The radiation that might cure a breast cancer may also raise a woman's risk of having a heart attack or heart disease later in life, according a new study that looked back at the cases of 2,168 women in Sweden and Denmark.

The risk "begins within a few years after exposure, and continues for at least 20 years," the researchers report in the New England Journal of Medicine. And that risk rises, they found, in proportion to the dose of radiation the heart receives.

The results are expected to be controversial because breast cancer is so common -- there are about 3 million survivors of breast cancer in the U.S. -- and because there is disagreement over the significance of the findings going forward since newer techniques block the heart from most exposure.

The researchers said the results should, in fact, be comforting to women because their risk of heart attack is so low to begin with and the data allow doctors to factor it into cancer treatment planning.

"Doctors can now estimate the risk and know that in most cases it will be very small so they can reassure their patients," lead author Dr. Sarah Darby of the University of Oxford said in a statement.

"The results of our study should not scare women off having radiotherapy," she told Reuters Health in an email. "It is a life-saving procedure."

Dr. Penny Anderson, a radiation oncologist at the