Desmoid Fibromatosis of the Gastric Wall Mimicking Needle Tract Seeding after Endoscopic Ultrasound‐Guided Fine‐Needle Biopsy for Pancreatic Cancer: A Case Report

Endoscopic ultrasound-guided fine-needle biopsy (EUS-FNB) is widely used for the pathological diagnosis of pancreatic diseases. However, needle tract seeding (NTS) remains a clinical concern, particularly when the puncture route is not included in the surgical resection field. We report a case in which distinguishing NTS from other pathologies was challenging. A woman in her 70s underwent EUS-FNB via the gastric antrum for a 10-mm pancreatic head lesion, which was diagnosed as adenocarcinoma (cT1bN0M0, Stage IA). She subsequently underwent pancreaticoduodenectomy (pT3N0M0, Stage IIA) without adjuvant chemotherapy. Two years postoperatively, a 30-mm submucosal tumor was detected in the stomach. Although positron emission tomography-computed tomography demonstrated fluorodeoxyglucose uptake, three separate sessions of EUS-FNB failed to yield a definitive diagnosis, with NTS being strongly suspected. Partial gastrectomy was performed. Histopathology revealed a fascicular proliferation of spindle-shaped cells with nuclear β-catenin positivity, leading to the diagnosis of a desmoid fibromatosis (DF). This case underscores that although NTS should be considered when a lesion arises along the EUS-FNB puncture tract, alternative diagnoses, including DF, must also be carefully evaluated.