Purpose: Radiotherapy (RT) dose escalation using stereotactic body radiation therapy (SBRT) may significantly improve both local control (LC) and overall survival (OS) for patients with inoperable pancreas cancer. However, ablative dose cannot be routinely offered because of the risk of causing severe injury to adjacent normal organs. Stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) represents a novel technique that may achieve safe delivery of ablative dose and improve long-term outcomes.

Methods and materials: We performed a single institution retrospective analysis of 35 consecutive pancreatic cancer patients treated with SMART in mid-inspiration breath hold on an MR-LINAC. Most had locally advanced disease (80%) and received induction chemotherapy (91.4%) for a median 3.9 months prior to SBRT. All were prescribed 5 fractions delivered in consecutive days to a median total dose of 50 Gy (BED10 100 Gy10), typically with a 120-130% hotspot. Elective nodal irradiation was delivered to 20 (57.1%) patients. No patient had fiducial markers placed and all were treated with continuous intrafraction MR visualization and automatic beam triggering.

Results: With median follow-up of 10.3 months from SMART, acute (2.9%) and late (2.9%) grade 3 toxicities were uncommon. One-year LC, distant metastasis-free survival, progression-free survival, cause-specific survival, and OS were 87.8%, 63.1%, 52.4%, 77.6% and 58.9%, respectively.

Conclusions: To our knowledge, this is the first report of 5-fraction pancreas SMART delivered on an MR-LINAC. We observed minimal severe treatment-related toxicity and encouraging early LC. Prospective confirmation of feasibility and long-term clinical outcomes of dose intensified SMART is warranted.

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