Primary Gastrointestinal Stromal Tumor of the Pancreatic Head: A Diagnostic Challenge Mimicking a Neuroendocrine Tumor.

Pancreatic hypervascular masses can be difficult to characterize on cross-sectional imaging, particularly when different entities share similar enhancement patterns. We report the case of a 63-year-old woman with a three-month history of abdominal pain and progressive distension. Contrast-enhanced computed tomography (CT) revealed a 49 × 60 mm irregular hypervascular mass in the pancreatic head with peripheral arterial-phase enhancement, a large central low-attenuation area compatible with necrosis, and mass effect with displacement of the first and second portions of the duodenum without luminal obstruction or mural invasion, associated with prominent gastroduodenal veins and early portal vein opacification. Based on the lesion's location and enhancement pattern, the initial radiologic impression was that of a pancreatic neuroendocrine tumor (NET). At laparotomy, the procedure was aborted because of severe intraoperative hemorrhage related to tumor hypervascularity, and only an incisional biopsy was obtained. Histopathology showed a mesenchymal neoplasm with a mixed spindle-cell and epithelioid pattern arranged in intersecting fascicles without necrosis. Immunohistochemistry demonstrated strong, diffuse DOG1 expression and negativity for epithelial, muscular, endothelial, and neural markers, with a Ki-67 index of approximately 1%, consistent with a low-risk gastrointestinal stromal tumor (GIST). After multidisciplinary discussion, pancreaticoduodenectomy and tyrosine kinase inhibitor therapy were proposed, but the patient declined and has been managed conservatively with close follow-up and symptomatic improvement. In this patient, the pancreatic mass mimicked a NET on imaging and was ultimately identified as an extragastrointestinal stromal tumor (GIST), underscoring the importance of histopathologic and immunohistochemical confirmation when evaluating hypervascular pancreatic lesions.