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Heterogeneous mass with patchy enhancement in the left posterior fossa, with surrounding edema and broad dural attachment.
The patient went on to have a resection.
Paraffin sections show a densely hypercellular tumor. This is composed of cells with pleomorphic round and oval vesicular nuclei, many with conspicuous nucleoli, and a moderate amount of eosinophilic cytoplasm, arranged in discrete, irregularly shaped islands in a fibrous stroma.
Frequent mitotic figures are noted and the majority of tumor islands are centrally necrotic. Tumor cells show strong cytoplasmic immunostaining for cytokeratin AE1/AE3 and strong surface staining for E-cadherin. Strong (+++) nuclear staining for estrogen receptor is seen in 10-20% of tumor cells and there is strong (+++) complete membrane staining for HER-2 in >90% of tumor cells. No staining for progesterone receptor, TTF-1 or tyrosinase is seen. The features are of metastatic undifferentiated carcinoma arising from breast.
Final diagnosis: metastatic undifferentiated carcinoma arising from breast.