Projected focal nodular hyperplasia of the liver with pronounced atypical ductular reaction resembling ductal plate and expressing KIT

A 26‐year‐old woman was found to have a left abdominal tumor in the space among the hepatic left lobe, stomach and spleen. A laparoscopic examination revealed that the tumor was a projected liver tumor, and resection of the tumor was performed. Grossly, the tumor was not encapsulated and measured 4 cm × 4 cm × 5 cm. Microscopically, the tumor was composed of mature hepatocytes, fibrous septae, abnormal vessels and ductular reaction (DR). A pathological diagnosis of projected focal nodular hyperplasia (FNH) was made. Characteristically, the cells of the DR showed atypical features such as small cells and hyperchromatic nuclei. The DR assumed the features of ductal plate‐like structures and immunohistochemically expressed KIT, suggesting that the cells of DR are stem cells and that when the stem cells proliferate they take a form of ductal plate‐like structures, similar to fetal bile duct development. Immunohistochemically, the cells of DR were positive for cytokeratin (CK) AE1/3, CK CAM5.2, CK7, CK8, CK18, CK19, carcinoembryonic antigen (CEA), CA19‐9, Ki‐67 (labeling = 3%) and KIT, but negative for CK20, p53, TTF‐1, CDX2, MUC1, MUC2, MUC5AC and MUC6. The hepatocytes were positive for CK CAM5.2, CK8, CK18 and Ki‐67 (labeling = 4%), but negative for CK AE1/3, CK7, CK19, CK20, CEA, CA19‐9, p53, KIT, TTF‐1, CDX2, MUC1, MUC2, MUC5AC and MUC6. In conclusion, the author reported a projected FNH. The DR of the FNH showed atypical features such as small cells and hyperchromatic nuclei. The DR assumed features of ductal plate‐like structures. KIT was positive in the DR in the FNH, suggesting that the cells of DR are liver stem cells, and proliferation of these cells take features of ductal plate‐like structures, similar to embryonic biliary development. MUC apomucins are negative in the DR.