Focal nodular hyperplasia of the liver
The reported prevalence of FNH in the general population ranges between 0.4 to 3%, probably increasing with age.
The most characteristic macroscopic feature is the central, stellate fibrovascular zone, as easily seen in the image, which has the historically entrenched names of “central scar,” “fibrous scar” or “scar-like fibrosis”.
The central scar usually consists of mature collagen and is numerous vascular channels, many of which are medium and large thick-walled arteries which often show fibromuscular hyperplasia, myointimal proliferation and myxomatous change, sometimes with significant luminal narrowing.
The pathogenesis is not fully understood, but it is highly accepted that an arterial abnormality, often a malformation, causing hypo- or hyperperfusion, which triggers reactive hyperplasia of otherwise normal hepatocytes.
Gene expression studies have, however, demonstrated molecular features supporting the concept of vascular abnormalities as the principal etiopathogenetic factor; FNH can have increased expression of ANGPT1, classically responsible for vessel formation, and ANGPT2, an antagonist to ANGPT1.
The diagnosis of FNH can be particularly challenging in biopsy material if the central scar is not included in the sample.
Larger and more symptomatic lesions are observed among patients taking oral contraceptives (OCP). Although FNH may be responsive to estrogens, it is clear that the use of OCPs is not required for the development of FNH.
Usually asymptomatic (80% of the cases), FNH not infrequently is incidentally diagnosed due to widespread use of radiologic examinations, as a mass noted at the time of a surgery or at autopsy. This lesion rarely grows or bleeds and has no malignant potential.
In the past, this tumor was resected due to the difficulty in distinguishing it from hepatic adenoma, but nowadays, with the improvement of imaging techniques and with a combination of imaging modalities it is now almost always accurately diagnosed with imaging and is not resected.
The evaluation of FNH by ultrasound (US) has a low sensitivity, however the lesion appears as a well demarcated, homogeneous and isoechoic mass relative to the liver parenchyma with a hyperechoic central scar.
Differential diagnosis of FNH comprises other hepatic tumors that present generally as single solid lesions, and their differentiation is crucial because of diverse therapeutic approaches and prognosis. Unfortunately, the distinction is not always straightforward, particularly with small lesions. Nevertheless, even in the presence of diagnostic evidences, the differential diagnosis should include: hepatic adenoma, HCC, Fibrolamellar HCC, hypervascular metastases, hemangioma and even focal steatosis.