A Simple Adaptive Planning Workflow for MR-Guided Radiotherapy of Prostate Cancer
X Chen*, E Paulson, E Ahunbay, E Omari, W Hall, C Lawton, M Straza, X Li, Medical College of Wisconsin, Milwaukee, WI
Presentations
PO-GePV-M-74 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: Two workflows, adaptive planning (adapt-to-shape, ATS) or repositioning (adapt-to-position, ATP) are used in MR-guided adaptive radiation therapy (MRgART) on a 1.5T MR-Linac. For MRgART of prostate cancer, ATP usually take 30 minutes, shorter than that for ATS (50 min), and is used for most fractions. However, ATP may not be sufficient if there is a large air-cavity in the rectum. We propose a simple adaptive workflow, Pseudo-ATS+, that can account for an air cavity change with a shorter time compared to ATS.
Methods: The Pseudo-ATS+ workflow includes: (1) segmenting the air cavity and external contours on daily air scan specifically designed to image air, (2) populating the obtained air and external contours, along with the reference contours for other structures to the daily MRI, and (3) optimizing an adaptive plan on daily images. To evaluate the performance of Pseudo-ATS+, we performed standard ATP, reconstructed doses of the ATP and Pseudo-ATS+ plans on daily images with fully updated contours and compared the obtained doses with the ATS plans for selected daily MRI sets with the presence of an air cavity in the rectum. Various DVH parameters, e.g., minimum dose (Dmin) for the PTV and volume coverage of 63Gy (V63Gy) for rectum and bladder, were compared.
Results: For Pseudo-ATS+, the differences in DVH parameters between the reconstructed dose and ATS plans were smaller than those for reconstructed ATP. The PTV Dmin of reconstructed Pseudo-ATS+ was 3.3% different from ATS, while the difference was 9.8% for reconstructed ATP from ATS for a case tested. Rectum V63Gy was within the planning goal for reconstructed Pseudo-ATS+ while reconstructed ATP plan dose was not. The Pseudo-ATS+ workflow required additional 5 minutes compared to ATP.
Conclusion: This proposed simple adaptive workflow, Pseudo-ATS+, can properly account for rectal air cavities changes without significantly extending patient-on-couch time during MRgART for prostate cancer.
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