Women who undergo breast-conserving surgery (lumpectomy) for early-stage breast cancer usually receive radiation therapy as a part of their treatment. Radiation kills cancer cells left behind after surgery and helps prevent them from causing a recurrence or spreading to other tissues. For example, in a 2011 study in The Lancet involving more than 10,000 women treated with breast-conserving surgery, radiation cut the 10-year recurrence rate by one-third to one-half. Numerous studies have found that mastectomy (which removes the entire breast) is no more effective than lumpectomy plus radiation in improving survival rates. (Radiation may also be given after mastectomy, depending on the size of the cancer or extent of its spread.)
The main serious downside of radiation is potential damage to the heart. Several studies have found that women who receive radiation for breast cancer have an increased risk of heart disease and death from cardiac causes. It's a special concern for women with left-sided breast cancer, because the heart is mostly in the left chest. Since the early 1990s, technical advances have been introduced to lower the risk of exposing the heart to radiation. But it's unclear how much these newer techniques help — partly because heart disease usually develops more than 10 years after exposure, and follow-up studies have been too short. Also, it hasn't been clear exactly how radiation damages the heart.
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