with either 3D-CRT or IMRT (p < 0.05). The maximum lung dose and the ipsilateral lung volume receiving 10 (V10) or 20Gy (V20) were significantly less with both V-MAT and IMRT (p < 0.05). The IMRT technique was superior to 3D-CRT and V-MAT of low dose distributions in ipsilateral lung (p < 0.05 in V5 and D5). The total mean monitor units (MUs) for V-MAT (621.0 ± 111.9) were 12.2% less than those for 3D-CRT (707.3 ± 130.9) and 46.5% less than those for IMRT (1161.4 ± 315.6) (p < 0.05). The average machine delivery time was 1.5 ± 0.2 minutes for the V-MAT plans, 7.0 ± 1.6 minutes for the 3D-CRT plans, and 11.5 ± 1.9 minutes for the IMRT plans, demonstrating much less delivery time for V-MAT. Based on this preliminary study, V-MAT and IMRT techniques offer improved dose conformity as compared with 3D-CRT techniques without increasing dose to the ipsilateral lung. In terms of MU and delivery time, V-MAT is significantly more efficient for APBI than for conventional 3D-CRT and static-beam IMRT. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the feasibility of a retrospective binning technique for 4-dimensional magnetic resonance imaging (4D-MRI) using body area (BA) as a respiratory surrogate. Methods and Materials: Seven patients with hepatocellular carcinoma (4 of 7) or liver metastases (3 of 7) were enrolled in an institutional review board-approved prospective study. All patients were simulated with both computed tomography (CT) and MRI to acquire 3-dimensinal and 4D images for treatment planning. Multiple-slice multiple-phase cine-MR images were acquired in the axial plane for 4D-MRI reconstruction. Image acquisition time per slice was set to 10-15 seconds. Single-slice 2-dimensinal cine-MR images were also acquired across the center of the tumor in orthogonal planes. Tumor motion trajectories from 4D-MRI, cine-MRI, and 4D-CT were analyzed in the superior–inferior (SI), anterior–posterior (AP), and medial–lateral (ML) directions, respectively. Their correlation coefficients (CC) and differences in tumor motion amplitude were determined. Tumor-to-liver contrast-to-noise ratio (CNR) was measured and compared between 4D-CT, 4D-MRI, and conventional T2-weighted fast spin echo MRI. Results: The means (±standard deviations) of CC comparing 4D-MRI with cine-MRI were 0.97 ± 0.03, 0.97 ± 0.02, and 0.99 ± 0.04 in SI, AP, and ML directions, respectively. The mean differences were 0.61 ± 0.17 mm, 0.32 ± 0.17 mm, and 0.14 ± 0.06 mm in SI, AP, and ML directions, respectively. The means of CC comparing 4D-MRI and 4D-CT were 0.95 ± 0.02, 0.94 ± 0.02, and 0.96 ± 0.02 in SI, AP, and ML directions, respectively. The mean differences were 0.74 ± 0.02 mm, 0.33 ± 0.13 mm, and 0.18 ± 0.07 mm in SI, AP, and ML directions, respectively. The mean tumor-to-tissue CNRs were 2.94 ± 1.51, 19.44 ± 14.63, and 39.47 ± 20.81 in 4D-CT, 4D-MRI, and T2-weighted MRI, respectively. Conclusions: The preliminary evaluation of our 4D-MRI technique results in oncologic patients demonstrates its potential usefulness to accurately measure tumor respiratory motion with improved tumor CNR compared with 4D-CT. [ABSTRACT FROM AUTHOR]

To quantitatively evaluate dosimetric effects of rotational offsets in stereotactic body radiation therapy (SBRT) for lung cancer. Overall, 11 lung SBRT patients (8 female and 3 male; mean age: 75.0 years) with medially located tumors were included. Treatment plans with simulated rotational offsets of 1°, 3°, and 5° in roll, yaw, and pitch were generated and compared with the original plans. Both clockwise and counterclockwise rotations were investigated. The following dosimetric metrics were quantitatively evaluated: planning target volume coverage (PTV V100%), max PTV dose (PTV Dmax), percentage prescription dose to 0.35cc of cord (cord D0.35cc), percentage prescription dose to 0.35cc and 5cc of esophagus (esophagus D0.35cc and D5cc), and volume of the lungs receiving at least 20Gy (lung V20). Statistical significance was tested using Wilcoxon signed rank test at the significance level of 0.05. Overall, small differences were found in all dosimetric matrices at all rotational offsets: 95.6% of differences were < 1% or < 1Gy. Of all rotational offsets, largest change in PTV V100%, PTV Dmax, cord D0.35cc, esophagus D0.35cc, esophagus D5cc, and lung V20 was − 8.36%, − 6.06%, 11.96%, 8.66%, 6.02%, and − 0.69%, respectively. No significant correlation was found between any dosimetric change and tumor-to-cord/esophagus distances (R2 range: 0 to 0.44). Larger dosimetric changes and intersubject variations were observed at larger rotational offsets. Small dosimetric differences were found owing to rotational offsets up to 5° in lung SBRT for medially located tumors. Larger intersubject variations were observed at larger rotational offsets. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the dosimetric effects of adaptive planning on lung stereotactic body radiation therapy (SBRT). Methods and Materials: Forty of 66 consecutive lung SBRT patients were selected for a retrospective adaptive planning study. CBCT images acquired at each fraction were used for treatment planning. Adaptive plans were created using the same planning parameters as the original CT-based plan, with the goal to achieve comparable comformality index (CI). For each patient, 2 cumulative plans, nonadaptive plan (PNON) and adaptive plan (PADP), were generated and compared for the following organs-at-risks (OARs): cord, esophagus, chest wall, and the lungs. Dosimetric comparison was performed between PNON and PADP for all 40 patients. Correlations were evaluated between changes in dosimetric metrics induced by adaptive planning and potential impacting factors, including tumor-to-OAR distances (d T-OAR ), initial internal target volume (ITV1), ITV change (ΔITV), and effective ITV diameter change (Δd ITV ). Results: 34 (85%) patients showed ITV decrease and 6 (15%) patients showed ITV increase throughout the course of lung SBRT. Percentage ITV change ranged from −59.6% to 13.0%, with a mean (±SD) of −21.0% (±21.4%). On average of all patients, PADP resulted in significantly (P=0 to .045) lower values for all dosimetric metrics. Δd ITV /d T-OAR was found to correlate with changes in dose to 5 cc (ΔD5cc) of esophagus (r=0.61) and dose to 30 cc (ΔD30cc) of chest wall (r=0.81). Stronger correlations between Δd ITV /d T-OAR and ΔD30cc of chest wall were discovered for peripheral (r=0.81) and central (r=0.84) tumors, respectively. Conclusions: Dosimetric effects of adaptive lung SBRT planning depend upon target volume changes and tumor-to-OAR distances. Adaptive lung SBRT can potentially reduce dose to adjacent OARs if patients present large tumor volume shrinkage during the treatment. [ABSTRACT FROM AUTHOR]

Purpose: Intensity modulated radiation therapy (IMRT) treatment planning can have wide variation among different treatment centers. We propose a system to leverage the IMRT planning experience of larger institutions to automatically create high-quality plans for outside clinics. We explore feasibility by generating plans for patient datasets from an outside institution by adapting plans from our institution. Methods and Materials: A knowledge database was created from 132 IMRT treatment plans for prostate cancer at our institution. The outside institution, a community hospital, provided the datasets for 55 prostate cancer cases, including their original treatment plans. For each “query” case from the outside institution, a similar “match” case was identified in the knowledge database, and the match case’s plan parameters were then adapted and optimized to the query case by use of a semiautomated approach that required no expert planning knowledge. The plans generated with this knowledge-based approach were compared with the original treatment plans at several dose cutpoints. Results: Compared with the original plan, the knowledge-based plan had a significantly more homogeneous dose to the planning target volume and a significantly lower maximum dose. The volumes of the rectum, bladder, and femoral heads above all cutpoints were nominally lower for the knowledge-based plan; the reductions were significantly lower for the rectum. In 40% of cases, the knowledge-based plan had overall superior (lower) dose–volume histograms for rectum and bladder; in 54% of cases, the comparison was equivocal; in 6% of cases, the knowledge-based plan was inferior for both bladder and rectum. Conclusions: Knowledge-based planning was superior or equivalent to the original plan in 95% of cases. The knowledge-based approach shows promise for homogenizing plan quality by transferring planning expertise from more experienced to less experienced institutions. [ABSTRACT FROM AUTHOR]

Purpose: To quantify uncertainties in delineating an internal target volume (ITV) and to understand how these uncertainties may be individually minimized for stereotactic body radiation therapy (SBRT) of early stage non-small cell lung cancer (NSCLC). Methods and Materials: Twenty patients with NSCLC who were undergoing SBRT were imaged with free-breathing 3-dimensional computed tomography (3DCT) and 10-phase 4-dimensional CT (4DCT) for delineating gross tumor volume (GTV)(3D) and ITV(10Phase) (ITV3). The maximum intensity projection (MIP) CT was also calculated from 10-phase 4DCT for contouring ITV(MIP) (ITV1). Then, ITV(COMB) (ITV2), ITV(10Phase+GTV3D) (ITV4), and ITV(10Phase+ITVCOMB) (ITV5) were generated by combining ITV(MIP) and GTV(3D), ITV(10phase) and GTV(3D), and ITV(10phase) and ITV(COMB), respectively. All 6 volumes (GTV(3D) and ITV1 to ITV5) were delineated in the same lung window by the same radiation oncologist. The percentage of volume difference (PVD) between any 2 different volumes was determined and was correlated to effective tumor diameter (ETD), tumor motion ranges, R(3D), and the amplitude variability of the recorded breathing signal (v) to assess their volume variations. Results: The mean (range) tumor motion (R(SI), R(AP), R(ML), and R(3D)) and breathing variability (v) were 7.6 mm (2-18 mm), 4.0 mm (2-8 mm), 3.3 mm (0-7.5 mm), 9.9 mm (4.1-18.7 mm), and 0.17 (0.07-0.37), respectively. The trend of volume variation was GTV(3D) Conclusion: Uncertainties in individualized ITVs for SBRT of early stage NSCLC could effectively be minimized by combining information from 3DCT, 4DCT, and MIP. If these images cannot be efficiently contoured, a combination of ITV(MIP) and GTV(3D) could be an effective alternative. [ABSTRACT FROM AUTHOR]

Purpose: To report the process and initial experience of remote credentialing of three-dimensional (3D) image guided radiation therapy (IGRT) as part of the quality assurance (QA) of submitted data for Radiation Therapy Oncology Group (RTOG) clinical trials; and to identify major issues resulting from this process and analyze the review results on patient positioning shifts. Methods and Materials: Image guided radiation therapy datasets including in-room positioning CT scans and daily shifts applied were submitted through the Image Guided Therapy QA Center from institutions for the IGRT credentialing process, as required by various RTOG trials. A centralized virtual environment is established at the RTOG Core Laboratory, containing analysis tools and database infrastructure for remote review by the Physics Principal Investigators of each protocol. The appropriateness of IGRT technique and volumetric image registration accuracy were evaluated. Registration accuracy was verified by repeat registration with a third-party registration software system. With the accumulated review results, registration differences between those obtained by the Physics Principal Investigators and from the institutions were analyzed for different imaging sites, shift directions, and imaging modalities. Results: The remote review process was successfully carried out for 87 3D cases (out of 137 total cases, including 2-dimensional and 3D) during 2010. Frequent errors in submitted IGRT data and challenges in the review of image registration for some special cases were identified. Workarounds for these issues were developed. The average differences of registration results between reviewers and institutions ranged between 2 mm and 3 mm. Large discrepancies in the superior-inferior direction were found for megavoltage CT cases, owing to low spatial resolution in this direction for most megavoltage CT cases. Conclusion: This first experience indicated that remote review for 3D IGRT as part of QA for RTOG clinical trials is feasible and effective. The magnitude of registration discrepancy between institution and reviewer was presented, and the major issues were investigated to further improve this remote evaluation process. [Copyright &y& Elsevier]

Purpose: To evaluate the reproducibility of tumor motion probability distribution function (PDF) in stereotactic body radiation therapy (SBRT) of lung cancer using cine megavoltage (MV) images. Methods and Materials: Cine MV images of 20 patients acquired during three-dimensional conformal (6–11 beams) SBRT treatments were retrospectively analyzed to extract tumor motion trajectories. For each patient, tumor motion PDFs were generated per fraction (PDFn) using three selected “usable” beams. Patients without at least three usable beams were excluded from the study. Fractional PDF reproducibility (Rn) was calculated as the Dice similarity coefficient between PDFn to a “ground-truth” PDF (PDFg), which was generated using the selected beams of all fractions. The mean of Rn, labeled as Rm, was calculated for each patient and correlated to the patient''s mean tumor motion rang (Am). Change of Rm during the course of SBRT treatments was also evaluated. Intra- and intersubject coefficient of variation (CV) of Rm and Am were determined. Results: Thirteen patients had at least three usable beams and were analyzed. The mean of Rm was 0.87 (range, 0.84–0.95). The mean of Am was 3.18 mm (range, 0.46–7.80 mm). Rm was found to decrease as Am increases following an equation of Rm = 0.17e−0.9Am + 0.84. Rm also decreased slightly throughout the course of treatments. Intersubject CV of Rm (0.05) was comparable to intrasubject CV of Rm (range, 0.02–0.09); intersubject CV of Am (0.73) was significantly greater than intrasubject CV of Am (range, 0.09–0.24). Conclusions: Tumor motion PDF can be determined using cine MV images acquired during the treatments. The reproducibility of lung tumor motion PDF decreased exponentially as the tumor motion range increased and decreased slightly throughout the course of the treatments. [Copyright &y& Elsevier]

Abstract: Purpose: To investigate (1) dosimetric differences between plans calculated using the anisotropic analytical algorithm (AAA) and pencil beam convolution (PBC) algorithm, (2) the plan quality achieved using AAA compared to PBC based on dosimetric parameters and (3) discrepancies with an independent MU verification calculation for breast treatment planning. Materials and methods: This study included 10 lumpectomy (Group I) and 10 mastectomy (Group II) cases. Target volumes were defined as breast for Group I and chest-wall for Group II based on the isodose distribution of PBC plans in order to evaluate plans. All plans were initially calculated with PBC. For study aim (1), plans were re-calculated using AAA with the same monitor units (MUs). For study aim (2), plans were calculated using AAA with modifications of wedges, subfields and beam weightings from the original plans to achieve optimal coverage. For study aim (3), independent MU verification was performed. A 3% difference between primary MUs and verification MUs was considered an action level. Results: (1) Plans using PBC overestimate the dose to the target volume compared to plans using AAA (Group I V 95% =90.4%:84.4%; Group II V 95% =83.0%:74.5%; PBC:AAA). (2) The new plans using AAA achieved similar target coverage to the original PBC plans based on dose–volume histograms (DVHs). Yet, the high-dose volume (V 105%) was significantly larger in AAA plans than PBC plans for Group II (V 105% =19.5%:24.0%). For both groups, there was a significant increase in the ipsilateral lung volume receiving low dose with AAA plans (Group I V 5Gy =23.6%:39.9%; Group II V 5Gy =21.2%:33.6%). Isodose distributions of AAA plans displayed insufficient coverage in the superior area. (3) In Group I, all PBC cases passed MU verification versus 30% of AAA cases. In Group II, 80% of PBC plans versus 65% of AAA plans were within the action level. Conclusions: Plans using AAA calculation can achieve a similar level of target coverage based on DVH as PBC calculation. Nevertheless, the dose distribution shows insufficient coverage in the superior area with AAA plans compared to PBC plans. The lung volume receiving the low-dose (i.e. 5Gy) is larger and the dose to the skin is greater in AAA plans than PBC plans. Compared with PBC calculation, a larger tolerance in discrepancy between AAA and independent MU verification should be allowed to account for the inadequate heterogeneity corrections in the latter. [Copyright &y& Elsevier]

Purpose: To develop a three-dimensional (3D) cone-beam computed tomography (CBCT) estimation method using a deformation field map, and to evaluate and optimize the efficiency and accuracy of the method for use in the clinical setting. Methods and Materials: We propose a method to estimate patient CBCT images using prior information and a deformation model. Patients’ previous CBCT data are used as the prior information, and the new CBCT volume to be estimated is considered as a deformation of the prior image volume. The deformation field map is solved by minimizing deformation energy and maintaining new projection data fidelity using a nonlinear conjugate gradient method. This method was implemented in 3D form using hardware acceleration and multi-resolution scheme, and it was evaluated for different scan angles, projection numbers, and scan directions using liver, lung, and prostate cancer patient data. The accuracy of the estimation was evaluated by comparing the organ volume difference and the similarity between estimated CBCT and the CBCT reconstructed from fully sampled projections. Results: Results showed that scan direction and number of projections do not have significant effects on the CBCT estimation accuracy. The total scan angle is the dominant factor affecting the accuracy of the CBCT estimation algorithm. Larger scan angles yield better estimation accuracy than smaller scan angles. Lung cancer patient data showed that the estimation error of the 3D lung tumor volume was reduced from 13.3% to 4.3% when the scan angle was increased from 60° to 360° using 57 projections. Conclusions: The proposed estimation method is applicable for 3D DTS, 3D CBCT, four-dimensional CBCT, and four-dimensional DTS image estimation. This method has the potential for significantly reducing the imaging dose and improving the image quality by removing the organ distortion artifacts and streak artifacts shown in images reconstructed by the conventional Feldkamp–Davis–Kress (FDK) algorithm. [ABSTRACT FROM AUTHOR]

Purpose: To explore an effective and efficient end-to-end patient-specific quality-assurance (QA) protocol for volumetric modulated arc radiotherapy (VMAT) and to evaluate the suitability of a stationary radiotherapy QA device (two-dimensional [2D] ion chamber array) for VMAT QA. Methods and Materials: Three methods were used to analyze 39 VMAT treatment plans for brain, spine, and prostate: ion chamber (one-dimensional absolute, n = 39), film (2D relative, coronal/sagittal, n = 8), and 2D ion chamber array (ICA, 2D absolute, coronal/sagittal, n = 39) measurements. All measurements were compared with the treatment planning system dose calculation either via gamma analysis (3%, 3- to 4-mm distance-to-agreement criteria) or absolute point dose comparison. The film and ion chamber results were similarly compared with the ICA measurements. Results: Absolute point dose measurements agreed well with treatment planning system computed doses (ion chamber: median deviation, 1.2%, range, −0.6% to 3.3%; ICA: median deviation, 0.6%, range, −1.8% to 2.9%). The relative 2D dose measurements also showed good agreement with computed doses (>93% of pixels in all films passing gamma, >90% of pixels in all ICA measurements passing gamma). The ICA relative dose results were highly similar to those of film (>90% of pixels passing gamma). The coronal and sagittal ICA measurements were statistically indistinguishable by the paired t test with a hypothesized mean difference of 0.1%. The ion chamber and ICA absolute dose measurements showed a similar trend but had disparities of 2-3% in 18% of plans. Conclusions: After validating the new VMAT implementation with ion chamber, film, and ICA, we were able to maintain an effective yet efficient patient-specific VMAT QA protocol by reducing from five (ion chamber, film, and ICA) to two measurements (ion chamber and single ICA) per plan. The ICA (Matrixx®, IBA Dosimetry) was validated for VMAT QA, but ion chamber measurements are recommended for absolute dose comparison until future developments correct the ICA angular dependence. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the feasibility of using lipiodol as a direct surrogate for target localization using cone-beam CT (CBCT) image guidance in radiotherapy (RT) of patients with unresectable liver tumors after transarterial chemoembolization. Methods and Materials: Forty-six patients with an unresectable solitary liver tumor were enrolled for RT using active breathing control (ABC) and CBCT image guidance after transarterial chemoembolization. Each patient had pre- and posttreatment CBCT in the first 10 fractions of treatment. Lipiodol retention was evaluated using daily CBCT scans, and volume of lipiodol retention in the liver was calculated and compared between planning CT and post-RT CT. Influence of lipiodol on dosimetry was evaluated by measuring doses using an ion chamber with and without the presence of lipiodol. Margin analysis was performed on the basis of both inter- and intrafractional target localization errors. Results: Twenty-eight patients successfully completed the study. The shape and size of lipiodol retention did not vary substantially during the course of treatment. The mean Dice similarity coefficient for the lipiodol volume in pretreatment CT and that in posttreatment CT was 0.836 (range, 0.817–0.885). The maximum change (ratio of the lipiodol volume in pretreatment CT to that in posttreatment CT) was 1.045. The mean dose changes with the presence of <10 mL lipiodol were −1.44% and 0.13% for 6 MV and 15 MV, respectively. With ABC and online CBCT image guidance, clinical target volume–planning target volume margins were determined to be 2.5 mm in the mediolateral direction, 2.9 mm in the anteroposterior direction, and 4.0 mm in the craniocaudal direction. Conclusions: Lipiodol could be used as a direct surrogate for CBCT image guidance to improve the localization accuracy for RT of liver tumors. Combination of ABC and CBCT image guidance with lipiodol can potentially reduce the clinical target volume–planning target volume margin. [ABSTRACT FROM AUTHOR]

We conducted a planning study to compare Varian's RapidArc (RA) and helical TomoTherapy (HT) for the treatment of pancreatic cancer. Three intensity-modulated radiotherapy (IMRT) plans were generated for 8 patients with pancreatic cancer: one using HT with 6-MV beam (Plan_HT6), one using single-arc RA with 6-MV beam (Plan_RA6), and one using single-arc RA with 15-MV beam (Plan_RA15). Dosimetric indices including high/low conformality index (CI100%/CI50%), heterogeneity index (HI), monitor units (MUs), and doses to organs at risk (OARs) were compared. The mean CI100% was statistically equivalent with respect to the 2 treatment techniques, as well as beam energy (0.99, 1.01, and 1.02 for Plan_HT6, Plan_RA6, and Plan_RA156, respectively). The CI50% and HI were improved in both RA plans over the HT plan. The RA plans significantly reduced MU (MURA6 = 697, MURA15 = 548) compared with HT (MUHT6 = 6177, p = 0.008 in both cases). The mean maximum cord dose was decreased from 29.6 Gy in Plan_HT6 to 21.6 Gy (p = 0.05) in Plan_RA6 and 21.7 Gy (p = 0.04) in Plan_RA15. The mean bowel dose decreased from 17.2 Gy in Plan_HT6 to 15.2 Gy (p = 0.03) in Plan_RA6 and 15.0 Gy (p = 0.03) Plan_RA15. The mean liver dose decreased from 8.4 Gy in Plan_HT6 to 6.3 Gy (p = 0.04) in Plan_RA6 and 6.2 Gy in Plan_RA15. Variations of the mean dose to the duodenum, kidneys, and stomach were statistically insignificant. RA and HT can both deliver conformal dose distributions to target volumes while limiting the dose to surrounding OARs in the treatment of pancreatic cancer. Dosimetric advantages might be gained by using RA over HT by reducing the dose to OARs and total MUs used for treatment. [ABSTRACT FROM AUTHOR]

Purpose: Breath-hold (BH) treatment minimizes internal target volumes (ITV) when treating sites prone to motion. Digital tomosynthesis (DTS) imaging has advantages over cone-beam CT (CBCT) for BH imaging: BH-DTS scan can be completed during a single breath-hold, whereas BH-CBCT is usually acquired by parsing the gantry rotation into multiple BH segments. This study evaluates the localization accuracy of DTS for BH treatment of liver tumors. Methods: Both planning CT and on-board DTS/CBCT images were acquired under BH, using the planning CT BH window as reference. Onboard imaging data sets included two independent DTS orientations (coronal and sagittal), and CBCT images. Soft tissue target positioning was measured by each imaging modality and translated into couch shifts. Performance of the two DTS orientations was evaluated by comparing target positioning with the CBCT benchmark, determined by two observers. Results: Image data sets were collected from thirty-eight treatment fractions (14 patients). Mean differences between the two DTS methods and the CBCT method were <1 mm in all directions (except the lateral direction with sagittal-DTS: 1.2 mm); the standard deviation was in the range of 2.1–3.5 mm for all techniques. The Pearson correlation showed good interobserver agreement for the coronal-DTS (0.72–0.78). The interobserver agreement for the sagittal-DTS was good for the in-plane directions (0.70–0.82), but poor in the out-of-plane direction (lateral, 0.26). Conclusions: BH-DTS may be a simpler alternative to BH-CBCT for onboard soft tissue localization of the liver, although the precision of DTS localization appears to be somewhat lower because of the presence of subtle out-of-plane blur. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the technical feasibility of volumetric modulated arc therapy (V-MAT) in the delivery of partial breast irradiation (PBI). Methods and Materials: V-MAT and the standard, three-dimensional conformal radiotherapy (3D-CRT), were compared retrospectively in 8 patients previously treated with PBI. These patients'' plans were replanned with a single partial arc using V-MAT that included partial blocking to minimize normal tissue dose. Dosimetric parameters were calculated to evaluate plan quality. Quality assurance studies included verifying both the point and the multiple planar doses. Total monitor units and delivery time were also evaluated, and collision clearance was analyzed. Results: Volumes of ipsilateral lung irradiated to 10 Gy (V10) and 20 Gy (V20) by V-MAT were significantly less than those of 3D-CRT (p = 0.03 for V10 and p = 0.025 for V20). The volume of ipsilateral breast irradiated to 5 Gy was significantly less by using V-MAT than with 3D-CRT (p = 0.02), with a ratio of integrated dose of <1.00. The total mean monitor units (489 ± 38) for V-MAT were significantly less than those for 3D-CRT (634 ± 123) (p = 0.017), with a 23% reduction. The average machine delivery time was 1.21 ± 0.10 min for the V-MAT plans and 6.28 ± 1.40 min for the 3D-CRT plans, resulting in a reduction factor of 80.1%. The conformity indexes were 1.3 in the V-MAT plans and 1.5 in the 3D-CRT plans (p = 0.102). Conclusions: V-MAT technology is feasible for PBI patients. Compared to a conventional 3D-CRT technique, it is more efficient, offers equivalent or better dose conformity, delivers lower doses to the ipsilateral lung and breast, and may potentially reduce intrafractional motion. [Copyright &y& Elsevier]

Abstract: Purpose: To investigate setup discrepancies measured with ExacTrac X-ray 6 degree-of-freedom (6D) and cone-beam computed tomography (CBCT) for patients under treatments of stereotactic body radiation therapy (SBRT). Materials and methods: In this work, phantom and patient studies were performed. In the phantom studies, an anthropomorphic phantom was placed with pre-defined positions, and imaged with ExacTrac X-ray 6D and CBCT to test the accuracy of the imaging systems. In the patient studies, 16 spinal SBRT patient cases were retrospectively analyzed. The patients were initially positioned in customized immobilization cradles and then aligned with ExacTrac X-ray 6D and CBCT. The setup discrepancies were computed and quantitatively analyzed. Results: This study indicates modest discrepancies between ExacTrac X-ray 6D and CBCT with spinal SBRT. The phantom experiments showed that translational and rotational discrepancies in root-mean-square (RMS) between ExacTrac X-ray 6D and CBCT were, respectively, <1.0mm and <1°. In the retrospective patient studies, translational and rotational discrepancies in RMS between ExacTrac X-ray 6D and CBCT were <2.0mm and <1.5°. Conclusions: ExacTrac X-ray 6D represents a potential alternative to CBCT; however, pre-caution should be taken when only ExacTrac X-ray 6D is used to guide SBRT with small setup margins. [Copyright &y& Elsevier]

Purpose: Dosimetric results and treatment delivery efficiency of RapidArc plans to those of conventional intensity-modulated radiotherapy (IMRT) plans were compared using the Eclipse treatment planning system for high-risk prostate cancer. Materials and Methods: This study included 10 patients. The primary planning target volume (PTVP) contained prostate, seminal vesicles, and pelvic lymph nodes with a margin. The boost PTV (PTVB) contained prostate and seminal vesicles with a margin. The total prescription dose was 75.6 Gy (46.8 Gy to PTVP and an additional 28.8 Gy to PTVB; 1.8 Gy/fraction). Three plans were generated for each PTV: Multiple-field IMRT, one-arc RapidArc (1ARC), and two-arc RapidArc (2ARC). Results: In the primary IMRT with PTVP, average mean doses to bladder, rectum and small bowel were lower by 5.9%, 7.7% and 4.3%, respectively, than in the primary 1ARC and by 3.6%, 4.8% and 3.1%, respectively, than in the primary 2ARC. In the boost IMRT with PTVB, average mean doses to bladder and rectum were lower by 2.6% and 4.8% than with the boost 1ARC and were higher by 0.6% and 0.2% than with the boost 2ARC. Integral doses were 7% to 9% higher with RapidArc than with IMRT for both primary and boost plans. Treatment delivery time was reduced by 2-7 minutes using RapidArc. Conclusion: For PTVs including prostate, seminal vesicles, and lymph nodes, IMRT performed better in dose sparing for bladder, rectum, and small bowel than did RapidArc. For PTVs including prostate and seminal vesicles, RapidArc with two arcs provided plans comparable to those for IMRT. The treatment delivery is more efficient with RapidArc. [Copyright &y& Elsevier]

Purpose: This clinical study evaluates the feasibility of using volumetric arc-modulated treatment (VMAT) for spine stereotactic body radiotherapy (SBRT) to achieve highly conformal dose distributions that spare adjacent organs at risk (OAR) with reduced treatment time. Methods and Materials: Ten spine SBRT patients were studied retrospectively. The intensity-modulated radiotherapy (IMRT) and VMAT plans were generated using either one or two arcs. Planning target volume (PTV) dose coverage, OAR dose sparing, and normal tissue integral dose were measured and compared. Differences in treatment delivery were also analyzed. Results: The PTV DVHs were comparable between VMAT and IMRT plans in the shoulder (D(99%)-D(90%)), slope (D(90%)-D(10%)), and tail (D(10%)-D(1%)) regions. Only VMAT(2arc) had a better conformity index than IMRT (1.09 vs. 1.15, p = 0.007). For cord sparing, IMRT was the best, and VMAT(1arc) was the worst. Use of IMRT achieved greater than 10% more D(1%) sparing for six of 10 cases and 7% to 15% more D(10%) sparing over the VAMT(1arc). The differences between IMRT and VAMT(2arc) were smaller and statistically nonsignificant at all dose levels. The differences were also small and statistically nonsignificant for other OAR sparing. The mean monitor units (MUs) were 8711, 7730, and 6317 for IMRT, VMAT(1arc), and VMAT(2arc) plans, respectively, with a 26% reduction from IMRT to VMAT(2arc). The mean treatment time was 15.86, 8.56, and 7.88 min for IMRT, VMAT(1arc,) and VMAT(2arc). The difference in integral dose was statistically nonsignificant. Conclusions: Although VMAT provided comparable PTV coverage for spine SBRT, 1arc showed significantly worse spinal cord sparing compared with IMRT, whereas 2arc was comparable to IMRT. Treatment efficiency is substantially improved with the VMAT. [ABSTRACT FROM AUTHOR]

Abstract: Background and purpose: To compare the residual setup errors measured with ExacTrac X-ray 6 degree-of-freedom (6D) and cone-beam computed tomography (CBCT) for a head phantom and patients receiving intracranial non-invasive fractionated stereotactic radiotherapy (SRT). Materials and methods: Setup data were collected on a Novalis Tx treatment unit for an anthropomorphic head phantom and 18 patients with intracranial tumors. Initial corrections were determined and corrected with the ExacTrac system only, and then the residual setup error was determined by means of three different procedures. These procedures included registrations of ExacTrac X-ray images with the corresponding digitally reconstructed radiographs (DRRs) using the ExacTrac 6D fusion, and registrations of CBCT images with the planning CT using both online 3D fusion and offline 6D fusion. The difference in residual setup errors between ExacTrac system and CBCT was computed. The impact of rotations on the difference was evaluated. Results: A modest difference in residual setup errors was found between ExacTrac system and CBCT. The root-mean-square (RMS) of the differences observed for translations was typically <0.5mm for phantom, and <1.5mm for patients, respectively. The RMS of the differences for rotation(s) was however <0.2 degree for phantom, and <1.0 degree for patients, respectively. The impact of rotation on the setup difference was minor but not negligible. Conclusions: This study indicates that there is a general agreement between ExacTrac system and CBCT. [Copyright &y& Elsevier]

Despite the widespread use of intensity-modulated radiation therapy (IMRT) for approximately a decade, a lack of adequate guidelines for documenting these treatments persists. Proper IMRT treatment documentation is necessary for accurate reconstruction of prior treatments when a patient presents with a marginal recurrence. This is especially crucial when the follow-up care is managed at a second treatment facility not involved in the initial IMRT treatment. To address this issue, an American Society for Radiation Oncology (ASTRO) workgroup within the American ASTRO Radiation Physics Committee was formed at the request of the ASTRO Research Council to develop a set of recommendations for documenting IMRT treatments. This document provides a set of comprehensive recommendations for documenting IMRT treatments, as well as image-guidance procedures, with example forms provided. [Copyright &y& Elsevier]

Purpose: To describe our experience and clinical strategy for stereotactic body radiotherapy (SBRT) of spinal lesions. Methods and Materials: Thirty-two patients with 33 spinal lesions underwent computed tomography–based simulation while free breathing. Gross/clinical target volumes included involved portions of the vertebral body and paravertebral/epidural tumor. Planning target volume (PTV) expansion was 6 mm axially and 3 mm radially; the cord was excluded from the PTV. Biologic equivalent dose was calculated using the linear quadratic model with α/β = 3 Gy. Treatment was linear accelerator based with on-board imaging; dose was adjusted to maintain cord dose within tolerance. Survival, local control, pain, and neurologic status were monitored. Results: Twenty-one patients are alive at 1 year (median survival, 14 months). Median follow-up is 6 months for all patients (7 months for survivors). Mean previous radiotherapy dose to 22 patients was 35 Gy, and median interval was 17 months. Renal (31%), breast, and lung (19% each) were the most common histologic sites. Three SBRT fractions (range, one to four fractions) of 7 Gy (range, 5–16 Gy) were delivered. Median cord and target biologic equivalent doses were 70 Gy3 and 34.3 Gy10, respectively. Thirteen patients reported complete and 17 patients reported partial pain relief at 1 month. There were four failures (mean, 5.8 months) with magnetic resonance imaging evidence of in-field progression. No dosimetric parameters predictive of failure were identified. No treatment-related toxicity was seen. Conclusions: Spinal SBRT is effective in the palliative/re-treatment setting. Volume expansion must ensure optimal PTV coverage while avoiding spinal cord toxicity. The long-term safety of spinal SBRT and the applicability of the linear-quadratic model in this setting remain to be determined, particularly the time-adjusted impact of prior radiotherapy. [Copyright &y& Elsevier]

Purpose: To evaluate digital tomosynthesis (DTS) technology for daily positioning of patients receiving accelerated partial breast irradiation (APBI) and to compare the positioning accuracy of DTS to three-dimensional cone-beam computed tomography (CBCT). Methods and Materials: Ten patients who underwent APBI were scanned daily with on-board CBCT. A subset of the CBCT projections was used to reconstruct a stack of DTS image slices. To optimize soft-tissue visibility, the DTS images were reconstructed in oblique directions so that the tumor bed, breast tissue, ribs, and lungs were well separated. Coronal and sagittal DTS images were also reconstructed. Translational shifts of DTS images were obtained on different days from the same patients and were compared with the translational shifts of corresponding CBCT images. Seventy-seven CBCT scans and 291 DTS scans were obtained from nine evaluable patients. Results: Tumor beds were best visible in the oblique DTS scans. One-dimensional positioning differences between DTS and CBCT images were 0.8–1.7 mm for the six patients with clips present and 1.2–2.0 mm for the three patients without clips. Because of the limited DTS scan angle, the DTS registration accuracy along the off-plane direction is lower than the accuracy along the in-plane directions. Conclusions: For patients receiving APBI, DTS localization offers comparable accuracy to CBCT localization for daily patient positioning while reducing mechanical constraints and imaging dose. [Copyright &y& Elsevier]

Purposes: To quantitatively compare two-dimensional (2D) orthogonal kV with three-dimensional (3D) cone-beam CT (CBCT) for target localization; and to assess intrafraction motion with kV images in patients undergoing stereotactic body radiotherapy (SBRT). Methods and Materials: A total of 50 patients with 58 lesions received 178 fractions of SBRT. After clinical setup using in-room lasers and skin/cradle marks placed at simulation, patients were imaged and repositioned according to orthogonal kV/MV registration of bony landmarks to digitally reconstructed radiographs from the planning CT. A subsequent CBCT was registered to the planning CT using soft tissue information, and the resultant “residual error” was measured and corrected before treatment. Posttreatment 2D kV and/or 3D CBCT images were compared with pretreatment images to determine any intrafractional position changes. Absolute averages, statistical means, standard deviations, and root mean square (RMS) values of observed setup error were calculated. Results: After initial setup to external marks with laser guidance, 2D kV images revealed vector mean setup deviations of 0.67 cm (RMS). Cone-beam CT detected residual setup deviations of 0.41 cm (RMS). Posttreatment imaging demonstrated intrafractional variations of 0.15 cm (RMS). The individual shifts in three standard orthogonal planes showed no obvious directional biases. Conclusions: After localization based on superficial markings in patients undergoing SBRT, orthogonal kV imaging detects setup variations of approximately 3 to 4 mm in each direction. Cone-beam CT detects residual setup variations of approximately 2 to 3 mm. [Copyright &y& Elsevier]

Purpose: To evaluate on-board digital tomosynthesis (DTS) for patient positioning vs. two-dimensional (2D) radiography and three-dimensional cone beam (CBCT). Methods and Materials: A total of 92 image sessions from 9 prostate cancer patients were analyzed. An on-board image set was registered to a corresponding reference image set. Four pairs of image sets were used: digitally reconstructed radiographs vs. on-board orthogonal paired radiographs for the 2D method, coronal-reference DTS vs. on-board coronal DTS for the coronal-DTS method, sagittal-reference DTS vs. on-board sagittal DTS for the sagittal-DTS method, and planning CT vs. CBCT for the CBCT method. The registration results were compared. Results: The systematic errors in all methods were <1 mm/1°. When registering the bony anatomy, the mean vector difference was 0.21 ± 0.11 cm between 2D and CBCT, 0.11 ± 0.08 cm between CBCT and coronal DTS, and 0.14 ± 0.07 cm between CBCT and sagittal DTS. The correlation between CBCT to DTS was stronger (coefficient = 0.92–0.95) than the correlation between 2D and CBCT or DTS (coefficient = 0.81–0.83). When registering the soft tissue, the mean vector difference was 0.18 ± 0.11 cm between CBCT and coronal DTS and 0.29 ± 0.17 cm between CBCT and sagittal DTS. The correlation coefficient of CBCT to sagittal DTS and to coronal DTS was 0.84 and 0.92, respectively. Conclusion: DTS could provide equivalent results to CBCT when the bony anatomy is used as landmarks for prostate image-guided radiotherapy. For soft tissue-based positioning verification, coronal DTS produced equivalent results to CBCT, but sagittal DTS alone was insufficient. DTS could allow for comparable soft tissue-based target localization with faster scanning time and a lower imaging dose compared with CBCT. [Copyright &y& Elsevier]