These reassuring results, which were reported Wednesday in the British journal The Lancet, are based on an analysis of 17 studies involving 10,801 women in Europe, Canada and the United States. All had cancers small enough to be treated with breast-conserving surgery, meaning a lumpectomy or other procedure to excise the cancer without removing the entire breast. Women were picked at random to receive radiation therapy, or not, and followed for a median of 9.5 years. Some were tracked for as long as 20 years.
Over all, radiation reduced the risk of a recurrence during the first 10 years after surgery to 19.3 percent from 35 percent, and reduced the risk of death from breast cancer to 21.4 percent from 25.2 percent in the first 15 years.
“It’s only been relatively recently realized that radiation has such a big effect on mortality,” Dr. Darby said, explaining that this positive outcome was at first not recognized because it takes five years to show up.
“These data reinforce the fact that the recurrence rate is reduced for the whole of the first decade,” she said. “That’s not been so clear. It’s not just for a year or the first few, but substantially reduced for a full decade.”
A researcher who was not part of the study, Dr. Thomas A. Buchholz, head of radiation oncology at the University of Texas M.D. Anderson Cancer Center, said: “The findings are extremely important. The group from Oxford are some of the most important breast cancer investigators. They’ve been able to get data from almost every single randomized controlled trial, both from the United States and Europe. The power is, you can combine the data to get very large sample sizes and answer fundamental questions.”
Earlier research by the Oxford group also found that radiation could prevent recurrence and increase survival, Dr. Buchholz said.
“Now, they’re confirming, with very long-term follow-up, the importance of radiation not just in preventing recurrences but in increasing your chance of being cured,” he said. “This is some of the most important data we have with respect to breast cancer treatment.”
The theory behind giving radiation after surgery for breast cancer is to kill any stray cancer cells that may have been left behind and to prevent them from growing into another tumor or spreading to vital organs.
But studies were needed to find out if the theory held up, especially since radiation has side effects like skin burns and fatigue.
Some researchers questioned whether radiation was needed, reasoning that even if cancer did recur in the breast, additional surgery would get rid of it, and that a recurrence did not necessarily mean the disease would spread. The new findings challenge that reasoning.
“If somebody has a local recurrence, yes, you can treat it, but before that happens, some of that residual cancer will have spread,” Dr. Darby said.
Some researchers thought that if a cancer had the tendency to spread, it would have done so even before the first operation, so radiation would be too late to stop it — another idea dispelled by the new findings.
Dr. Darby’s analysis found that after 15 years, for every four recurrences prevented, one death was prevented.
Most of the women in the studies — 7,287 — had no cancer in their lymph nodes, which is generally a favorable sign. In 1,050 others, there were positive nodes, and in 2,464 there was no information on the nodes. Some women took tamoxifen, a hormone-blocking drug given to women whose tumors may be fed by estrogen. And many of the women with cancerous lymph nodes also got chemotherapy. Regardless of what other treatments they received, all the women still benefited from radiation.
In women with clear lymph nodes, radiation cut the 10-year recurrence risk in half, to 15.6 percent from 31 percent, and cut the 15-year risk of death to 17.2 percent from 20.5 percent. In women with diseased lymph nodes, radiation cut the recurrence rate to 42.5 percent from 63.7 percent, and cut the death risk to 42.8 percent from 51.3 percent.
Earlier studies had found that radiation for breast cancer could harm the heart and increase a woman’s risk of dying from heart disease. But techniques have improved in recent years to avoid exposing the heart and lungs to radiation, and Dr. Darby’s study found no significant differences between women who did or did not receive radiation when it came to death rates from diseases other than breast cancer.