Primary central nervous system large B-cell lymphoma with prolific, mixed T-cell and macrophage infiltrates, mimicking multiple sclerosis.

Although tissue confirmation is essential for a diagnosis of primary central nervous system large B-cell lymphoma (PCNSBL), accurate assessment may still be difficult, even when tissue is obtained. We report a 59-year-old man, first diagnosed as multiple sclerosis by open biopsy at another institution, who was then correctly diagnosed as PCNSBL after stereotactic biopsy at our hospital. The initial biopsy showed heavy lymphoid and macrophage influx with visible demyelination. On rebiopsy, a diffuse infiltrate of small to medium-sized lymphocytes was prominent and largely stained as T cells (CD3) by immunohistochemistry. There was also an admixture of macrophages, but this time, relatively low numbers of large malignant cells were also identified. The latter stained as B cells (CD20), enabling a diagnosis of B-cell lymphoma, and the condition responded fully to high-dose methotrexate. It is thus possible for PCNSBL to be histologically misinterpreted as a result of ancillary inflammation, characterized here as a profusion of T cells and macrophages.