18 F-FAPI-42 Versus 18 F-FDG PET/MRI in a Case of Primary Peripheral T-Cell Lymphoma of the Skeletal Muscles.

Primary skeletal muscle lymphoma is rare. We describe 18 F-FAPI-42 and 18 F-FDG PET/MRI findings in a case of primary peripheral T-cell lymphoma of the skeletal muscles with brain involvement. The multiple skeletal muscle tumors and one larger cerebral tumor showed intense FDG uptake and mild to moderate FAPI uptake. FDG PET was superior to FAPI PET in delineating the muscle tumors because of significantly higher FDG uptake of the muscle tumors than FAPI uptake. FAPI PET was superior to FDG PET in delineating the cerebral lesion because of a very low background FAPI activity in the brain parenchyma.