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(WUSA 9) -- Breast cancer patients already have one major battle to fight. But now,some may be at risk for another... heart disease.

Radiotherapy, used in breast cancer patients to destroy cancer cells, has now been linked to an increase in the risk of heart disease, according to the New England Journal of Medicine.

Radiation therapy is measured in units called gray. In recent years, there has been a decrease in heart exposure to radiation, but most women with breast cancer still receive doses between one and five gray. For one increase in gray, there is a 7.4 percent increase in the risk of a major coronary event.

The study researched women in Sweden and Denmark, who had breast cancer and received radiation therapy. In Sweden, the women were all younger than 70 years old, and in Denmark, the women were all younger than 75 years old.

Out of the 963 women in the study who suffered a major coronary event, ischemic heart disease (reduced blood flow to the heart) played a role in 54 percent of deaths.

And women who have radiation in their left breast may need to take even more precaution.

The study found higher rates of coronary events in patients with radiation of the left breast versus the right breast.

The study found that the risk of heart disease increases within the first five years after radiation exposure, and continues for another 20 years.

"I don't believe this should alarm people," Dr. Penny Anderson, a radiation oncologist at the Fox Chase Cancer Center in Philadelphia, told Reuters Health. Anderson told Reuters, that with current technology, "The heart is exposed to minimal radiation, if at all. Our criteria is less than one percent exposure. It would have been a lot higher 10 years ago."

With one million new diagnoses of breast cancer each year, it is the leading cancer in women around the world. With these new findings, doctors are being warned to take precaution in prescribing radiation therapy, especially in patients who have pre-existing heart conditions.

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