A Case of Hepatocellular Carcinoma Arising within a Hepatocellular Adenoma in a Patient with Familial Adenomatous Polyposis

Hepatocellular adenoma is a rare benign liver tumor, but certain molecular subtypes carry a risk of malignant transformation, particularly in male patients and large tumors. Familial adenomatous polyposis (FAP) may predispose to hepatocellular neoplasms through β-catenin pathway dysregulation, although reported cases are uncommon. A 35-year-old man with FAP was found to have multiple high-echoic hepatic masses up to 12 cm on ultrasonography, and magnetic resonance imaging showed features suggestive of both focal nodular hyperplasia and hepatocellular adenoma. Ultrasound-guided biopsy was compatible with hepatic adenoma, but some immunohistochemical features partially overlapped with focal nodular hyperplasia. Because malignant transformation could not be excluded, segmentectomy was performed. Surgical pathology demonstrated hepatocellular carcinoma arising within hepatocellular adenoma, with approximately 20% of the lesion showing malignant transformation, and resection margins were tumor-free. This case highlights that imaging and biopsy findings may mimic focal nodular hyperplasia in patients with familial adenomatous polyposis, and surgical resection should be considered when risk factors for malignant transformation are present.