Brain FDG-PET after radiation therapy is classically used to differentiate 
between tumor recurrence and radiation-related tumor necrosis. Little is known 
about FDG-PET in patients with radiation-induced leukoencephalopathy without 
radiological aspect of necrosis. We present a 69-year-old woman who had 
preventive whole brain radiation after a diagnosis of paraneoplastic 
Lambert-Eaton syndrome related to small cell lung cancer Five months after 
radiation therapy, she developed radiation-induced leukoencephalopathy 
manifested by ataxia. Profound cerebellar hypometabolism on FDG-PET was in 
contrast with the presence of only discrete cerebellar white matter changes on 
MRI. FDG-PET abnormalities seem to correlate better with clinical signs related 
to radiation-associated brain toxicity than MRI.