The patient proceeded to craniotomy and resection.
HISTOPATHOLOGY
MICROSCOPIC DESCRIPTION: Paraffin sections show fragments of a densely hypercellular tumour. This is composed of cells with pleomorphic round and oval vesicular nuclei, many with conspicuous nucleoli and a variable amount of pale cytoplasm. These are arranged in variably sized solid aggregates within a dense fibrous stroma. Frequent mitotic figures are noted and there are several foci of tumour necrosis. Some of these show dystrophic calcification. Immunohistochemistry shows strong nuclear staining in tumour cells for oestrogen receptor (+++; >90%) and strong membrane staining for E-cadherin. HER-2 is negative (+; <5%). No staining for progesterone receptor, TTF-1, CDX-2, GCDFP or cytokeratin CK7 is seen in tumour cells. The features are of metastatic undifferentiated carcinoma and are consistent with an origin from breast.
DIAGNOSIS: "Posterior fossa tumour": Metastatic undifferentiated carcinoma consistent with an origin from breast.