Women who get radiation during treatment for breast cancer are more likely to be alive 15 years later, compared with those who don't get radiation.

So says an international team of researchers who reviewed 78 studies of more than 42,000 women with breast cancer.

Their key findings:

15-year survival was more than 5% better with radiation therapy
Radiation therapy particularly improved rate of recurrence 5 years after treatment
The study appears in The Lancet. The researchers were from the Early Breast Cancer Trialists' Collaborative Group (EBCTCG), which includes scientists from around the world.

Radiation therapy is given to kill any breast cancer cells that linger after surgery. It's often paired with chemotherapy to knock out as many remaining cancer cells as possible.

The goal is to improve patients' survival and to prevent cancer's return.

Better Long-Term Survival

Doctors already knew that women who get radiation after lumpectomy (cancer surgery that saves as much of the breast as possible) are less likely to get cancer again in that same breast over the next five years.

The new study estimates how radiation affects breast cancer survival in the long run.

"Now we know that it also reduces the long-term chances of dying from the disease," says Sarah Darby, PhD, in a news release.

Darby worked on The Lancet report. She's a professor at Oxford University's Clinical Trial Service Unit.

The study included women who had lumpectomies and mastectomies (total removal of the breast).

Over 15 years, about 36% of those who had gotten radiation survived for 15 years, compared with nearly 31% of those who did not get radiation. Radiation's benefits beyond 15 years aren't known, the study states.

Survival Improves With Time

"The differences in breast cancer mortality are greater at 15 years than at five years," the researchers write.

After crunching the numbers, they came up with this estimate: For every four local recurrences that are avoided by radiation therapy, there is one fewer breast cancer death. A local recurrence is breast cancer's return to the breast that was originally treated.

The study does show some risk of heart disease, lung problems, or breast cancer in previously unaffected breasts for patients who got radiation. New techniques have cut down on (but not eliminated) those risks, the researchers note.

They stress that they aren't making any recommendations about treatment.