Abstract

A wide range of denomination has been used for inflammatory myofibroblastic tumor (IMT). IMT is not entirely homogeneous, even though it shows some overlapping histologic features such as haphazard proliferation of spindle cell and polymorphic chronic inflammatory cell infiltraion. The spindle cell is considered to be of myofibroblastic origin but follicular dendritic cell origin was reported recently. IMT is known as nonneoplastic, aberrant inflammatory response. However, IMT could show local invasion, recurrence, vascular invasion, and malignant transformation, and clonal characteristics and aneuploidy of IMT support the hypothesis that IMT may be a neoplastic process. In order to define the nature of spindle cell of IMT, immunohistochemical stains for smooth muscle actin (SMA), vimentin (VMT), lysozyme, S-100 protein, cytokeratin, CD21 were done. Additional immunohistochemical stains for MIB-1 for proliferating activity and LMP (latent membrane protein) for Epstein-Barr virus (EBV) were done. IMTs were composed of each 2 cases from lung, liver and lymph node and one case from common bile duct, maxillary sinus, bladder and thigh, and were histologically subclassified according to Coffin et al. Nine cases (90%) were positive for SMA and VMT, but no correlation between SMA and VMT immunoreactivity and histologic types was identified. Five cases (50%) were positive for lysozyme and S-100 protein, and histologic type III was negative for lysozyme and S-100 protein, and immunoreactivity for S-100 protein was different according to the histologic subtypes. All 11 cases were negative for CD21 and EBV LMP. MIB-1 labelling index was less than 1% in all cases. In summary, the spindle cell is regarded as myofibroblastic origin rather than follicular dendritic cell origin. Relationship with EBV is not clear, and negligible MIB-1 reaction suggests that IMT might have a good prognosis.

• Keywords: Inflammatory myofibroblastic tumor;Immunohistochemistry