Metaplastic carcinoma of the breast with rhabdomyosarcomatous element: aspiration cytology with histological, immunohistochemical, and ultrastructural correlations.

We studied a metaplastic breast carcinoma with a rhabdomyosarcomatous element from a 62-yr-old woman. In the fine-needle aspirates processed by Ultrafast Papanicolaou stain, there were small undifferentiated cells with scanty cytoplasm and differentiated cells with red macronucleoli and abundant cytoplasm. Some differentiated cells contained a brown inclusion with a blue rim. Ultrastructurally, the brown inclusion correlated to a central aggregate of sarcomeres and the blue rim correlated to actin filaments surrounding the sarcomeres. The differentiated cells without cytoplasmic inclusions expressed cytokeratin and contained tonofilaments. A transitional cell type containing both sacromeres and tonofilaments was absent. Immunohistochemically, the small undifferentiated cells expressed vimentin diffusely and showed >90% MIB-1 proliferating index, whereas the differentiated cells expressed cytokeratin, actin, or myoglobin and had virtually absent MIB-1 nuclear labeling. Histologically, the small cells were more concentrated along the capsule and the large cells were more concentrated in the center of the tumor. These findings suggest the bidirectional differentiation of the small undifferentiated cells into carcinoma cells and rhabdomyosarcoma cells in this tumor. Copyright 2003 Wiley-Liss, Inc.