Inflammatory pseudotumors (IPTs) of the lymph node and spleen are an uncommon, benign cause of
lymphadenopathy and/or
splenomegaly that often bear striking clinicopathologic similarities to the inflammatory myofibroblastic
tumors (IMTs) found in soft tissues. These
tumors have classically been grouped together under the umbrella category of "
inflammatory pseudotumor." Recent evidence shows that IMTs are in fact
neoplastic processes that often harbor balanced
chromosomal translocations involving the
ALK kinase gene. These translocations result in expression of
ALK kinase in IMTs as assessed by immunohistochemical studies. However, the relationship between IMT and IPT of the lymph node and spleen is uncertain. To determine if ALK
tyrosine kinase expression is also present in IPT, 13 cases of IPT (9 involving lymph nodes, 4 splenic lesions) were examined for the presence of ALK
tyrosine kinase by immunohistochemical staining on
paraffin-embedded tissue. In addition, in situ hybridization studies for Epstein-Barr virus--encoded RNAs (EBER) and immunoperoxidase studies for human herpesvirus-8 (HHV8)--specific
proteins were performed. All cases had clinical, morphologic, and immunophenotypic findings typical of IPT and had varying proportions of fibroblastic and inflammatory components. Age ranged from 11 to 75 (median, 40) years; 8 subjects were male, and 5 were female. None of the cases (0 of 13) had positive staining for
ALK kinase or HHV8, and in 1 a lymph node (1 of 13) was focally positive for EBV (EBER) by in situ hybridization. The absence of
ALK kinase as detected by immunohistochemical studies in IPT of the lymph node and spleen suggests that this entity is biologically distinct from the histologically similar IMT.