Focal Nodular Hyperplasia
Reviewer(s): Dharam Ramnani, MD
Focal nodular hyperplasia (FNH) is not a true neoplasm but a tumor-like mass of benign hyperplastic hepatocytes. It is the 2nd most common benign liver nodule (after hemangioma). It is a regenerative response of liver to a localized vascular anomaly/injury. Following associations have been noted: vascular lesions, such as cavernous hemangioma, epithelioid hemangioendothelioma, and hereditary hemorrhagic telangiectasia; hepatic venous outflow obstruction; orthotopic liver transplantation; and chronic alcohol abuse. There is no convincing relationship with oral contraceptive use. Grossly, FNH is a subcapsular, pale yellow-brown or gray, solitary, well-circumscribed (but unencapsulated), firm, coarsely nodular lesion with a depressed central stellate scar. Fibrous strands radiate from the scar and divide the lesion into nodules. Hemorrhage and necrosis are uncommon. Contrast-enhanced CT or MRI of liver show FNH as a hypervascular lesion that enhances during the arterial phase with a non-enhancing central scar. FNH comprises nodules of normal hepatocytes separated by fibrous bands that radiate from the central scar. There is some resemblance to cirrhosis. The fibrovascular stroma contains thick-walled dystrophic arteries, bile ductular proliferations, and inflammatory cell infiltrates. FNH lacks normal portal tracts but may contain intermediate- to large- caliber bile ducts within the central fibrous zone. Map-like staining pattern with glutamine synthetase is an important distinguishing feature from hepatocellular adenoma. The differential diagnosis includes: hepatocellular adenoma, cirrhosis, and well-differentiated hepatocellular carcinoma. FNH can be safely left alone and monitored with periodic ultrasound. Larger or symptomatic lesions are removed by segmental resection or enucleation. References:1. Goldblum, J. R. et al (2018). Rosai and Ackerman's Surgical Pathology - 11th Edition. Philadelphia, PA. Elsevier.2. WHO Classification of Tumours. Digestive System Tumors, 5th Edition, 2019; IARC, Lyon, France.3. Feldman, M., Friedman, L. S., & Brandt, L. J. (2016). Sleisenger & Fordtran's Gastrointestinal & Liver Disease - 10th Edition. Philadelphia, PA. Elsevier Saunders.