A new Mayo Clinic study casts doubt on past medical reports that repeated low-level medical radiation causes leukemia in adults.
The Mayo report, published today in the New England Journal of Medicine, covers only medical radiation. And it does not deal with the X-raying of pregnant women, which a substantial body of evidence has linked to leukemia in the offspring.
But the Mayo data, said one author, Dr. Joel Gray, show that low levels of radiation from any source -- X-rays, nuclear reactors, nuclear accidents like Three Mile Island or any other -- "have little if any effect on adult health."
The Mayo study is significant because of the unusually detailed information the researchers, Gray Athena Linos, Alan Orvis, Robert Kyle, W. Michael O'Fallon and Leonard Kurland, had on their subjects.
The Mayo Clinic cares for virtually the entire 90,000 population of Rochester, Minn., and surrounding Olmstead County, and keeps a full history for everyone treated anywhere in the county.
The researchers compared the radiation histories of the 138 men and women who developed leukemia between 1955 and 1974 with those of others of the same age and sex and found no overall difference.
After dropping a few persons from both groups because they had received massive radiation for cancer, the Mayo group concluded that even repeated low-level radiation "most probably" caused no leukemia in the population they studied.
Even if this conclusion is wrong, the Mayo group adds, the effect would "almost surely" be less than double the average person's risk of developing leukemia, currently 9.5 chances in 100,000 a year.
"How much less, we can't say, because our study population wasn't large enough," said another author.
Indeed, the Mayo study by no means eliminates the possibility that low-level radiation still causes some cancers.
A National Academy of Sciences study group said a year ago that the chance of developing any cancer after low-level radiation is low, but that there is more chance of thyroid, lung, digestive system or breast cancer than ther is of leukemia.
This study is now being rewritten, however, because many committee members protested that it exaggerated the possible dangers. The rewrite, due shortly, is expected to be much less precise and say only that, given the lack of precise facts, opinions on the seriousness of low-level radiation differ.
The main problem is that no truly large human population has ever been studied long and closely enough to settle the question. Most of the information from animal studies is based on larger doses.
The Mayo authors reviewed several past reports that found excess leukemia in persons who underwent low-level medical radiation. These include a famed study by Dr. Alice Stewart, who found an increased risk in children whose mothers were X-rayed while their babies were still in the womb.
"This result has been verified. There is a danger from X-ray exposure in the womb," said Dr. Linos.
But the Mayo scientists said past reports on adult leukemia were based on unreliable estimates, many from patients' memories, of radiation received. The Mayo Clinic, in contrast, has a record of every X-ray procedure.
"I'd say we still need more study. We need to look at the whole problem harder," Dr. Gray said.
"But, given the information we have, I'd say there is not a significant problem," and "definitely not" a good reason for the panic Pennsylvanians have felt over Three Mile Island's radiation after the accident at the nuclear power plant in March 1979.