A case of primary central nervous system lymphoma presenting diffuse infiltrative leukoencephalopathy.

A 58-year-old immunocompetent man gradually developed loss of appetite, cognitive decline, gait disturbances, and personality changes over 4 months. Brain magnetic resonance imaging (MRI) revealed bilateral diffuse leukoencephalopathy without mass formation on admission. His condition progressively deteriorated, and we treated him with intravenous high-dose steroids. His symptoms improved rapidly, but exacerbated when therapy was withdrawn. A brain biopsy was performed, and the diagnosis of primary central nervous system lymphoma (PCNSL) was confirmed. He was successfully treated with high-dose methotrexate therapy. Although it is difficult to diagnose PCNSL without mass formation in the early stages, steroid responsiveness is important and brain biopsy is essential for the correct diagnosis of PCNSL.