Answer: Malignant phyllodes tumor

Histology: Much of this lesion has the appearance of high grade malignant spindle cell neoplasm. These spindle cells focally form whorls, the significance of which is not clear. The lesion does not label for cytokeratins of low, intermediate, and high molecular weight, and does not label for p63. The findings would suggest a high grade sarcoma, not otherwise specified; however, on further sectioning of the specimen, one can see residual lower grade phyllodes tumor evident, which indicates that of this lesion represents malignant stromal overgrowth of a phyllodes tumor.

Discussion: Metaplastic and sarcomatoid carcinoma are used essentially interchangeably in the breast, and these should label with cytokeratins or be associated with a carcinomatous component on further sampling of the lesion. High grade sarcoma, not otherwise specified is essentially a diagnosis of exclusion in the breast, since a malignant phyllodes tumor may have stromal overgrowth which can be indistinguishable from essentially any sarcoma. Thorough sampling of the specimen (i.e. going back to the bucket) is often the best way to establish this diagnosis, as a residual benign or lower grade phyllodes tumor component may be only focally present. Similarly, finding a small carcinomatous component in the face of a high grade malignant spindle cell neoplasm can help establish the diagnosis of metaplastic carcinoma when immunohistochemical stains for cytokeratins are negative in the spindle cell component, as can rarely happen.