The higher the radiation dose, the higher the risk of these heart problems. "This study has shown for the first time that as the radiation dose to the heart increases, so does the risk of radiation-induced heart disease," Taylor said.
In recent years, Taylor said, improvements in radiation machines have made it possible to deliver radiation more accurately so the heart receives less exposure.
Taylor evaluated each woman's average dose of radiation to the heart. The overall average was 4.9 units, or grays. Women with cancer in the left breast -- which is closer to the heart -- averaged more heart exposure (6.6 grays). Today, Taylor said, the average overall ranges from 2 grays to 10 grays.
The rate of heart problems increased by 7.4 percent per gray. No starting threshold was found.
The risk started within five years after treatment and persisted into the third decade.
The proportional increase was similar in women with and without heart disease risk factors (such as high blood pressures) at the start. Those with pre-existing risk factors, however, had greater absolute increases in risk.
To put the finding in perspective, for a 50-year-old woman with no pre-existing heart disease risk factors, a dose to the heart of 3 grays would increase the risk of death from ischemic heart disease before age 80 from 1.9 percent to 2.4 percent, Taylor said.
Doctors should identify women with pre-existing heart disease risk factors and those with a small distance between heart and breast and give the radiation in a way that minimizes heart exposure, Taylor said.
Editorial author Moslehi, who also is an instructor of medicine at Harvard Medical School, said the study results are important, but not entirely reassuring. "It suggests that women should worry about heart disease risk after radiation therapy," he said. The new findings "may be just the tip of the iceberg," he sa
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