Patterns of Failure After Definitive Ablative 5-Fraction Stereotactic Body Radiation Therapy for Inoperable Pancreatic Ductal Adenocarcinoma

PURPOSE: Definitive stereotactic body radiation therapy (SBRT) for inoperable pancreatic ductal adenocarcinoma (PDAC) is often delivered with a nonablative dose to gross tumor alone despite high rates of locoregional failure (LRF). The purpose of this study was to characterize patterns of failure after definitive ablative SBRT and evaluate the impact of clinical target volume (CTV) design. METHODS AND MATERIALS: We performed a retrospective cohort study of nonmetastatic PDAC treated with definitive ablative SBRT on a 0.35 Tesla MR-Linac between 2018-2024. Patients who had surgery were excluded. The median prescribed gross tumor volume and CTV doses were 50 Gy and 33 Gy in 5 fractions, respectively. CTV coverage evolved from no CTV or limited perivascular coverage to larger anatomically derived volumes eventually including the "triangle volume." The first diagnostic scan showing LRF was registered to the simulation scan on which the recurrence was contoured. LRFs were classified as in-field, marginal, or out-of-field. RESULTS: Among 121 consecutive patients, 87.6% received induction chemotherapy, and 92.6% were treated with a CTV. Median follow-up after SBRT was 12.0 months. LRF occurred in 17 patients (14.0%) at a median of 14.3 months and no LRF was observed in patients treated to the "triangle volume." In-field failures were rare (2.5%) as were marginal (6.6%) and out-of-field (5.0%) failures. LRF involved the primary tumor (n = 5; 29.4%), paraaortic lymph nodes (n = 4; 23.5%), porta hepatis (n = 4; 23.5%), superior mesenteric artery (n = 3; 17.6%), or celiac artery (n = 1; 5.9%). Larger CTV size was associated with a nonstatistically significant increase in acute grade 1 to 2 nausea. CONCLUSIONS: This is the first study to characterize patterns of LRF following induction chemotherapy and definitive ablative SBRT for inoperable PDAC. Our findings suggest that routine use of an anatomically derived CTV should be considered including the "triangle volume."