Z Chang

TH-EF-BRA-08: A Novel Technique for Estimating Volumetric Cine MRI (VC-MRI) From Multi-Slice Sparsely Sampled Cine Images Using Motion Modeling and Free Form Deformation

PURPOSE To develop a technique to estimate on-board VC-MRI using multi-slice sparsely-sampled cine images, patient prior 4D-MRI, motion-modeling and free-form deformation for real-time 3D target verification of lung radiotherapy. METHODS A previous method has been developed to generate on-board VC-MRI by deforming prior MRI images based on a motion model(MM) extracted from prior 4D-MRI and a single-slice on-board 2D-cine image. In this study, free-form deformation(FD) was introduced to correct for errors in the MM when large anatomical changes exist. Multiple-slice sparsely-sampled on-board 2D-cine images located within the target are used to improve both the estimation accuracy and temporal resolution of VC-MRI. The on-board 2D-cine MRIs are acquired at 20-30frames/s by sampling only 10% of the k-space on Cartesian grid, with 85% of that taken at the central k-space. The method was evaluated using XCAT(computerized patient model) simulation of lung cancer patients with various anatomical and respirational changes from prior 4D-MRI to onboard volume. The accuracy was evaluated using Volume-Percent-Difference(VPD) and Center-of-Mass-Shift(COMS) of the estimated tumor volume. Effects of region-of-interest(ROI) selection, 2D-cine slice orientation, slice number and slice location on the estimation accuracy were evaluated. RESULTS VCMRI estimated using 10 sparsely-sampled sagittal 2D-cine MRIs achieved VPD/COMS of 9.07±3.54%/0.45±0.53mm among all scenarios based on estimation with ROI_MM-ROI_FD. The FD optimization improved estimation significantly for scenarios with anatomical changes. Using ROI-FD achieved better estimation than global-FD. Changing the multi-slice orientation to axial, coronal, and axial/sagittal orthogonal reduced the accuracy of VCMRI to VPD/COMS of 19.47±15.74%/1.57±2.54mm, 20.70±9.97%/2.34±0.92mm, and 16.02±13.79%/0.60±0.82mm, respectively. Reducing the number of cines to 8 enhanced temporal resolution of VC-MRI by 25% while maintaining the estimation accuracy. Estimation using slices sampled uniformly through the tumor achieved better accuracy than slices sampled non-uniformly. CONCLUSIONS Preliminary studies showed that it is feasible to generate VC-MRI from multi-slice sparsely-sampled 2D-cine images for real-time 3D-target verification. This work was supported by the National Institutes of Health under Grant No. R01-CA184173 and a research grant from Varian Medical Systems.

TU-AB-BRA-09: A Novel Method of Generating Ultrafast Volumetric Cine MRI (VC-MRI) Using Prior 4D-MRI and On-Board Phase-Skipped Encoding Acquisition for Radiotherapy Target Localization

PURPOSE To develop a technique generating ultrafast on-board VC-MRI using prior 4D-MRI and on-board phase-skipped encoding k-space acquisition for real-time 3D target tracking of liver and lung radiotherapy. METHODS The end-of-expiration (EOE) volume in 4D-MRI acquired during the simulation was selected as the prior volume. 3 major respiratory deformation patterns were extracted through the principal component analysis of the deformation field maps (DFMs) generated between EOE and all other phases. The on-board VC-MRI at each instant was considered as a deformation of the prior volume, and the deformation was modeled as a linear combination of the extracted 3 major deformation patterns. To solve the weighting coefficients of the 3 major patterns, a 2D slice was extracted from VC-MRI volume to match with the 2D on-board sampling data, which was generated by 8-fold phase skipped-encoding k-space acquisition (i.e., sample 1 phase-encoding line out of every 8 lines) to achieve an ultrafast 16-24 volumes/s frame rate. The method was evaluated using XCAT digital phantom to simulate lung cancer patients. The 3D volume of end-ofinhalation (EOI) phase at the treatment day was used as ground-truth onboard VC-MRI with simulated changes in 1) breathing amplitude and 2) breathing amplitude/phase change from the simulation day. A liver cancer patient case was evaluated for in-vivo feasibility demonstration. RESULTS The comparison between ground truth and estimated on-board VC-MRI shows good agreements. In XCAT study with changed breathing amplitude, the volume-percent-difference(VPD) between ground-truth and estimated tumor volumes at EOI was 6.28% and the Center-of-Mass-Shift(COMS) was 0.82mm; with changed breathing amplitude and phase, the VPD was 8.50% and the COMS was 0.54mm. The study of liver patient case also demonstrated a promising in vivo feasibility of the proposed method CONCLUSION: Preliminary results suggest the feasibility to estimate ultrafast VC-MRI for on-board target localization with phase skipped-encoding k-space acquisition. Research grant from NIH R01-184173.

SU-F-J-155: Evaluation of Transcytolemmal Water Exchange Analysis For Therapeutic Response Assessment Using Dynamic Contrast-Enhanced MRI

PURPOSE To compare the performance of shutter-speed(SS) model with transcytolemmal water exchange analysis against the Tofts model in the study of the efficacy of an anti-angiogenesis drug METHODS: 16 mice with LS-174T implanted were randomly assigned into treatment/control groups (n=8/group) and received bevacizumab/saline three times (Day1/Day4/Day8). All mice received one pre- (Day0) and two post-treatment (Day2/Day9) DCE scans. For each scan, the CA extravasation rate constant KTtrans /KStrans from the Tofts/SS model were calculated. The intracellular water residence time τi which reflects limited transcytolemmal water exchange between cell and extravascular-extracellular-space were also analyzed using SS model. A biological subvolume(BV) within the tumor was automatically segmented based on the τi intensity distribution, and the SS model parameters within the BV (KS,BVtrans and τi, BV ) were analyzed. Rank-sum tests were conducted to assess the differences of each parameters statistics (mean value/coefficient-of-variation (CV) /kurtosis/skewness/heterogeneity indices d1 and d2 ) between treatment/control groups. Experiment using support vector machine in a leave-one-out approach were performed to validate the use of the analyzed biomarkers for treatment/control classification. RESULTS The SS model was a better fit for all scans in terms of Bayesian information criterion. At Day9, the treatment group had significantly higher mean KTtrans (p=0.021), KStrans (p=0.021) and τi (p=0.045). In the identified BV, the treatment group had significantly higher mean KS,BVtrans at both Day2(p=0.038) and Day9(p=0.007). Additionally, at Day9, the treatment group had significantly higher mean τi , BV (p=0.045) and higher KS,BVtrans heterogeneity indices d1 (p=0.010) and d2 (p=0.021) values. When using KS,BVtrans statistics for treatment/control group classification, the highest accuracy was 68.8%/87.5% at Day2/Day9; this result was better than the result of 62.5%/87.5% using KStrans statistics and 50.0%/87.5% using KTtrans statistics. CONCLUSION The SS model parameters may be more reliable than the Tofts model parameters for therapeutic assessment. The proposed biological subvolume in this work may be useful for early therapeutic effect monitoring.

TU-AB-202-04: Development of a DeformableImage Registration (DIR) Error Correction Method Employing Kolmogorov-Zurbenko(KZ) Filter.

PURPOSE This study aims to develop a DIR error correction method capable of utilizing sparse ground-truth motion information and recovering missing data with the Kolmogorov-Zurbenko (KZ) filter. METHODS The error correction method employs a two-step approach. First, sparse ground-truth displacement vectors are integrated into a pre-correction deformable vector field (DVF) to estimate a post-correction DVF with coarse resolution. Second, the coarse post-correction DVF is boosted to a full-resolution DVF through convolution with the KZ filter. To validate the use of the KZ filter for missing data recovery, recovery errors were determined by comparing a DVF recovered from down-sampling with the original full-resolution DVF. The entire error correction method was tested on an in-house developed digital lung motion phantom consisting of a primary volume, a DVF, and a secondary volume synthesized by applying the DVF on the primary volume. Five pre-correction DVFs were obtained by performing DIR between the two volumes using Velocity, MIM, ILK and OHS algorithms in DIRART toolbox, and Elastix, and then corrected. Primary volumes were synthesized with pre- and post-correction DVFs, respectively. The error correction method was evaluated with pre- and post-correction registration errors, and intensity errors in synthesized primary volumes. RESULTS Our test results for sparsely down-sampled (<0.4%) DVFs showed that the KZ filter outperformed the cubic polynomial interpolation method for whole lung DVF map recovery in terms of median error (0.60mm vs 0.73mm) and mean error (1.18mm vs 1.29mm). Pre- and post-correction 3D registration errors per voxel for Velocity, MIM, ILK, OHS, and Elastix are reduced by 2.39 mm on average. Pre- and post-correction intensity errors are reduced by 0.37 in unit of image intensity on average. CONCLUSION We have implemented a two-step method capable of utilizing sparse ground-truth displacement vectors for DIR error reduction, allowing DIR accuracy improvement utilizing clinically available motion data. This study is supported by NIH grant 1R21CA165384.

SU-F-R-15: Establishing Relevant ADC-Based Texture Analysis Metrics for Quantifying Early Treatment-Induced Changes in Head and Neck Squamous Cell Carcinoma

PURPOSE The purpose of this study is to identify texture analysis metrics from apparent diffusion coefficient (ADC) maps that would provide quantifiable changes with treatment in patients with head and neck squamous cell carcinoma (HNSCC). We discerned which imaging metrics were relevant using baseline agreement and variations during early treatment. METHODS We retrospectively analyzed diffusion-weighted MRI scans in 9 patients with stages II-IV HNSCC. ADC maps were generated from two baseline scans, performed 1 week apart, and one early treatment scan, obtained during the 2nd week of chemoradiation. Regions of interest (ROI) consisting of primary and nodal disease were drawn on the resampled ADC maps. Four 3D texture matrices describing local and regional relationships between voxel intensities in the ROIs were generated. From these, 38 texture metrics and 7 histogram features were calculated for each patient, including the mean and median ADC. To identify metrics with good agreement between baseline studies, we compared all metrics using the intra-class correlation coefficient (ICC). For metrics with ICC ≥ 0.80, the Wilcoxon signed-rank test was used to test if the difference between the mean of the baselines and the early treatment was non-zero. RESULTS Nine of the 45 metrics had an ICC ≥ 0.80. Six of these 9 metrics had a p-value < 0.05: run length non-uniformity, ADC median, texture strength, ADC mean, zone percentage, and variance. Only 1 of the 9 metrics remained of interest, after applying the Holm correction to the alpha levels: run length non-uniformity (p = 0.004) in the Gray Level Run Length Matrix. CONCLUSION The feasibility of texture analysis is dependent on the baseline agreement of each metric, which disqualifies many texture characteristics. Consequently, only a few metrics are reproducible and qualify for future studies that provide quantitative assessment of early treatment changes for HNSCC.

SU-G-201-13: Investigation of Dose Variation Induced by HDR Ir-192 Source Global Shift Within the Varian Ring Applicator Using Monte Carlo Methods

PURPOSE The Varian tandem and ring applicators are used to deliver HDR Ir-192 brachytherapy for cervical cancer. The source path within the ring is hard to predict due to the larger interior ring lumen. Some studies showed the source could be several millimeters different from planned positions, while other studies demonstrated minimal dosimetric impact. A global shift can be applied to limit the effect of positioning offsets. The purpose of this study was to assess the necessities of implementing a global source shift using Monte Carlo (MC) simulations. METHODS The MCNP5 radiation transport code was used for all MC simulations. To accommodate TG-186 guidelines and eliminate inter-source attenuation, a BrachyVision plan with 10 dwell positions (0.5cm step sizes) was simulated as the summation of 10 individual sources with equal dwell times for simplification. To simplify the study, the tandem was also excluded from the MC model. Global shifts of ±0.1, ±0.3, ±0.5 cm were then simulated as distal and proximal from the reference positions. Dose was scored in water for all MC simulations and was normalized to 100% at the normalization point 0.5 cm from the cap in the ring plane. For dose comparison, Point A was 2 cm caudal from the buildup cap and 2 cm lateral on either side of the ring axis. With seventy simulations, 108 photon histories gave a statistical uncertainties (k=1) <2% for (0.1 cm)3 voxels. RESULTS Compared to no global shift, average Point A doses were 0.0%, 0.4%, and 2.2% higher for distal global shifts, and 0.4%, 2.8%, and 5.1% higher for proximal global shifts, respectively. The MC Point A doses differed by < 1% when compared to BrachyVision. CONCLUSION Dose variations were not substantial for ±0.3 cm global shifts, which is common in clinical practice.

MO-B-BRC-02: Ultrasound Based Prostate HDR

Brachytherapy has proven to be an effective treatment option for prostate cancer. Initially, prostate brachytherapy was delivered through permanently implanted low dose rate (LDR) radioactive sources; however, high dose rate (HDR) temporary brachytherapy for prostate cancer is gaining popularity. Needle insertion during prostate brachytherapy is most commonly performed under ultrasound (U/S) guidance; however, treatment planning may be performed utilizing several imaging modalities either in an intra- or post-operative setting. During intra-operative prostate HDR, the needles are imaged during implantation, and planning may be performed in real time. At present, the most common imaging modality utilized for intra-operative prostate HDR is U/S. Alternatively, in the post-operative setting, following needle implantation, patients may be simulated with computed tomography (CT) or magnetic resonance imaging (MRI). Each imaging modality and workflow provides its share of benefits and limitations. Prostate HDR has been adopted in a number of cancer centers across the nation. In this educational session, we will explore the role of U/S, CT, and MRI in HDR prostate brachytherapy. Example workflows and operational details will be shared, and we will discuss how to establish a prostate HDR program in a clinical setting. LEARNING OBJECTIVES 1. Review prostate HDR techniques based on the imaging modality 2. Discuss the challenges and pitfalls introduced by the three imagebased options for prostate HDR brachytherapy 3. Review the QA process and learn about the development of clinical workflows for these imaging options at different institutions.

TU‐D‐BRD‐01: Image Guided SBRT II: Challenges ' Pitfalls

Stereotactic body radiation therapy (SBRT) has been effective treatment for the management of various diseases, which often delivers high radiation dose in a single or a few fractions. SBRT therefore demands precise treatment delivery to the tumor while sparing adjacent healthy tissue. Recent developments in image guidance enable target localization with increased accuracy. With such improvements in localization, image-guided SBRT has been widely adopted into clinical practice. In SBRT, high radiation dose is generally delivered with small fields. Therefore, it is crucial to accurately measure dosimetric data for the small fields during commissioning. In addition, image-guided SBRT demands accurate image localization to ensure safety and quality of patient care. Lately, the reports of AAPM TG 142 and TG 104 have been published and added recommendations for imaging devices that are integrated with the linear accelerator for SBRT. Furthermore, various challenges and potential pitfalls lie in the clinical implementation of image-guided SBRT. In this lecture, these challenges and pitfalls of image-guided SBRT will be illustrated and discussed from dosimetric, technical and clinical perspectives.Being a promising technique, image-guided SBRT has shown great potentials, and will lead to more accurate and safer SBRT treatments. LEARNING OBJECTIVES 1. To understand dosimetric challenges and pitfalls for IGRT application in SBRT. 2. To understand major clinical challenges and pitfalls for IGRT application in SBRT. 3. To understand major technical challenges and pitfalls for IGRT application in SBRT.