Case Report: Resectability-guided management of an indeterminate periduodenal spindle cell tumor initially suspected to be a gastrointestinal stromal tumor.

Duodenal mesenchymal spindle cell tumors are uncommon and may overlap radiologically with gastrointestinal stromal tumors (GISTs) and retroperitoneal lesions. We report a 55-year-old woman with abdominal discomfort, nausea, and vomiting. Contrast-enhanced computed tomography demonstrated an approximately 6.0 × 3.8 cm mass posterior to the pancreatic head that was interpreted as a probable duodenal stromal tumor. Gastroscopic biopsy was non-representative. Given the symptoms, tumor size, uncertain origin, and vascular proximity, surgical exploration was undertaken. The tumor was firm and densely adherent to the inferior vena cava, and pancreaticoduodenectomy was performed. Supplementary pathological review showed a submucosal spindle cell tumor with 2 mitoses per 50 high-power fields, hemorrhage and inflammatory exudates, focal infiltrative growth, and limited invasion of adjacent pancreatic tissue. DOG-1, CD117, desmin, and CD34 were negative; SMA was positive, and β-catenin showed only weak cytoplasmic staining without nuclear accumulation. The expanded panel remained insufficient for definitive lineage assignment, and the lesion was designated an unclassified duodenal mesenchymal spindle cell tumor with SMA expression. Four abdominal computed tomography examinations and one magnetic resonance imaging examination showed no definite recurrence or metastasis through May 28, 2025. This case illustrates the limitations of superficial biopsy and shows that management of an anatomically complex periduodenal tumor may need to integrate diagnostic uncertainty, resectability, vascular proximity, and operative safety.