[A case of methotrexate-associated diffuse large B-cell lymphoma with splenial lesions of the corpus callosum on brain MRI after complete remission with chemotherapy].

An 86-year-old woman in a wheelchair was accompanied by her husband and son as she visited our outpatient clinic due to disturbed consciousness and fever. Twenty-seven years earlier, she had been diagnosed with rheumatoid arthritis and had been treated with methotrexate (MTX) and low-dose prednisolone (PSL). She stopped taking MTX four years previously when she was diagnosed with diffuse large B cell lymphoma of the paranasal sinus. Her lymphoma went into remission after six cycles of systemic immunochemotherapy. MRI after hospitalization revealed a lesion in the splenium of the corpus callosum that was hyperintense on diffusion-weighted imaging and which had low apparent diffusion coefficient values. An analysis of the cerebrospinal fluid revealed no atypical cells. The MRI findings were atypical, but her consciousness disturbance improved, leading to the diagnosis of mild encephalitis/encephalopathy with a reversible splenial lesion, which would be associated with a transient consciousness disturbance with a good course. However, her consciousness worsened over the next 3 weeks. One month later, a contrast-enhanced MRI showed an enlarged lesion in the callosum as well as new lesions, and the diagnosis of secondary CNS lymphoma was made. Brain biopsy is often not feasible. Less invasive and highly accurate diagnostic methods are needed, such as the identification of a spinal fluid tumor marker.