Cytological Features of Pancreatic Medullary Carcinoma Diagnosed by Endoscopic Ultrasound‐Guided Fine Needle Aspiration

Pancreatic medullary carcinoma is an extremely rare tumor classified as a histological subtype of pancreatic ductal adenocarcinoma. We report a case of a pancreatic medullary carcinoma and describe its cytological features. A 75-year-old woman presented with lower back pain, epigastric discomfort, and a tumor-like lesion in the pancreatic body. Cytologic specimens obtained via endoscopic ultrasound-guided fine-needle aspiration revealed relatively uniform small round cells with a high nuclear-to-cytoplasmic ratio, appearing as isolated scattered cells or loosely aggregated small clusters in a bloody background containing mature lymphocytes. Histological examination showed cells similar to those observed in the cytological smear, and molecular testing revealed wild-type KRAS and high microsatellite instability. Consequently, the patient was diagnosed with pancreatic medullary carcinoma. To our knowledge, only 33 cases of pancreatic medullary carcinoma have been reported to date, and this is the first report to describe its cytological features.