A case of small undifferentiated pancreatic carcinoma detected during follow-up for focal pancreatic parenchymal atrophy with fatty replacement.

Undifferentiated carcinoma (UC) is a rare and aggressive subtype of pancreatic cancer, typically diagnosed at an advanced stage. We report a case of UC detected as a small lesion (10 mm on imaging) during follow-up for focal pancreatic parenchymal atrophy (FPPA). A 75-year-old woman with a history of nontuberculous mycobacterial disease underwent routine computed tomography, revealing focal parenchymal atrophy in the pancreatic body without any abnormality in the main pancreatic duct (MPD). During follow-up, CA19-9 levels increased, and diffusion-weighted magnetic resonance imaging identified a small lesion. Endoscopic ultrasonography-guided fine-needle aspiration yielded a pathological diagnosis suggestive of UC. Surgical resection was performed, and pathology confirmed anaplastic-type UC arising within the FPPA region with fatty replacement, along with adenocarcinoma in situ in the same area. The tumor measured 14 mm in size. Despite curative resection, the patient developed liver metastases and died 7 months postoperatively. This case illustrates that early malignant lesions, including UC, may develop in areas of FPPA even without MPD abnormalities. Careful surveillance using imaging and tumor markers may facilitate early detection of such aggressive tumors. To our knowledge, this is the first reported case of resectable UC arising from an area of FPPA with fatty replacement.