[Sarcoidosis-lymphoma syndrome showing abnormal FDG uptake in lymph nodes and muscles upon post-chemotherapy PET/CT imaging].

A 65-year-old woman was diagnosed with rheumatoid arthritis in 2010 and was treated with methotrexate (MTX). In 2012, she was diagnosed with sarcoidosis and underwent a follow-up therapy for mild peripheral neuropathy due to neurosarcoidosis. In 2018, she experienced primary splenic diffuse large B-cell lymphoma (DLBCL) and was diagnosed with sarcoidosis-lymphoma syndrome (SLS). MTX was discontinued, and six cycles of rituximab were administered combined with chemotherapy. Positron emission tomography combined with computed tomography performed 18 weeks after the last cycle of chemotherapy showed new abnormal fluoro-2-deoxy-D-glucose (FDG) uptake in the mediastinal and hilar lymph nodes and skeletal muscles. Sarcoidosis was suspected because of increased serum angiotensin-converting enzyme levels and magnetic resonance imaging findings in the lower limb muscles. However, pathological findings of DLBCL and sarcoidosis were not confirmed in the hilar lymph node biopsy. Therefore, malignant lymphoma can be distinguished from sarcoidosis using abnormal FDG uptake after chemotherapy for SLS.