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 in 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 a million cases are diagnosed each year worldwide. When it's confined to the breast, most women get surgery to remove the lump, followed by several weeks of radiation to kill any lingering cancer cells and sometimes hormone or chemotherapy.
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 suffered 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. They used hospital records and treatment plans to figure how many gray units actually reached each woman's heart and one artery often involved in heart attacks.
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.