Spindle cell tumours of the breast: practical approach to diagnosis.

Spindle cell tumours of the breast are uncommon and often present diagnostic challenges. The most important is the sarcomatoid/metaplastic carcinoma, which has monophasic and biphasic variants. Each of these groups presents special diagnostic difficulties. In the monophasic variant the mesenchymal component predominates and the epithelial element forms a minor component often detected only after immunohistochemical study. The spindle cell areas may be bland and therefore under-diagnosed as nodular fasciitis or fibromatosis. Alternatively they may be highly malignant with a pattern that is misinterpreted as primary sarcoma of the breast. In the biphasic variant, the difficulty is in distinguishing between sarcomatoid carcinoma, myoepithelial carcinoma or malignant phyllodes tumour. Other spindle cell lesions of the breast include the various myofibroblastic tumours, the spindle cell variant of adenomyoepithelioma, the varied primary breast sarcomas, metastatic tumours with spindle cell morphology and, finally, the very rare follicular dendritic cell tumour. A simple practical approach to the diagnosis of spindle cell lesions is presented to help the general surgical pathologist to compile a differential diagnosis and to arrive at the correct conclusion