Metaplastic breast carcinoma. A diagnostic problem in fine needle aspiration biopsy.

Fine needle aspiration (FNA) was used to evaluate a breast lump and enlarged lymph nodes in a woman with a prior history of lumpectomy on the contralateral breast and a recent negative mammogram. The FNA cytologic findings included markedly atypical fibroblast-like cells lying singly and in groups in a myxoid background, highly atypical multinucleated cells and numerous mitoses, features that were interpreted as a high-grade malignant mesenchymal tumor. The carcinomatous cells in the aspirates were only fully appreciated after histologic examination of the mastectomy specimen and the axillary lymph node metastases showed a dual differentiation consisting of both epithelial and mesenchymal components, leading to a final diagnosis of metaplastic carcinoma. Electron microscopic study of histologic samples confirmed the dual differentiation, and both keratin and vimentin intermediate filaments were recognized by immunohistochemical staining. The regional lymph node metastases were predominantly sarcomatous, which apparently is a rare event. The entity of metaplastic carcinoma is discussed in relation to other mixed epithelial-mesenchymal lesions of the breast, and the previous literature on this entity is briefly reviewed.