[A Case of Malignant Thyroid Lymphoma That Was Difficult to Differentiate from Methotrexate-Associated Lymphoproliferative Disorders(MTX-LPD)].

A female patient in her 60s was going to get treatment for rheumatoid arthritis(RA). Considering the possibility of using biologics, CT examination was performed for screening of malignant diseases. A mass shadow in the left lobe of the thyroid gland was detected. The patient was followed up, and ultrasonography did not reveal any malignant findings. She was treated with methotrexate(MTX), and 1 year later, the thyroid mass was enlarged on CT. Ultrasonography revealed an enlarged hypoechoic region. Fine needle aspiration cytology revealed malignant lymphoma. Excisional biopsy was performed to determine the treatment plan. The pathological diagnosis was follicular lymphoma, and the possibility of methotrexate- associated lymphoproliferative disorders(MTX-LPD)persisted. It was difficult to discontinue MTX because of the high activity of RA. She was treated with rituximab for malignant lymphoma and concurrently with MTX for RA. The thyroid tumor disappeared for 3 months. Four years later, there is no sign of tumor recurrence.