Diagnostic utility of immunohistochemistry in hepatocellular carcinoma, its variants and their mimics.

Hepatocellular carcinoma (HCC) is known for its histomorphologic heterogeneity. Immunohistochemistry (IHC) can help in the comparative morphologic evaluation of HCC, its variants and their mimics. Some of these diagnostic challenges can be attributed to (i) the variety of neoplasms that can arise from the hepatic stem cell lineage; (ii) the spectrum of well-differentiated hepatocellular nodular lesions; (iii) the liver being a target for metastases with some of these histologic entities mimicking variants of HCC or actually arising in the liver; and (iv) the limitations of serum alpha-fetoprotein (AFP). The role of IHC is in the distinction of benign hepatocellular nodules from reactive hepatocytes; WD-HCC from benign hepatocellular nodules; poorly differentiated HCC from cholangiocarcinoma and metastases; and determination of histogenesis of malignant tumor; and of primary site of origin of malignant tumor. A panel of antibodies has more discriminant value. AFP expression usually indicates malignancy in a hepatocellular nodule and hepatocytic histogenesis of a malignancy. Polyclonal carcinoembryonic antigen (pCEA) and CD10 stain bile canaliculi in better-differentiated HCC. HepPar1 is generally accepted as a hepatocytic marker. However, not all HCC stain uniformly and not all HepPar1-positive tumors are of hepatocytic origin or arise in the liver. Mature hepatocytes and hepatocellular nodules stain with CAM 5.2, CK 8, and 18 but not with CK 7, 19, 20, or AE1/AE3. Biliary epithelium expresses CK 7 and 19. CD 34 highlights sinusoidal capillarization. AFP, pCEA/CD10, and CD34 are useful for ascertainment of malignancy in hepatocellular nodules; HepPar1 and cytokeratins to be included if histogenesis is the issue. IHC results should be interpreted in the larger context of the case.