Inflammatory pseudotumor (IPT) is a heterogeneous group of lesions occurring in various organs, which is histologically characterized by fibroblastic and myofibroblastic proliferation with inflammatory infiltrate. Inflammatory myofibroblastic
tumor (IMT) is a neoplastic counterpart of IPT, which shows aberrant expression of ALK and its gene translocation. In contrast, the concept "
immunoglobulin (Ig)G4-related IPT" in the lung, liver, and pancreas has recently been proposed as a member of
IgG4-related sclerosing disease. In this study, we compared the histopathologic features with an emphasis on
IgG4 expression between 22 cases of IMT and 16 cases of
IgG4-related sclerosing disease, including chronic sclerosing
sialadenitis (n=8), mass-forming
autoimmune pancreatitis (n=3),
sclerosing cholangitis (n=1),
retroperitoneal fibrosis (n=2), and chronic sclerosing
dacryoadenitis (n=2). Bland-looking spindle cell proliferation with
fibrosis and inflammatory infiltrate of lymphocytes and plasma cells was the common morphologic feature in both lesions. Obstructive
phlebitis was observed in all of the IgG4-related sclerosing lesions, but in only 1/22 (4.5%) of IMT. The immunohistochemical expression of ALK was observed in 15/22 (68.2%) of IMT and 0/16 (0%) of
IgG4-related sclerosing disease. The number of IgG4-positive plasma cells and the ratio of
IgG4+/ IgG+ plasma cells were each significantly lower in IMT than in
IgG4-related sclerosing disease [mean 6.4/HPF vs. 178.3/HPF (P<0.0001), 3.0% vs. 67.5% (P<0.0001), respectively]. The results suggest that
IgG4 does not play an important role in the pathogenesis of IMT. In addition, the evaluation of IgG4+ plasma cells and the ratio of
IgG4+/IgG+ plasma cells and the presence of obstructive
phlebitis may be useful for the differential diagnosis between IMT and
IgG4-related sclerosing disease.