Wilms' tumor protein (WT1) in mammary myofibroblastoma: An immunohistochemical study

Wilms' tumor protein (WT1) has been immunohistochemically detected in thecytoplasm of some developing, adult normal and neoplastic human tissues,suggesting its complex regulator activity in transcriptional/translationalprocesses. Among neoplastic tissues, WT1 has been documented in the cytoplasm of benign and malignant vascular tumors and in rhabdomyosarcoma, while there are no available studies about its expression in myofibroblastic tumors. Accordingly, westudied immunohistochemically the potential expression of WT1 in mammarymyofibroblastoma (MFB), a prototypical myofibroblastic tumor. A series of 18cases of mammary MFB, including several morphological variants (classic,fibrotic, myxoid, lipomatous, Schwannian-like, and epithelioid variants), weretested with antibodies against the N-terminal of WT1. The most striking findingwas a diffuse and strong WT1 cytoplasmic immunostaining restricted to the"epithelioid cell MFB", a rare and diagnostically challenging variant. Converselythe other variants of MFB, including the classic-type, were negative or onlyfocally positive. The present study shows that mammary epithelioid cell MFBshould be added to the list of mesenchymal tumors which express WT1 in thecytoplasm of neoplastic cells. Accordingly, we suggest that the detection of WT1 cytoplasmic immunoreactivity is of complementary diagnostic value to conventionalmyofibroblastic markers in identifying epithelioid cell myofibroblastoma.