Evaluation of loco-regional recurrences using deformable image registration after isotoxic high dose stereotactic body radiotherapy in localised pancreatic cancer.

Background and Purpose: Pancreatic ductal adenocarcinoma is a highly aggressive malignancy with poor survival outcomes. Neoadjuvant treatments, including radiotherapy, have been developed to improve resectability and survival rates. This study evaluates loco-regional recurrence (LRR) patterns after isotoxic high-dose stereotactic body radiotherapy (iHD-SBRT) for localized pancreatic cancer using deformable image registration (DIR). Materials and Methods: Patients with borderline/locally advanced pancreatic adenocarcinoma treated between January 2018 and January 2021 were included. Treatment involved modified FOLFIRINOX chemotherapy, iHD-SBRT, and surgery, if feasible. LRRs identified via CT/MRI during follow-up were mapped back onto initial radiotherapy planning using validated DIR workflows. Recurrences were classified as in-field (IF), marginal (M), or out-of-field (OF). Results: Among 41 patients, LRRs were identified in 17 (10 via CT, 7 via MRI). The majority of LRRs were classified as OF (53 %, n = 9), with 23.5 % (n = 4) each in IF and M categories. Marginal recurrences were located near major abdominal vessels, such as the superior mesenteric artery and coeliac artery. DIR quality metrics, including DICE and MDA, validated the accuracy of the mapping process. Conclusion: Image based evaluation of loco-regional recurrence is an important part of the assessment of patients treated with iHD-SBRT for localised pancreatic cancer. DIR-based analysis emphasized the importance of expanding the tumor-vessel interface (TVI) structure in radiotherapy planning to include the full circumference of adjacent vessels with a 5 mm margin. DIR-based analysis highlighted that several marginal recurrences occurred in proximity to perivascular regions, suggesting a potential benefit of expanding perivascular target coverage in future treatment planning.