Focal nodular hyperplasia (FNH) is the second-most-common benign hepatic tumor,
constituting about 8% of cases. The pathogenesis of FNH is a matter of debate. In the past, it was thought to be either a hamartoma or a neoplasm. Currently, it is thought to represent a nonneoplastic, hyperplastic response to a congenital vascular malformation. FNH is found predominantly in women of child-bearing age, with a female-to-male ratio of 6 to 8:1. Although an association with oral contraceptives has been suggested, the correlations are much lower than are
Clinical manifestations of FNH are rare.
Epigastric or right-upper-quadrant pain with a palpable mass is present only in a small minority of patients. Spontaneous rupture with hemorrhage is extremely rare. Malignant degeneration has not been reported, but it is critical to distinguish FNH from the fibrolamellar variant of hepatocellular carcinoma (HCC), a malignant lesion with a similar central scar. It is important to note that the latter’s scar is usually large and eccentric, with broad fibrous bands and calcifications.
Although ultrasound is often the imaging study that first detects focal hepatic lesions, it does not discriminate FNH from other pathology well. FNH lesions have an echogenicity very similar to that of surrounding normal liver.
However, distinguishing characteristics on multiphasic CT can be readily identified. In the late arterial phase, FNH has a bright homogeneous enhancement with a hypodense central scar. Delayed-phase images may show hyperattenuation of the central scar. Occasionally, the radiating septa may also be visualized.
When MRI is employed, the central scar appears hyperintense on T2-weighted images, and when contrast is used, the enhancement pattern is similar to that seen on CT. Superparamagnetic iron oxide (SPIO) is an MR contrast agent that undergoes phagocytosis by the reticuloendothelial system (RES) (the Kupffer cells in the liver). On SPIO-enhanced T2-weighted images, FNH is hypointense but with a bright central scar.
On hepatic scintigraphy with 99m Tc-sulfur colloid, FNH has variable colloid uptake compared with the normal liver. However, intense colloid uptake (10% of cases, related to the number of Kupffer cells present) is a very specific finding for FNH. In combination, use of different imaging modalities, especially MRI, yields a precise diagnosis of FNH in 70% to 90% of cases. When the diagnosis remains in doubt, histologic examination is indicated.
Elective resection is not indicated in asymptomatic patients when studies differentiate FNH from adenoma or malignant lesions. When the lesion is unresectable, it can be treated with transarterial embolization. Oral contraceptives should be stopped. There is no contraindication to pregnancy with this lesion, but close observation for tumor growth during pregnancy and the postpartum period is prudent.