Feasibility of Stereotactic Body Radiation Therapy for Pancreatic Tumors Abutting Organs at Risk Using Magnetic Resonance Guided Adaptive Radiation Therapy.

PURPOSE: Stereotactic body radiation therapy (SBRT) has historically been contraindicated in patients with tumors abutting gastrointestinal (GI) organs due to risk of toxicity. Adaptive magnetic resonance (MR) guided SBRT (MRgSBRT) is an increasingly used treatment paradigm to prescribe ablative doses to pancreatic tumors. Here we present our institutional experience of adaptive MRgSBRT for pancreatic tumors abutting or invading GI organs at risk (OARs).

METHODS AND MATERIALS: Forty-eight patients with pancreatic adenocarcinoma tumors abutting or invading GI OARs who received MRgSBRT to 50 Gy in 5 fractions at our institution between 2018-2019 were reviewed. Dosimetric variables were compared pre- and postadaptation to determine adequacy of target coverage, reasons for online adaptation, and resulting changes in GI OAR and constraints.

RESULTS

Patients' mean age was 67 years, 50% female, 63% with ECOG 0-1, and with majority of tumors being locally advanced (52%) and located in the pancreatic head, uncinate process, or neck (92%). Tumors abutted or invaded GI OARs in 100% and 21% of cases, respectively. Of the 240 fractions evaluated, 99% required online adaptation and 77% underwent normalization. The mean PTV_opt (PTV minus a 5mm-expansion of GI OAR contours as the plan optimization structure) receiving prescription dose was 93%. The predicted and adapted critical volume (V36Gy ≤0.5 cc) for OARs were found to be statistically significantly different (P < .001). The duodenum had the highest volume receiving 36 Gy for both preadapted (mean 3.4 cc) and postadapted (mean 0.33 cc) plans. Plans for pancreatic head, uncinate process, or neck tumors frequently exceeded duodenum dose constraints and plans for pancreatic body or tail tumors more often exceeded stomach constraints (P < .001).

CONCLUSIONS

Adaptive MR guidance may permit SBRT for pancreatic tumors abutting or invading OARs with minimal toxicity.