On the narrower question of differentiating between Alzheimer's and fronto-temporal dementia, however, the agency found PET scans reasonably worthwhile. Most patients with that form of dementia -- who have not been shown to benefit from Alzheimer's drugs -- show metabolic deficits near the front of the brain; in Alzheimer's, the deficits are in other regions.
The Medicare office announced in June its intention to approve reimbursement for this limited purpose and sought public comments.
Several doctors and other medical professionals wrote to oppose coverage, fearing that once PET was approved for a narrow use, it would be marketed more broadly as a diagnostic test for Alzheimer's generally. The disease affects more than 4 million Americans today, and the number is expected to multiply rapidly as the population ages.
The Alzheimer's Association, the largest patient advocacy organization for the disease, supported the approval with reservations.
"It is important to reiterate that unnecessary PET scanning has a number of potentially serious consequences, including unnecessary exposure of patients to radiation, misdiagnosis and unnecessary use of medical resources," wrote the association's president, Sheldon Goldberg.
To address such concerns, the office will pay only for PET scans in patients whose symptoms of "social disinhibition, awkwardness, and difficulties with language" are more prominent than their memory loss, and who have undergone a full physical exam, two mental status examinations over six months, and the usual laboratory and imaging tests done to diagnose dementia.
PET providers have also agreed to collaborate in a large study with the agency and the National Institute on Aging to clarify the ways in which PET may be useful for patients suspected of having early Alzheimer's.
Sean Tunis, the Centers for Medicare and Medicaid Service's chief medical officer, said that although the final design of the study has not been decided, it will almost certainly involve randomly assigning some people with early dementia to get PET scans in addition to the usual medical work-up. At least 1,000 patients would be enrolled in 30 to 50 medical centers, he speculated.