Methotrexate-associated lymphoproliferative disorder arising in the retromolar triangle and lung of a patient with rheumatoid arthritis

We report an extremely rare case of massive methotrexate-associated lymphoproliferative disorder (MTX-LPD) arising in the retromolar triangle and lung of a patient with rheumatoid arthritis. The patient was a 75-year-old woman who was referred to our department because of severe pain associated with a unilateral ulcer on the left retromolar triangle. The tumor had an extranodal location in the retromolar triangle and in the right lung. A clinicopathologic examination found a lymphocytic infiltrate with increasingly atypical histopathologic features. Atypical large cells were strongly positive in Epstein-Barr virus-encoded small RNA in situ hybridization and in staining with CD20 antibodies. MTX-LPD was diagnosed based on the medical history and histopathologic results. The lesion responded well to withdrawal of MTX followed by R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, prednisone) chemotherapy. There have been no signs of recurrence for 4 years since withdrawal of MTX.