Treatment Outcomes and Toxicity Profile of MRI-Guided Gated-Real Time Online Adaptive Stereotactic Body Radiotherapy to Patients with Pancreatic Tumors.

PURPOSE/OBJECTIVE(S): Stereotactic body radiation therapy (SBRT) to patients with inoperable pancreas cancer has shown promising outcomes. Dose escalation is challenging given the associated risks to nearby organs at risk and requires treatment plan adaptation and motion management. This study reports the treatment related adverse events and treatment outcomes of MRI-guided SBRT utilizing gated-real time online adaptive radiotherapy to patients with pancreatic tumors.

MATERIALS/METHODS: This is a single institution retrospective IRB approved study and included patients with pancreatic tumors who were treated with MRI-guided SBRT utilizing gated-real time online adaptive radiotherapy, consecutively between 2019 and 2022. All patients were treated with 0.35T MRI linear accelerator. All patients' charts and radiation plans were reviewed for this study.

RESULTS: The study included 25 patients treated consecutively to 125 radiation fractions with MRI-guided SBRT. The study included 15 females and 10 males, 19 white, 2 Asians, 1 Latin American and 2 African Americans. The median age was 74, and all patients had pancreatic adenocarcinoma (92%) except two patients who had biopsy proven renal cell carcinoma metastatic to the pancreas without evidence of disease elsewhere. Most patients (68%) received chemotherapy (FOLFIRINOX or Gemcitabine / Protein-bound paclitaxel) prior to SBRT. The median dose to gross target volume (GTV) was 50 Gy, and to planning target volume (PTV) was 35 Gy. All patients were treated in 5 radiation fractions. Thirteen patients (52%) received elective nodal irradiation (ENI). All patients had at least a single real-time online adaptive replanning (median 3 fractions, range 1-5 fractions). All treatment fractions were gated (100%) with MRI guidance. The 1-, 2-, & 3- year local-regional control (LRC) were 81%, 57%, & 57% respectively (Median 16 months). The 1-, 2-, & 3- year Distant Control (DC) were 81%, 74%, & 56% respectively (Median 14 months). The 1-, 2-, 3-year overall survival (OS) were 77%, 35%, & 24% (Median 21 months). Planning Target Volume (PTV) dose ≥40Gy was significantly associated with improved local-regional control (p = 0.02). Gross Target Volume (GTV) dose <50Gy was significantly associated with lower DC (p = 0.03). ENI showed a trend towards improved local-regional control but was not statistically significant (p = 0.1). None of the patients experienced ≥ grade 3 toxicities. Most common adverse event were grades 1 or 2 fatigue (24%), nausea (16%), and abdominal discomfort (12%).

CONCLUSION: In this cohort, MRI-guided SBRT utilizing gated-real time online adaptive radiotherapy was a safe and effective approach for patients with pancreatic tumors who are not surgical candidates. In this study, dose escalation was significantly associated with improved local-regional (PTV ≥40 Gy) and distant control (GTV ≥50Gy). None of the patients experienced grade ≥3 radiation induced adverse events.