Brain cancer has increased as much as 500 percent among elderly Americans, a trend that some scientists regard as alarming because the disease is almost always fatal.

"It once was considered that brain tumors reached a peak rate {among people in their thirties} and then would rapidly decline in the older population, but it now appears that the incidence continues to increase with age," said Nigel H. Greig, a National Institute on Aging researcher. "I think it is alarming."

In a study published recently in the Journal of the National Cancer Institute, Greig and three co-authors report that the rate of brain cancer appears to have increased as much as sixfold among the elderly surveyed in 1985, compared to the rates measured a decade earlier.

For those ages 75 to 79, the rate increased 187 percent, the study found. For those ages 80 to 84, the rate went up by 394 percent, and the rate of increase was 501 percent for those ages 85 and older.

Primary brain cancer rates showed little change in 1985 among younger Americans, Greig said.

"In virtually all other age groups, the incidence rates were approximately the same as in 1974," he said. "But {the elderly} had a dramatic increase. It is a large rise over a 10-to-15-year period."

Brain cancer is relatively rare in the population as a whole, representing only about 1.5 percent of the total new cancers diagnosed annually.

The incidence of all types of cancer among all age groups rose 10.7 percent during the study period, Greig said.

The study measured only primary brain tumors -- cancers that started in the brain and did not spread there from tumors elsewhere in the body. The most common primary brain tumors among the elderly were glioblastoma multiforme and astrocytoma, two extremely virulent forms of cancer.

"It is alarming from the point of view that it is a lethal disease with a dismal prognosis," said Greig. "It is also alarming because treatment is not particularly good right now."

Improved diagnostic techniques may explain some of the increase in brain cancers found among the elderly, Greig said. Since the use of X-ray computed tomography, or CAT scan, became widespread in the late 1970s, brain tumors are more easily detected. Precise diagnosis before CAT scans depended upon invasive techniques, such as intercranial biopsies, that were usually not performed on the elderly, he said.

Stanley I. Rapoport, a co-author of the study, said the increase may also stem from a greater interest in the health of the elderly which has occurred since the 1970s.

"People then essentially gave up on the elderly," he said. "They didn't spend much time evaluating their mental dysfunction."

But Greig and Rapoport said they believe at least part of the detected increase in brain cancer is not a result of sharper diagnostics.

"There is a possibility that it is {linked to} environmental factors to which we are all exposed," Rapoport said, "but we don't know that yet."