FDG-Avid Intrathecal Inflammation Following Administration of Intrathecal Methotrexate.

Intrathecal methotrexate is a standard option for central nervous system-directed therapy in patients with stage III and IV diffuse large B-cell lymphoma. We present a case of a 50-year-old man with stage IV diffuse large B-cell lymphoma with widespread lymphomatous involvement including the central nervous system, who received 7 doses of intrathecal methotrexate via Ommaya catheter. Posttherapy F-FDG-PET/CT imaging demonstrated diffuse, intense intrathecal FDG avidity, without correlative findings on MR spinal imaging. FDG avidity resolved on follow-up. These PET/CT findings are most consistent with methotrexate-induced thecal inflammation, which needs to be distinguished from intrathecal malignancy.