Radiation and Cancer Physics

SS 37 - Physics 11 - Online Imaging and Motion Management

267 - Magnitude and Dosimetric Impact of?Inter- and Intra-Fraction Anatomic Variability During MRI-Guided Online Adaptive Radiation Therapy (MRgART) to the Abdomen

Wednesday, October 24
11:30 AM - 11:40 AM
Location: Room 302

Magnitude and Dosimetric Impact of Inter- and Intra-Fraction Anatomic Variability During MRI-Guided Online Adaptive Radiation Therapy (MRgART) to the Abdomen
L. E. Henke1, O. L. Green2, A. Curcuru1, Y. Lu1, S. Mutic2, P. J. Parikh2, and R. Kashani3; 1Washington University School of Medicine, Department of Radiation Oncology, St. Louis, MO, 2Washington University School of Medicine, St. Louis, MO, 3University of Michigan, Ann Arbor, MI

Purpose/Objective(s): Online adaptive radiotherapy (ART) using MRI-guided radiotherapy (MRgRT) is being used to deliver ablative doses to abdominal malignancies. The reported time frame for online ART planning is 30 minutes, raising concerns that intra-fraction motion of abdominal organs-at-risk (OAR) might erase dosimetric gains of ART. We compared the magnitude of inter-fraction OAR motion to motion occurring during the timeframe of a single treatment. We then analyzed the fractional dosimetric benefit of online ART for OAR sparing, accounting for intra-fraction motion.

Materials/Methods: Exhale breath-hold setup 0.35T MRIs were acquired for 10 patients undergoing abdominal MRgART (pancreas, liver, nodal oligometastases) for initial daily setup, and then repeated at 30–60 min, while patients remained on-table in the treatment position. Stomach (STOM), duodenum (DUOD), small bowel (SB), and large bowel (LB) contours from the initial daily setup MRI (fxA1 contours) were used as the baseline and compared to contours from the same-fraction repeat MRI (fxA2) to delineate intra-fraction OAR motion. Inter-fraction OAR motion was determined by comparing fxA1 contours to contours from another fraction’s MRI (fxB), acquired 1-2 days apart from fxA1. Similarity metrics including dice coefficient (DC) and modified Hausdorff distance (MHD) were compared between fxA1 and fxA2 vs fxA1 and fxB contours. Dosimetric impact of inter- and intra-fraction motion was evaluated by comparing dose delivered to OARs when pre-existing non-adaptive plans were applied to fxA1 contours vs. when online-adaptive plans were applied to fxA2 contours.

Results: Across all OARs, intra-fraction motion between fxA1 and fxA2 contours was significantly less than inter-fraction motion between fxA1 and fxB contours, as measured by both average DC (0.74 vs. 0.55, P<0.05) and average MHD (3.9mm vs. 8.3mm, P<0.001). For individual OARs, inter-fraction motion exceeded intra-fraction motion for both DC and MHD (P<0.05), except for the SB, for which DC similarity was significantly better for intra-fraction motion (P=0.045) but MHD was not significantly less (P=0.06). When non-adaptive plans were applied to fxA1 contours, 15 OAR violations occurred, comprising 6 STOM, 3 DUOD, 5 SB and 1 LB violations. All were resolved with ART. When adaptive plans were applied to fxA2 contours, 6 violations occurred (5 SB, 1 DUOD). Average magnitude of OAR dose violations (defined as [max OAR point dose in Gy/OAR point dose constraint in Gy] x100%) was less for intra-fraction violations (107%), than for inter-fraction violations (116%).

Conclusion: In this study, we found that intra-fraction abdominal OAR motion was significantly less than inter-fraction motion, validating an online adaptation approach. There was preservation of the dosimetric gains for online ART throughout the timeframe of a treatment fraction for all OARs with the exception of the small bowel. Further work is needed to assess the necessary PRV and tolerance for small bowel in MRgART.

Author Disclosure: L.E. Henke: Research Grant; ViewRay, Inc. Speaker's Bureau; ViewRay, Inc. Travel Expenses; ViewRay, Inc. O.L. Green: Honoraria; ViewRay, Inc. Travel Expenses; ViewRay, Inc. A. Curcuru: None. Y. Lu: None. S. Mutic: Honoraria; ViewRay Inc, Varian Medical Systems, Inc, American Society for Radiation Oncology (ASTRO). Advisory Board; ViewRay Inc. Travel Expenses; ViewRay Inc, Varian Medical Systems, Inc, American Society for Radiation Oncology (ASTRO). Stock; Radialogica. Partnership; TreatSafely. Chief Technical Officer; Radialogica, LLC. P.J. Parikh: Research Grant; Varian Medical, Viewray. Honoraria; Viewray. Speaker's Bureau; Sirtex. Advisory Board; Sirtex. Stock; Holaira.

Lauren Henke, MD, MS

Disclosure:
Employment
Barnes Jewish Hospital: Hospital Employee: Employee

Compensation
ViewRay, Inc.: Research Grants, Speaker's Bureau, Travel Expenses

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267 - Magnitude and Dosimetric Impact of?Inter- and Intra-Fraction Anatomic Variability During MRI-Guided Online Adaptive Radiation Therapy (MRgART) to the Abdomen



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