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 that it is 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, and the lead author of the study, published earlier this week 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 NewYork-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 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.