Is the diagnosis at hand?

The differential diagnoses were: viral encephalitis; acute disseminated encephalomyelitis; diffusely infiltrative neoplasia; or immune reconstitution inflammatory syndrome (IRIS) of pre-existing progressive multifocal leucoencephalopathy, toxo plasmosis, or HIV-associated neurocognitive disorder (HAND). A brain biopsy sample taken on day 24 showed perivascular large CD20-positive B cells (figure B) with 25% proliferation fraction, but Epstein-Barr virus (EBV) encoded RNA negative on in-situ hybridisation.1 There was also marked peri vascular and parenchymal T-cell infiltration (predominantly CD8-positive) and microgliosis (CD68-positive).