The increased risk is present throughout a person's lifetime, according to the study, which appears in tomorrow's issue of the Journal of the National Cancer Institute. The risk was also higher if exposure occurred during childhood.
Though scientists have been aware of a link between radiation and various forms of cancer, including breast and lung, little is known about the relationship between radiation and tumors of the central nervous system.
Because central nervous system cells do not divide the same way as cells from tissue elsewhere in the body, you can't automatically assume that what is true in other tissues is also true of the central nervous system, explains Dr. Allan Novetsky, director of medical oncology at Maimonides Medical Center in Brooklyn, N.Y.
Much of what scientists know about the effects of radiation has come from the Radiation Effects Research Foundation (RERF) in Hiroshima, which conducted the present study. RERF's studies of atomic bomb survivors have been ongoing since the United States dropped atomic bombs on the Japanese cities of Nagasaki and Hiroshima at the end of World War II. They provide "clear evidence of radiation-associated increases in the risks of leukemia and a broad spectrum of solid cancers," says Dale Preston, lead author of the study and a member of RERF's department of statistics.
Recent evidence from RERF suggests the risk for other illnesses such as heart disease, stroke and liver disease may also be increased by radiation exposure.
In this study, Preston and his colleagues analyzed information from Hiroshima and Nagasaki tumor registries, medical records and death certificates for 80,160 Japanese atomic bomb survivors to identify benign and malignant tumors of the primary nervous system and pituitary gland that were diagnosed between 1958 and 1995.
Radiation exposure was associated with an increased risk of all types of nervous system tumors, with the risk being especially high for schwannomas, which are benign tumors of the nervous system. Men have a slightly higher risk than women, while people exposed as children had a higher risk than people exposed as adults.
The findings raise the specter that other forms of radiation, including therapeutic ones, may also have a harmful effect.
"It raises a red flag, a caution that central nervous system tissue may be susceptible to the cancer-producing effect of radiation like other tissues are and thus should not be used -- especially in the pediatric population -- with a sense of immunity," Novetsky says. "With the increasing use of X-ray procedures and with CT scans of the brain being done at the drop of a hat, it raises at least a question as to whether one should just order CT scans of the brain."
Though most of the tumors identified are benign, they are nevertheless of concern.
Preston notes the risks associated with low-dose radiation exposure are not large and "should not dissuade people from justified diagnostic medical procedures that involve low-dose radiation exposures."
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