Development of myeloid sarcoma after long-term methotrexate use for rheumatoid arthritis.

Myeloid sarcoma (MS) in the complete absence of bone marrow disease is an extremely rare phenomenon. We report the case of a 78-year-old woman with multiple subcutaneous lung and liver nodules, including mediastinal and peritoneal lymph node swelling, who had been receiving methotrexate (MTX) for 10 years for rheumatoid arthritis (RA). She was initially diagnosed with ALK-negative anaplastic large cell lymphoma. After one course of an anthracycline-containing regimen, pathologic cells were identified as CD68 (Kp-1)-positive with myeloid-lineage tumor cells and abnormal karyotypes with 8q21 and 21q22. Subsequent treatment was changed to acute myelogenous leukemia (AML) induction chemotherapy. Although the lesions were partially reduced in size following treatment for lymphoma, complete response (CR) was obtained only after AML chemotherapy. The patient remained in CR over 3 years after the last chemotherapy. This case may indicate an association between long-term MTX use and MS. An early diagnosis and adequate therapy may be important for improving survival outcomes in MS. This report demonstrates that CD68 staining is important for the differential diagnosis of MS and lymphoma. Careful follow-up is necessary for this patient, who may be the first case of MS after methotrexate use for RA.