The discovery turns standard medical practice on its head. Surgeons have been removing lymph nodes from under the arms of breast cancer patients for 100 years, believing it would prolong women’s lives by keeping the cancer from spreading or coming back.

Now, researchers report that for women who meet certain criteria—about 20 per cent of patients, or 40,000 women a year in the United States—taking out cancerous nodes has no advantage. It does not change the treatment plan, improve survival or make the cancer less likely to recur. And it can cause complications like infection and lymphedema, a chronic swelling in the arm that ranges from mild to disabling.

Removing the cancerous lymph nodes proved unnecessary because the women in the study had chemotherapy and radiation, which probably wiped out any disease in the nodes, the researchers said. Those treatments are now standard for women with breast cancer in the lymph nodes, based on the realisation that once the disease reaches the nodes, it has the potential to spread to vital organs and cannot be eliminated by surgery alone.

Experts say that the new findings, combined with similar ones from earlier studies, should change medical practice for many patients. Some centres have already acted on the new information.

Memorial Sloan-Kettering Cancer Center in Manhattan changed its practice in September, because doctors knew the study results before they were published. But more widespread change may take time, experts say, because the belief in removing nodes is so deeply ingrained.

“This is such a radical change in thought that it’s been hard for many people to get their heads around it,” said Dr Monica Morrow, chief of the breast service at Sloan-Kettering and an author of the study.

Doctors and patients alike find it easy to accept more cancer treatment on the basis of a study, Dr Morrow said, but get scared when the data favour less treatment.

The new findings are part of a trend to move away from radical surgery for breast cancer. Rates of mastectomy, removal of the whole breast, began declining in the 1980s after studies found that for many patients, survival rates after lumpectomy and radiation were just as good as those after mastectomy.

The trend reflects an evolving understanding of breast cancer. In decades past, there was a belief that surgery could “get it all”—eradicate the cancer before it could spread to organs and bones. But research has found that breast cancer can begin to spread early, even when tumours are small, leaving microscopic traces of the disease after surgery.

The modern approach is to cut out obvious tumours—because lumps big enough to detect may be too dense for drugs and radiation to destroy—and to use radiation and chemotherapy to wipe out microscopic disease in other places.

But doctors have continued to think that even microscopic disease in the lymph nodes should be cut out to improve the odds of survival. And until recently, they counted cancerous lymph nodes to gauge the severity of the disease and choose chemotherapy. But now the number is not so often used to determine drug treatment, doctors say. What matters more is whether the disease has reached any nodes at all. If any are positive, the disease could become deadly. Chemotherapy is recommended, and the drugs are the same, no matter how many nodes are involved.