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Women treated with radiation for breast cancer are more likely to develop heart problems later, even with the lower doses used today, troubling new research suggests. The risk comes from any amount of radiation, starts five years after treatment and lasts for decades, doctors found.

Patients shouldn’t panic; radiation has improved cancer survival, and that is the top priority, doctors say. The chance of experiencing a radiation-induced heart problem is fairly small for most women.

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.

Women also can cut their risk by controlling their weight, cholesterol and blood pressure.

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Still, the study shows that the potential harm from radiation runs deeper than many medical experts may have realized, especially for women who have cardiac-risk factors such as diabetes.

Some chemotherapy drugs are known to harm the heart muscle, but the new study shows radiation can hurt arteries, making them prone to harden and clog and cause a heart attack. Women who receive both treatments have both types of risk.

The study “will raise the antenna” about the need to do more to prevent this, said Dr. David Slosky, a cardiologist at Vanderbilt University, one of the growing number of medical centers with special “cardio-oncology” programs for cancer survivors.

With today’s lower radiation doses, “it is less of a problem, but it is not going away,” he said.

The artery-related problems that the study tracked may be just the most visible of many risks because radiation also can cause valve, rhythm and other heart troubles, said Dr. Javid Moslehi. He is co-director of the cardio-oncology program at the Dana-Farber Cancer Institute in Boston.

Like cancer, heart disease develops after “a number of strikes that go against you,” such as high cholesterol, he said. “The radiation is just another hit.”

He wrote an editorial that appears with the study in Thursday’s New England Journal of Medicine. British government agencies and private foundations paid for the research.

Breast cancer is the most common cancer in women; more than 1 million cases are diagnosed each year worldwide. About 3 million women in the United States have been treated for it. Although radiologists try to spare the heart, it still gets some of the dose, especially when the left breast is treated. When it’s confined to the breast, most women get surgery to remove the lump, followed by several weeks of radiation in attempt to kill lingering cancer cells and sometimes hormone or chemotherapy. Radiation can damage the linings of blood vessels and scar the heart muscle.

What heart-disease risks come from what specific doses isn’t known. The new study, led by Dr. Sarah Darby of the University of Oxford in England, sought to measure that.

It involved 2,168 breast-cancer patients from Sweden and Denmark diagnosed between 1958 and 2001 and treated with radiation. They included 963 women who had a heart attack, needed an artery-opening procedure or died of heart artery-related causes in the years after their radiation treatment. The other 1,205 were similar patients who did not develop these heart problems.

Researchers compared the women’s radiation exposures using gray units, a measure of how much is absorbed by the body.

Most women treated today get doses that result in 1 to 5 gray units reaching the heart, more if the cancer is in the left breast. Patients in the study got an average of five gray units; the doses ranged from 1 to 28.

The risk of a heart attack, need for an artery-opening procedure or dying of heart disease rose about 7 percent per gray unit and no “safe” level was seen. The risk started to rise within five years of treatment and continued for at least 20 years.

What to do?

Don’t forgo radiation if it’s recommended because it is lifesaving and doctors increasingly have ways to shield the heart from exposure, said Dr. Bruce Haffty, associate director of the Cancer Institute of New Jersey.

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 all doctors who are treating such patients.

But she and other experts also said 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.

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