Hepatocellular Carcinoma Arising From Well-Differentiated Hepatocellular Neoplasm of Uncertain Malignant Potential in a Patient With Abernethy Malformation.

Abernethy malformation is a rare congenital extrahepatic mesocaval shunt (CEMS) in which mesenteric venous blood bypasses the liver completely (type I) or partially (type II) and drains directly into the vena cava. In complete CEMS (type I), the intrahepatic portal vein is absent. The resulting lack of portal flow is associated with the development of hepatic nodules, such as focal nodular hyperplasia (FNH), hepatocellular adenoma (HCA), and hepatocellular carcinoma (HCC). These lesions have a higher risk of malignant transformation than conventional adenomas. The term hepatocellular neoplasm of uncertain malignant potential (HUMP) has been proposed for borderline lesions between HCA and well-differentiated HCC, diagnosed based on atypical histological, genetic, or clinical features suggesting a possible malignant potential. However, the exact risk of malignant transformation is unknown. We present the case of a 32-year-old man with Type I Abernethy malformation who developed malignant transformation of a HUMP over a 15-year period. Serial hepatic resections performed at other institutions demonstrated histological progression from HUMP to HCC with subsequent lung metastasis. This gradual transformation has been documented in several surgical specimens. This case highlights the need for careful recognition of HUMP in patients with Abernethy malformation, as timely liver transplantation may be essential to prevent malignant transformation and achieve curative outcomes.