J Kim¹*, P Parikh¹, P Lee², MD Chuong³, M Fuss⁴, D Low⁵, (1) Henry Ford Health System, Detroit, MI, (2) MD Anderson Cancer Center, (3) MCI, Miami, FL, (4) Viewray, (5) UCLA, Los Angeles, CA

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  J Kim

TH-B-206-5 (Thursday, 7/14/2022) 8:30 AM - 9:30 AM [Eastern Time (GMT-4)]

Room 206

Purpose: The goal of the multi-institutional phase II Stereotactic MR-guided Adaptive Radiation Therapy (SMART) for pancreatic cancer was to deliver an ablative prescription dose using an isotoxic approach with on-table adaptive replanning on a 0.35T MR-Linac. Quality assurance was done to evaluate adaptive plans for contour accuracy and their ability to reach an ablative dose (BED >70 Gy) even while sacrificing coverage due to the isotoxic approach.

Methods: We evaluated 120 patients enrolled across 10 institutions. Prescription dose was 50 Gy in 5 fractions, but limited by protocol-specified OAR constraints. GI viscous hollow structures (stomach, duodenum, small bowel, and large bowel) were re-contoured using the localization MRI. Full course dose was recalculated on the day’s anatomy. Plan adaptation occurred if the volume receiving 33Gy or greater (V33) of any contoured OARs exceeded 1cc or for insufficient target coverage. GI OAR contour accuracy was evaluated retrospectively on the first adapted fraction for each patient by 3 physician PIs. Major deviations were contour inaccuracies potentially leading to substantial overlap of non-contoured regions with the 33Gy isodose.

Results: 102 (85%) patients were contoured per protocol or with minor deviations, and 18 (15%) had major deviations. 537 (89.5%) fractions required adaptation. The duodenum (N=443) most frequently exceeded constraints. After adaptation, V33 was < 1.0cc for all OARs. After excluding overlapping OARs, mean D98% to the remaining PTV was 40.2 Gy. Overall, the mean CTV D95% was 41.3 Gy (82.6% of Rx dose), resulting in an approximate BED of 75.4 Gy.

Conclusion: Frequency of contouring deviations was similar to comparable protocols, even with the higher complexity of the daily MR imaging and plan adaptation. Patients successfully received ablative radiation while maintaining the isotoxicity of surrounding OARs. Further work will complete contour evaluation and separately assess association between dose and contour deviations on patient outcomes.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by funding from Viewray, Inc.

Keywords

MRI, Image Guidance, Clinical Trials

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI protocols for therapy

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