Pathological classification of hepatic inflammatory pseudotumor with respect to IgG4-related disease

Recently, much attention has focused on IgG4-related disease, which is characterized by abundant IgG4-positive plasma cell infiltration and high serum IgG4 levels. IgG4-related disease sometimes manifests as tumorous lesions, and its relationship to inflammatory pseudotumor has been suggested. In this study, we examined clinicopathological features of a total of 16 cases of hepatic inflammatory pseudotumor (11 men and 5 women with an average age of 67 years) with respect to IgG4-related disease. The tumors could be pathologically classified into two types: fibrohistiocytic (10 cases) and lymphoplasmacytic (6 cases). Fibrohistiocytic inflammatory pseudotumors were characterized by xanthogranulomatous inflammation, multinucleated giant cells, and neutrophilic infiltration, and mostly occurred in the peripheral hepatic parenchyma as mass-forming lesions. In contrast, lymphoplasmacytic inflammatory pseudotumors showed diffuse lymphoplasmacytic infiltration and prominent eosinophilic infiltration, and were all found around the hepatic hilum. In addition, venous occlusion with little inflammation and cholangitis without periductal fibrosis were frequently observed in the fibrohistiocytic type, whereas obliterative phlebitis and cholangitis with periductal fibrosis were common features of the lymphoplasmacytic type. Interestingly, IgG4-positive plasma cells were significantly more numerous in the lymphoplasmacytic than fibrohistiocytic type. However, two of the fibrohistiocytic inflammatory pseudotumors had relatively many IgG4-positive plasma cells. In conclusion, hepatic inflammatory pseudotumor could be classified into two types based on clinicopathological characteristics. The lymphoplasmacytic type is unique, and could belong to the so-called IgG4-related diseases. In contrast, the fibrohistiocytic type might still be a heterogeneous group of disorders. This latter type seems pathologically different from IgG4-related disease, although cases with relatively abundant IgG4-positive plasma cells should be differentiated from IgG4-related disease with secondary histopathologic modifications.