Granular Cell Tumor of the Bile Duct: A Challenging Cytologic Diagnosis on Endoscopic Ultrasound‐Guided Fine Needle Aspiration

Granular cell tumor (GCT) is a rare mesenchymal neoplasm that is believed to originate from Schwann‐like mesenchymal cells. It affects more women than men with an F:M ratio of 5:4 and can involve various body sites such as peripheral soft tissues, trunk, and head and neck. Multiple lesions can be seen in up to 15% patients. Most of GCT are benign with indolent behavior. Rare cases show cytological atypia and considered histologically malignant; or show metastasis and considered clinically malignant. Endoscopic ultrasound‐guided fine needle aspiration (EUS‐FNA) is widely used nowadays for the diagnosis of epithelial malignancies of the gastrointestinal tract and pancreatobiliary tract. Its efficacy for submucosal and intramural mesenchymal lesions is focused primarily on gastrointestinal stromal tumors (GIST) because of their higher incidence compared with other mesenchymal lesions. Biliary tract GCT is rarely reported in the literature. Preoperative diagnosis of GCT has important therapeutic and prognostic value. Formulating such a diagnosis using cytologic smears alone is extremely challenging, but a definitive and accurate diagnosis is possible when cytomorphology is coupled with optimal cellularity and immunohistochemistry studies on the concurrent cellblock or tissue biopsy. Here we report the first case of biliary GCT diagnosed by EUS‐FNA cytology and concurrent biopsy without surgical resection, which endorses the clinical utility of EUS‐FNA cytology on evaluating biliary lesions and broaden our differential diagnosis of biliary neoplasms.