Atypical Focal Nodular Hyperplasia: Don't Let It Fool You

Abstract Focal nodular hyperplasia (FNH) is a benign lesion occurring in a background of normal liver, often in younger women. It is thought to be secondary to hepatocyte hyperplastic growth response around a localized vascular malformation. While the typical features of FNH are well known, those with atypical appearances could pose a diagnostic challenge. Since FNH is rarely symptomatic and carries no risk for malignant transformation or life-threatening complications, it is essential to distinguish it from other lesions such as hepatocellular adenoma (HCA), hepatocellular carcinoma (HCC), and hypervascular liver metastases to avoid unnecessary interventions. Some of the atypical features to be aware of include presence of microscopic/macroscopic fat, calcification, changes in T2 signal, unusual patterns of hepatobiliary phase contrast uptake, as well as abnormal growth and enhancement. Lesions histologically identical to FNH arising in an abnormal liver are called FNH-like lesions and usually result from conditions affecting the vascular supply of the liver such as Budd-Chiari disease, sinusoidal obstruction syndrome, chronic portal vein thrombosis, and others.