Radiation treatment for breast cancer can increase a woman's risk of heart disease, doctors have long known. But the size of the added risk has not been clear.

Now, a new study offers a way to estimate the risk. It finds that for most women the risk is modest and outweighed by the benefit from the treatment, which can halve the recurrence rate and lower the death rate from breast cancer by about one-sixth.

According to the study, a 50-year-old woman with no cardiovascular risk factors has a 1.9 percent chance of dying of heart disease before she turns 80. Radiation treatment for breast cancer would increase that risk by 2.4 percent to 3.4 percent, depending on how much radiation hits the heart.

"It would be a real tragedy if this put women off having radiotherapy for breast cancer," said Sarah Darby, a professor of medical statistics at the University of Oxford in Britain. She is the lead author of the study, published Wednesday in the New England Journal of Medicine.

Dr. Silvia Formenti, the chairwoman of radiation oncology at New York University Langone Medical Center, said she worried that women with cancer would misconstrue the findings to mean that radiation is dangerous and that they should have their breasts removed instead of having lumpectomies, in order to avoid radiation.

At the same time, however, she and other experts say that the cardiovascular risk is real and that when radiation is given, every effort should be made to minimize exposure of the heart. In addition, women who have had radiation treatment need to be especially vigilant about controlling other factors that increase the odds of heart disease, such as high blood pressure and cholesterol.

Dr. Lori Mosca, the director of preventive cardiology at New York-Presbyterian Hospital/Columbia University Medical Center, who was not involved in the study, said the findings meant that a history of breast irradiation should be added to the list of risk factors for heart disease and taken into consideration by doctors who are treating such patients.

"We absolutely need to put on our radar screen that prior radiation to the breast may be a new and important risk factor for women," Mosca said.

But she and other experts also warned that the results needed to be verified because the study was not a controlled experiment but was based on an analysis of records and estimates of radiation exposure to the heart.

Dr. Javid Moslehi, co-director of the cardio-oncology program at the Dana-Farber Cancer Institute in Boston and the author of an editorial accompanying the study, said the research was the first to provide risk estimates correlated with doses in breast cancer treatment over a long time period. "This is a huge paper, both in terms of how many women it impacts and how it opens the door for new studies that need to be done," he said.

Moslehi said the study reflected the fact that many people with cancer are now living long enough to encounter long-term effects of radiation and chemotherapy. They have given rise to a fast-growing medical field, cardio-oncology.

About 3 million women in the United States have been treated for breast cancer, and the majority have had radiation. Although radiologists try to spare the heart, it still gets some of the dose, especially when the left breast is treated. Radiation can damage the linings of blood vessels and scar the heart muscle.

Darby's study is based on the records of 2,168 women who had radiation for breast cancer from 1958 to 2001 in Sweden and Denmark; 963 of the women had "major cardiac events" sometime after their cancer treatment, meaning a heart attack or clogged coronary arteries that needed treatment or caused death.

From the treatment records, the researchers estimated the radiation dose to the women's hearts. They found that the risk began to increase within a few years after exposure and that it continued to rise for at least 20 years. The higher the dose, the higher the risk, and there was some increase in risk at even the lowest level of exposure.

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