In this review, the authors focus on the use of immunohistochemistry to characterize the different types and subtypes of benign hepatocellular nodules. They describe the classical and currently accepted features leading to the easy and formal diagnosis of focal nodular hyperplasia (FNH) and hepatocellular adenoma (HCA). In addition, they report some atypical features and difficulties in the interpretation of section staining analyses, which represent important parameters for pathologists. A significant contribution of molecular biology to the characterization of FNH has been to reclassify some cases of FNH as inflammatory HCA. Furthermore, the pattern of overexpression of glutamine synthetase (GS), a target gene of β-catenin has been successfully used to identify FNH by immunohistochemistry. Molecular approaches have demonstrated that HCA is a heterogeneous entity. Genotype classification of HCA has allowed the identification of three subtypes: HNF1A-mutated HCA (H-HCA) in 35% of cases, β-catenin-mutated HCA (b-HCA) in 10%, and inflammatory HCA (IHCA) in 55%. Following molecular data, the diagnosis of H-HCA relies on the lack of liver fatty acid binding protein (LFABP) immunostaining. The diagnosis of b-HCA is straightforward when GS is strongly and diffusely expressed by lesional hepatocytes, and is accompanied by nuclear β-catenin immunoreactivity. In IHCA, serum amyloid protein and C- reactive protein are strongly and usually diffusely expressed by tumoral hepatocytes with a sharp limit with the surrounding nontumoral liver. IHCA can also be β-catenin activated (10%). Due to the strong association of b-HCA with hepatocellular carcinoma transformation, the identification of this HCA subtype is extremely important.
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