J Roper

Single-Isocenter Multiple-Target Stereotactic Radiosurgery: Risk of Compromised Coverage

PURPOSE To determine the dosimetric effects of rotational errors on target coverage using volumetric modulated arc therapy (VMAT) for multitarget stereotactic radiosurgery (SRS). METHODS AND MATERIALS This retrospective study included 50 SRS cases, each with 2 intracranial planning target volumes (PTVs). Both PTVs were planned for simultaneous treatment to 21 Gy using a single-isocenter, noncoplanar VMAT SRS technique. Rotational errors of 0.5°, 1.0°, and 2.0° were simulated about all axes. The dose to 95% of the PTV (D95) and the volume covered by 95% of the prescribed dose (V95) were evaluated using multivariate analysis to determine how PTV coverage was related to PTV volume, PTV separation, and rotational error. RESULTS At 0.5° rotational error, D95 values and V95 coverage rates were ≥95% in all cases. For rotational errors of 1.0°, 7% of targets had D95 and V95 values <95%. Coverage worsened substantially when the rotational error increased to 2.0°: D95 and V95 values were >95% for only 63% of the targets. Multivariate analysis showed that PTV volume and distance to isocenter were strong predictors of target coverage. CONCLUSIONS The effects of rotational errors on target coverage were studied across a broad range of SRS cases. In general, the risk of compromised coverage increased with decreasing target volume, increasing rotational error and increasing distance between targets. Multivariate regression models from this study may be used to quantify the dosimetric effects of rotational errors on target coverage given patient-specific input parameters of PTV volume and distance to isocenter.

Technical Note: Unified imaging and robotic couch quality assurance

PURPOSE To introduce a simplified quality assurance (QA) procedure that integrates tests for the linacs imaging components and the robotic couch. Current QA procedures for evaluating the alignment of the imaging system and linac require careful positioning of a phantom at isocenter before image acquisition and analysis. A complementary procedure for the robotic couch requires an initial displacement of the phantom and then evaluates the accuracy of repositioning the phantom at isocenter. We propose a two-in-one procedure that introduces a custom software module and incorporates both checks into one motion for increased efficiency. METHODS The phantom was manually set with random translational and rotational shifts, imaged with the in-room imaging system, and then registered to the isocenter using a custom software module. The software measured positioning accuracy by comparing the location of the repositioned phantom with a CAD model of the phantom at isocenter, which is physically verified using the MV port graticule. Repeatability of the custom software was tested by an assessment of internal marker location extraction on a series of scans taken over differing kV and CBCT acquisition parameters. RESULTS The proposed method was able to correctly position the phantom at isocenter within acceptable 1 mm and 1° SRS tolerances, verified by both physical inspection and the custom software. Residual errors for mechanical accuracy were 0.26 mm vertically, 0.21 mm longitudinally, 0.55 mm laterally, 0.21° in pitch, 0.1° in roll, and 0.67° in yaw. The software module was shown to be robust across various scan acquisition parameters, detecting markers within 0.15 mm translationally in kV acquisitions and within 0.5 mm translationally and 0.3° rotationally across CBCT acquisitions with significant variations in voxel size. Agreement with vendor registration methods was well within 0.5 mm; differences were not statistically significant. CONCLUSIONS As compared to the current two-step approach, the proposed QA procedure streamlines the workflow, accounts for rotational errors in imaging alignment, and simulates a broad range of variations in setup errors seen in clinical practice.

MO-D-BRB-03: Quality Assurance of SBRT

Increased use of SBRT and hypofractionation in radiation oncology practice has posted a number of challenges to medical physicist, ranging from planning, image-guided patient setup and on-treatment monitoring, to quality assurance (QA) and dose delivery. This symposium is designed to provide current knowledge necessary for the safe and efficient implementation of SBRT in various linac platforms, including the emerging digital linacs equipped with high dose rate FFF beams. Issues related to 4D CT, PET and MRI simulations, 3D/4D CBCT guided patient setup, real-time image guidance during SBRT dose delivery using gated/un-gated VMAT/IMRT, and technical advancements in QA of SBRT (in particular, strategies dealing with high dose rate FFF beams) will be addressed. The symposium will help the attendees to gain a comprehensive understanding of the SBRT workflow and facilitate their clinical implementation of the state-of-art imaging and planning techniques. LEARNING OBJECTIVES 1. Present background knowledge of SBRT, describe essential requirements for safe implementation of SBRT, and discuss issues specific to SBRT treatment planning and QA. 2. Update on the use of multi-dimensional and multi-modality imaging for reliable guidance of SBRT. 3. Discuss treatment planning and QA issues specific to SBRT. 4. Provide a comprehensive overview of emerging digital linacs and summarize the key geometric and dosimetric features of the new generation of linacs for substantially improved SBRT. NIH/NCI; Varian Medical Systems; F. Yin, Duke University has a research agreement with Varian Medical Systems. In addition to research grant, I had a technology license agreement with Varian Medical Systems.

Does size matter? Investigating the optimal planning target volume margin for postoperative stereotactic radiosurgery to resected brain metastases

OBJECTIVEThe optimal margin size in postoperative stereotactic radiosurgery (SRS) for brain metastases is unknown. Herein, the authors investigated the effect of SRS planning target volume (PTV) margin on local recurrence and symptomatic radiation necrosis postoperatively.METHODSRecords of patients who received postoperative LINAC-based SRS for brain metastases between 2006 and 2016 were reviewed and stratified based on PTV margin size (1.0 or > 1.0 mm). Patients were treated using frameless and framed SRS techniques, and both single-fraction and hypofractionated dosing were used based on lesion size. Kaplan-Meier and cumulative incidence models were used to estimate survival and intracranial outcomes, respectively. Multivariate analyses were also performed.RESULTSA total of 133 patients with 139 cavities were identified; 36 patients (27.1%) and 35 lesions (25.2%) were in the 1.0-mm group, and 97 patients (72.9%) and 104 lesions (74.8%) were in the > 1.0-mm group. Patient characteristics were balanced, except the 1.0-mm cohort had a better Eastern Cooperative Group Performance Status (grade 0: 36.1% vs 19.6%), higher mean number of brain metastases (1.75 vs 1.31), lower prescription isodose line (80% vs 95%), and lower median single fraction-equivalent dose (15.0 vs 17.5 Gy) (all p < 0.05). The median survival and follow-up for all patients were 15.6 months and 17.7 months, respectively. No significant difference in local recurrence was noted between the cohorts. An increased 1-year rate of symptomatic radionecrosis was seen in the larger margin group (20.9% vs 6.0%, p = 0.028). On multivariate analyses, margin size > 1.0 mm was associated with an increased risk for symptomatic radionecrosis (HR 3.07, 95% CI 1.13-8.34; p = 0.028), while multifraction SRS emerged as a protective factor for symptomatic radionecrosis (HR 0.13, 95% CI 0.02-0.76; p = 0.023).CONCLUSIONSExpanding the PTV margin beyond 1.0 mm is not associated with improved local recurrence but appears to increase the risk of symptomatic radionecrosis after postoperative SRS.

WE‐DE‐201‐04: Cross Validation of Knowledge‐Based Treatment Planning for Prostate LDR Brachytherapy Using Principle Component Analysis

PURPOSE To validate a knowledge-based algorithm for prostate LDR brachytherapy treatment planning. METHODS A dataset of 100 cases was compiled from an active prostate seed implant service. Cases were randomized into 10 subsets. For each subset, the 90 remaining library cases were registered to a common reference frame and then characterized on a point by point basis using principle component analysis (PCA). Each test case was converted to PCA vectors using the same process and compared with each library case using a Mahalanobis distance to evaluate similarity. Rank order PCA scores were used to select the best-matched library case. The seed arrangement was extracted from the best-matched case and used as a starting point for planning the test case. Any subsequent modifications were recorded that required input from a treatment planner to achieve V100>95%, V150<60%, V200<20%. To simulate operating-room planning constraints, seed activity was held constant, and the seed count could not increase. RESULTS The computational time required to register test-case contours and evaluate PCA similarity across the library was 10s. Preliminary analysis of 2 subsets shows that 9 of 20 test cases did not require any seed modifications to obtain an acceptable plan. Five test cases required fewer than 10 seed modifications or a grid shift. Another 5 test cases required approximately 20 seed modifications. An acceptable plan was not achieved for 1 outlier, which was substantially larger than its best match. Modifications took between 5s and 6min. CONCLUSION A knowledge-based treatment planning algorithm for prostate LDR brachytherapy is being cross validated using 100 prior cases. Preliminary results suggest that for this size library, acceptable plans can be achieved without planner input in about half of the cases while varying amounts of planner input are needed in remaining cases. Computational time and planning time are compatible with clinical practice.

SU-F-BRA-13: Knowledge-Based Treatment Planning for Prostate LDR Brachytherapy Based On Principle Component Analysis

Purpose: To create a knowledge-based algorithm for prostate LDR brachytherapy treatment planning that standardizes plan quality using seed arrangements tailored to individual physician preferences while being fast enough for real-time planning. Methods: A dataset of 130 prior cases was compiled for a physician with an active prostate seed implant practice. Ten cases were randomly selected to test the algorithm. Contours from the 120 library cases were registered to a common reference frame. Contour variations were characterized on a point by point basis using principle component analysis (PCA). A test case was converted to PCA vectors using the same process and then compared with each library case using a Mahalanobis distance to evaluate similarity. Rank order PCA scores were used to select the best-matched library case. The seed arrangement was extracted from the best-matched case and used as a starting point for planning the test case. Computational time was recorded. Any subsequent modifications were recorded that required input from a treatment planner to achieve an acceptable plan. Results: The computational time required to register contours from a test case and evaluate PCA similarity across the library was approximately 10s. Five of the ten test cases did not require any seed additions, deletions, or moves to obtain an acceptable plan. The remaining five test cases required on average 4.2 seed modifications. The time to complete manual plan modifications was less than 30s in all cases. Conclusion: A knowledge-based treatment planning algorithm was developed for prostate LDR brachytherapy based on principle component analysis. Initial results suggest that this approach can be used to quickly create treatment plans that require few if any modifications by the treatment planner. In general, test case plans have seed arrangements which are very similar to prior cases, and thus are inherently tailored to physician preferences.

SU-E-I-38: Improved Metal Artifact Correction Using Adaptive Dual Energy Calibration

Purpose: The empirical dual energy calibration (EDEC) method corrects for beam-hardening artifacts, but shows limited performance on metal artifact correction. In this work, we propose an adaptive dual energy calibration (ADEC) method to correct for metal artifacts. Methods: The empirical dual energy calibration (EDEC) method corrects for beam-hardening artifacts, but shows limited performance on metal artifact correction. In this work, we propose an adaptive dual energy calibration (ADEC) method to correct for metal artifacts. Results: Highly attenuating copper rods cause severe streaking artifacts on standard CT images. EDEC improves the image quality, but cannot eliminate the streaking artifacts. Compared to EDEC, the proposed ADEC method further reduces the streaking resulting from metallic inserts and beam-hardening effects and obtains material decomposition images with significantly improved accuracy. Conclusion: We propose an adaptive dual energy calibration method to correct for metal artifacts. ADEC is evaluated with the Shepp-Logan phantom, and shows superior metal artifact correction performance. In the future, we will further evaluate the performance of the proposed method with phantom and patient data.

MRI-Based Computed Tomography Metal Artifact Correction Method for Improving Proton Range Calculation Accuracy

PURPOSE Computed tomography (CT) artifacts can severely degrade dose calculation accuracy in proton therapy. Prompted by the recently increased popularity of magnetic resonance imaging (MRI) in the radiation therapy clinic, we developed an MRI-based CT artifact correction method for improving the accuracy of proton range calculations. METHODS AND MATERIALS The proposed method replaces corrupted CT data by mapping CT Hounsfield units (HU number) from a nearby artifact-free slice, using a coregistered MRI. MRI and CT volumetric images were registered with use of 3-dimensional (3D) deformable image registration (DIR). The registration was fine-tuned on a slice-by-slice basis by using 2D DIR. Based on the intensity of paired MRI pixel values and HU from an artifact-free slice, we performed a comprehensive analysis to predict the correct HU for the corrupted region. For a proof-of-concept validation, metal artifacts were simulated on a reference data set. Proton range was calculated using reference, artifactual, and corrected images to quantify the reduction in proton range error. The correction method was applied to 4 unique clinical cases. RESULTS The correction method resulted in substantial artifact reduction, both quantitatively and qualitatively. On respective simulated brain and head and neck CT images, the mean error was reduced from 495 and 370 HU to 108 and 92 HU after correction. Correspondingly, the absolute mean proton range errors of 2.4 cm and 1.7 cm were reduced to less than 2 mm in both cases. CONCLUSIONS Our MRI-based CT artifact correction method can improve CT image quality and proton range calculation accuracy for patients with severe CT artifacts.

SU-E-T-369: Experience of Using 6D Robotic Couch Top in the Treatment of Intracranial Tumors Utilizing Frameless Stereotactic Radiosurgery (SRS) Technique

PURPOSE To investigate the extent and necessity of 6 DOF corrections for intracranial frameless Stereotactic Radiosurgery METHODS: Civco Protura 6D robotic couch top was fitted to the Novalis TX in 2012. The couch enables ± 3 ° rotations in pitch, roll and yaw, ±50 mm in lateral and longitudinal shifts and ±25 mm in vertical shifts. Patient sets up using the room laser; then two orthogonal kV images are taken for confirmation. A CBCT is acquired and registered to the planning CT using two independent systems. The calculated rotational and translational shifts are applied. A second CBCT is acquired to assess the residual translational and rotational errors. The treatment will be carried out if residual rotational shifts are ≤ 0.3 degrees. We treated 113 patients utilizing 6D couch to align a total of 160 targets. Some of the targets were fractionated, with total alignments of 252. Geometrical analysis is performed to assess the systems accuracy and extent of shifts. RESULTS After the planar kV images alignment, a CBCT was acquired and registered to the planning CT, the average required rotational shifts were (yaw=1.03 °± 0.8, roll=1.16°± 0.9 and Pitch= 0.9°± 0.7). A second CBCT was taken to verify the match and the previous shifts, the residual rotational errors on average were 0.37°± 0.6, 0.27°± 0.28 and 0.24°± 0.29 in the yaw, roll, and pitch directions, respectively. The translational residual shifts (mm) were 0.68 ± 0.57, 0.68 ± 0.57, 0.68 ± 0.57 in lateral, vertical, and longitudinal directions, respectively. CONCLUSION The 6D couch is capable of aligning targets with an accuracy of ≤ 0.4 ° in any rotational direction and ≤ 0.7 mm in any translational directions, and not applying the rotational corrections could lead to compromised target dose coverage and may lead to excessive dose to OARs.

TH‐A‐9A‐11: Single‐Isocenter Multiple‐Target SRS: Risk of Compromised Coverage

PURPOSE To characterize the risks of compromised coverage in single-isocenter multiple-lesion VMAT SRS. METHODS Fifty patients were selected retrospectively from our SRS program. Each patient had two lesions treated to ≥ 21 Gy. Single-isocenter VMAT SRS plans were created in Eclipse. PTV volumes and distances from isocenter were recorded. PTV coverage (D95 and V95) was evaluated across rotational setup errors of 0.5, 1.0, or 2° applied to three axes. Coverage rates were analyzed versus volume, distance, and rotation. For a rotational error of 2°, lesion size and separation distance were compared across coverage rate levels using ANOVA. A multivariate logistic regression model was fit using generalized estimating equations (GEE), modeling the probability of a 95% V95/D95 rate or higher given lesion size and separation distance while accounting for intra-patient correlation. The estimated probabilities from the GEE model were used to capture the operating conditions in a receiver operating characteristic (ROC) curve; area under the curve (AUC) was estimated. RESULTS Mean lesion volume and distance to isocenter are 0.96±1.25cc and 3.53±1.61cm. V95/D95 proportions above 95% range from 92-100% when rotational errors are ≤1°. At 2.0° rotation, V95/D95 are >95% in only 62-64% of cases; V95 falls to 75% for <0.3cc lesions at 4cm yet remains >90% up to 6cm for lesions >0.9cc. V95 is <40% in an extreme case. The logistic regression analysis shows that lesion volume and distance to isocenter are independent predictors (p< 0.001) of V95/D95 rates exceeding 95%. The ROC derived from a GEE multivariate model has an AUC of 0.87. CONCLUSION PTV coverage can be compromised substantially by rotational setup errors of 2°, in particular for <0.3cc lesions at distances >4cm from isocenter. Statistical analysis suggests that lesion volume and distance to isocenter could be used to select patients who are good candidates for single-isocenter multiple-lesion SRS.