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.