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 suffering a radiation-induced heart problem is fairly small.

For example, 4 to 5 of every 100 women who are 50 years old and free of heart risks will develop a major cardiac problem by age 80, and radiation treatment would add one more case, the research suggests.

Women also can do a lot to cut their risk by keeping weight, cholesterol and blood pressure under control.

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

And it comes amid greater awareness of overtreatment — that many women are being treated for cancers that would never prove fatal, leading to trouble down the road such as heart disease.

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.