And 15 years after diagnosis, 25 percent of women with radiation were alive, vs. 21 percent in the control group. At that time, the absolute risk reduction in all-cause mortality was 3 percent, the study found.
The numbers varied according to age, grade of tumor, estrogen-receptor status, whether the patient had taken the estrogen-suppressing drug tamoxifen and how extensive the surgery had been.
"The magnitude of the benefit got smaller with more favorable factors, but there were benefits in every subgroup, which translated into benefit in terms of recurrence risk, which translated into overall survival," Hayes said.
"Across the board, no matter what those features were, women benefited from radiation," Hayes said.
For some time, doctors have wondered whether radiation was really necessary for all women, said study co-author Sarah Darby, professor of medical statistics at the University of Oxford in England.
"These findings will enable both doctors and patients to have a better idea of the benefit that is likely to be gained from radiotherapy on a patient-by-patient basis," she said.
The U.S. National Cancer Institute has more on breast cancer treatments.
SOURCES: Sarah Darby, Ph.D., professor, medical statistics, University of Oxford, England; Shelly Hayes, M.D., radiation oncologist, Fox Chase Cancer Center, Philadelphia; Jay R. Harris, M.D., professor and chairman, radiation oncology, Dana-Farber Cancer Institute and Brigham & Women's Hospital, Boston; Oct. 20, 2011, The Lancet, online
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