Inflammatory Pseudotumor of the Orbit: A Histopathologic and Immunohistochemical Study of 32 Cases

Background and Objective: Inflammatory pseudotumor is a lesion composed of proliferating spindle cells with mixed inflammatory infiltrates. Some authors have proposed the name inflammatory myofibroblastic tumor as a proper descriptive term rather than the vague inflammatory pseudotumor. The aim of this study was to verify the myofibroblastic origin of spindle cells in idiopathic orbital inflammatory pseudotumor by immunohistochemical staining. Materials and Methods: We reviewed a series of 32 inflammatory pseudotumors arising in orbit for expression of smooth muscle actin, vimentin, desmin and anaplastic lymphoma kinase using immunohistochemical staining. Results: There were 21 females and 11 males aged 3 to 64 years with a mean age of 31. Immu-nohistochemically, spindle cells of 51.75%, 79.3%, and 17.2% of lesions expressed smooth muscle actin (15/29), vimentin (23/29) and desmin (5/29). All lesions (32/32) were negative for anaplastic lymphoma kinase Conclusion: In this study, reactivity for smooth muscle actin in spindle cells can be demonstrated as myofibroblastic differentiation. The absence of anaplastic lymphoma expression in all cases of orbital inflammatory pseudotumor in this study strongly suggests that these lesions, albeit histologically similar, are biologically distinct from their soft tissue counterparts or those inflammatory myofibroblastic tumor that negative for anaplastic lymphoma immunoreactivity may be characterized by one or more chromosomal aberration involving regions other than 2p23 is as yet unknown.