Diagnosis: C. Metaplastic carcinoma

Histology:
The tumor is composed atypical spindle cells with variable cellularity; hypercellular regions alternate with hypocellular regions, the latter of which displays expanded, dense, eosinophilic collagen. Within the hypercellular regions, there are also small bands of the brightly eosinophilic, dense collagen. At the ends of the tissue sections, foci of malignant squamous cell carcinoma are identified. eaf-like architecture are seen.

Discussion:
This lesion is a metaplastic carcinoma of the breast, characterized in this case by intermediate-grade malignant spindle cells admixed with squamous cell carcinoma. Metaplastic carcinomas of the breast are more common than phyllodes tumors, which are more common than metastatic sarcoma to the breast or primary breast sarcomas. The presence of a malignant epithelial component (i.e., the squamous cell carcinoma) enables making the diagnosis of metaplastic carcinoma on the basis of the H&E sections. This malignant epithelial differentiation can be focal. Immunostains for cytokeratins such as AE1/AE3, CK903, and CAM5.2 can be used to highlight epithelial differentiation within the malignant spindle cells; however, cytokeratin labeling can be seen in malignant phyllodes tumor, so focal cytokeratin labeling alone in malignant spindle cells is not enough to differentiate a metaplastic carcinoma from a malignant phyllodes tumor. In the absence of the malignant epithelial component, the most useful next step is often to take additional tumor sections to look for an unsampled epithelial component.

Reference:
Charu V, Cimino-Mathews A. Spindle cell lesions of the breast. Am J Surg Pathol: Reviews & Reports. 2017 Mar-Apr;22(2):116-124