Introduction

Focal nodular hyperplasia (FNH) is a common benign liver lesion composed of nodules of proliferating hepatocytes, malformed vessels, and bile ductular proliferation. Classically, FNH has been reported in livers which are normal or near normal. Recently, however, FNH-like lesions have been reported in cirrhotic livers and, as such, FNH should be considered in the clinicopathologic differential diagnosis of hepatocellular carcinoma (HCC) and other nodular liver lesions.1 ^– 3

As most of these lesions are detected by imaging studies, it is useful to review the imaging characteristics of FNH. Classical FNH usually has typical imaging characteristics. On unenhanced scans, the lesion is homogenous and well defined and may be either hypo- or iso-dense. It shows bright enhancement on the early arterial phase before becoming iso-dense on the portal phase. The characteristic stellate central scar is hypo-dense on early arterial and portal phases before becoming hyper-dense on delayed scans. This, however, is only seen in <50% of patients and its lack can make a definitive preoperative diagnosis difficult to make on imaging alone.4 Atypical imaging features include lesion heterogenicity, fat accumulation, non-visualisation of the central scar, pseudocapsular enhancement, and non-enhancement of the central scar.5

Differentials for an FNH with atypical imaging features include HCC, hepatocellular adenoma (HA), and intrahepatic cholangiocarcinoma (ICC). For example, the arterial enhancement with subsequent washout during the portal or …