Methotrexate-associated Intravascular Large B-cell Lymphoma in a Patient with Rheumatoid Arthritis: A Very Rare Case.

We herein report a rare case of methotrexate (MTX)-associated intravascular large B-cell lymphoma (IVLBCL) in a man with rheumatoid arthritis. Two episodes of a fever of unknown origin accompanied by elevated levels of serum lactate dehydrogenase and the soluble interleukin-2 receptor occurred within a year, so the patient was suspected of having an MTX-associated lymphoproliferative disorder. His clinical symptoms resolved after the cessation of MTX. However, after treatment with iguratimod, another disease-modified anti-rheumatic drug, markedly similar symptoms recurred, and random skin biopsies resulted in a diagnosis of IVLBCL. The patient received a rituximab-containing chemotherapy and achieved complete remission.