Cell membrane and cytoplasmic staining for MIB-1 in hyalinizing trabecularadenoma of the thyroid gland

The monoclonal MIB-1 antibody reacts with the nuclei of cells in the late G 1, S, C2, and M phases of the cell cycle. Previously, we found two cases of hyalinizing trabecular adenoma that showed cell membrane and cytoplasmic i mmunopositivity for the antibody. The purpose of this investigation was to confirm this exceptional reactive pattern of MIB-1 in hyalinizing trabecula r adenoma. For the study, we collected 13 additional hyalinizing trabecular adenomas and stained a total of 15 tumors using MIB-1 antibody. Ten cases of papillary thyroid carcinoma were studied similarly. All hyalinizing trab ecular adenomas showed strong positivity for the antibody in 90% or more of the tumor cells, localized especially to the cell membrane and also to the cytoplasm. There was no cell membrane or cytoplasmic MIB-1 positivity amon g the 10 papillary carcinomas. Luminal border of normal extratumoral thyroi d follicles rarely showed faint immunopositivity. Our findings indicate tha t strong cell membrane and cytoplasmic immunoreactivity for MIB-1 is a char acteristic of the hyalinizing trabecular adenoma. Staining for MIB-1 will b e useful in differentiating hyalinizing trabecular adenoma from papillary c arcinoma, which shares a number of cytologic and histologic findings with h yalinizing trabecular adenoma.