Hyalinizing trabecular adenoma of the thyroid: Its unusual cytoplasmic immunopositivity for MIB1

The monoclonal antibody Ki‐67 reacts with a human nuclear cell proliferation‐associated antigen that is expressed in all active parts of the cell cycle and is well established as a marker of cell proliferation. However, the Ki‐67 method requires fresh frozen material. Recently, MIB1 has been reported to give an immunohistochemical staining pattern identical to Ki‐67 on paraffin‐embedded tissue sections, frozen sections and cytological samples_? This proliferation‐associated antigen is apparently localized in the nucleus. Recently, we performed immunohistochemical staining using the monoclonal MIB1 antibody upon a variety of tumors and non‐neoplastic conditions of the thyroid. Tumor cells of hyalinizing trabecular adenoma revealed an intense cytoplasmic immunopositivity for MIB1. In contrast, cytoplasmic immunostaining for MIB1 was negative in all other thyroid tumors and non‐neoplastic lesions. Because of this unusual staining pattern, we repeated the staining of all cases and found the results to be reproducible. Therefore, we believe that positive cytoplasmic immunostaining for MIB1 is a characteristic finding of hyalinizing trabecular adenoma and is useful in differentiating it from other thyroid tumors.