Supplementary Material for: Pancreatic Extragastrointestinal Stromal Tumour: A Case Report of an Extremely Rare Entity

Introduction: Primary pancreatic gastrointestinal stromal tumours (GISTs) are extremely rare, comprising less than 1% of pancreatic neoplasms. Their atypical location poses significant diagnostic challenges, often mimicking other pancreatic tumours. Diagnosis relies on pathological and immunohistochemical analysis. Case Presentation: We report the case of a 74-year-old female with unintentional weight loss and anorexia. Past medical history was relevant for new-onset diabetes. Contrast-enhanced computed tomography (CT), followed by endoscopic ultrasound (EUS), revealed a large, hypervascular and heterogeneous mass with necrotic areas in the pancreatic head and uncinate process, displacing gastric antrum and duodenum. These exams showed normal pancreatic parenchyma and no biliary or pancreatic duct dilation. EUS-guided FNB revealed glandular cells without cytological evidence of malignancy. A second biopsy using contrast-enhanced EUS (CE-EUS) guidance yielded similarly inconclusive results. DOTANOC-PET demonstrated no significant uptake, making a neuroendocrine tumour less likely. Due to the lesion’s size, necrotic features and hypervascular pattern on EUS, and the impossibility to exclude malignancy/potential of malignancy by biopsies, surgical resection was decided. The patient underwent pancreatoduodenectomy, and histopathology confirmed a low-risk pancreatic GIST with spindle cell morphology, low mitotic activity, and DOG-1 positivity. Discussion/Conclusion: This case highlights the importance of considering pancreatic GISTs in the differential diagnosis of pancreatic masses. Advanced imaging and multidisciplinary discussion are crucial for timely and appropriate therapeutic decision-making. Illustrative iconography is presented, contributing to the existing literature and supporting diagnosis and management of these rare tumours.