The patient went on to have surgery.
MICROSCOPIC DESCRIPTION: 1&2: Paraffin sections show fragments of cerebral white matter. These are densely infiltrated by a population of atypical large mononuclear cells with round and oval vesicular nuclei, many with conspicuous nucleoli and a variable amount of basophilic cytoplasm. These are arranged in diffuse sheets and are seen to infiltrate the walls of small calibre blood vessels. Scattered mitotic figures are noted. There is prominent reactive astrocytic gliosis in adjacent white matter. Immunohistochemistry shows the atypical cells are CD20+, MUM1+, bcl-2+, bcl-6+, CD23-, CD3-, cyclin D1-, CD10-, EBV-. About 95% of the tumour cells are c-myc immunostain positive. About 50% of the cells are p53 positive.
FINAL DIAGNOSIS: non Hodgkin's large B cell CNS lymphoma - non germinal centre phenotype.
Although haemorrhage in PCNSL is very uncommon, seen more frequently in immunocompromised individuals, the non-haemorrhagic lesion in this case, and the distribution still makes lymphoma the most likely diagnosis. Multicentric high grade glioma would be an alternative ddx.