A case of FGFR2 fusion-positive pancreatic head tumor with difficult differential diagnosis between cholangiocarcinoma and pancreatic ductal adenocarcinoma.

FGFR2 fusions and rearrangements are genetic abnormalities commonly observed in intrahepatic cholangiocarcinoma. A 42-year-old woman with a history of breast cancer presented to our hospital with jaundice. The patient was clinically diagnosed with distal cholangiocarcinoma (DCCA), and underwent pancreaticoduodenectomy. The tumor extensively invaded the pancreatic head, and intraductal spread was found in a branch of the pancreatic duct. Therefore, the patient was postoperatively diagnosed with pancreatic ductal adenocarcinoma (PDAC). Multiple lung metastases appeared 51 months after surgical resection. Partial lung resection confirmed histologically that the lung metastasis originated from PDAC. Furthermore, comprehensive genomic profiling (CGP) of lung resection specimens revealed FGFR2-SYCP1 fusion. However, common genetic alterations in PDAC, such as KRAS, SMAD4, TP53, and CDKN2A, were not detected in CGP. A re-evaluation of the resected pancreatic tissue showed that the histological features were possible for DCCA; however, it was difficult to make a conclusive diagnosis. Based on CGP analysis, the patient is currently receiving futibatinib, and the lung metastatic lesions have shrunk or partly disappeared. CGP can assist in selecting an appropriate therapeutic strategy when histological differentiation between DCCA and PDAC is challenging.