The Use of Dual Isocenter MR-Guided Adaptive Radiation Therapy for Large Field Sarcomas
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PO-GePV-M-228 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Soft tissue sarcomas are large and heterogeneous with radiation response difficult to predict, making them well-suited for MR-guided adaptive radiation therapy (MRgART). However, these bulky lesions may extend past the maximum clinical field size, necessitating the use of multiple isocenters to facilitate treating > 25cm. This work evaluates the dosimetric accuracy and effects of subsequent adaptations on the region of overlap between the two isocenters to support large-field MRgART.
Methods: The dual isocenter treatment plans for 6 sarcoma patients were retrospectively investigated. Measurements were performed using a 3D detector array. Dose measurements were compared to the corresponding planned dose distribution using gamma index with criteria of 3%/2mm, 10% dose threshold focusing on the region of overlap between the fields associated with the superior and inferior isocenters. The gross tumor volume (GTV) for Patient 6 was updated daily to reflect changes in patient anatomy. In this study, the change in tumor volume was analyzed for this patient and an adaptive plan was measured to confirm robustness of the adaptive process utilizing dual isocenter treatment plan.
Results: All plans passed with >95% of points meeting the criteria with pass rates ranging from 95.3%- 98.7%. The volume of the GTV contour for Patient 6 ranged from 2963.3cc at simulation to 3855.3cc at the final treatment, necessitating ART. The gamma pass rate for the adaptive plan was 96.9% which was comparable to the gamma pass rate of the original treatment plan (98.7%). No appreciable variations in dose were observed at the junction of the two isocenters for the original or ART deliveries as compared to the planned geometry.
Conclusion: Dual isocenter planning enables the treatment of large sarcomas with MRgART despite field size limitations. The overlap region was accurately represented in the planning system and was found to be robust to daily adaptation.
Funding Support, Disclosures, and Conflict of Interest: Research collaborations with Philips Healthcare, GE Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189 and R01HL153720.
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