That finding comes from a new study of nearly 7,500 older women with early stage, estrogen-sensitive breast cancer.
The study shows that women who got radiation after a lumpectomy were about half as likely as other women to later need to have their whole breast removed because of cancer.
The study appears in the journal Cancer.
New Breast Cancer Radiation Study
Ten years after their diagnosis, 3.2% women who'd had radiation after lumpectomy had gone on to get a mastectomy, compared to 6.3% in the group that did not receive radiation.
When researchers dug deeper into the numbers to try to learn which women were getting the most benefit, they found that it was women aged 70 to 75 who had high-grade tumors. A tumor's grade refers to the way its cells look under a microscope. High-grade tumors have very abnormal-looking cells.
Patients in the study with high-grade tumors who chose not to get radiation therapy after their surgeries had a 15% chance of having a mastectomy within 10 years.
Women over 75 with tumors that didn't look very aggressive didn't appear to benefit from radiation.
Guidelines Say Radiation Doesn’t Benefit Most Older Patients
Breast cancer becomes more likely with age. The majority of breast tumors are fueled by hormones, and most breast cancers are caught early, before they've spread to the lymph nodes or other organs.
"There are lots of patients like this that we see in our clinic all the time," says researcher Benjamin Smith, MD, an assistant professor in the radiation oncology department at MD Anderson Cancer Center in Houston.
Since 2005, the nonprofit National Comprehensive Cancer Network has advised against radiation therapy after surgery to remove early estrogen-sensitive breast cancers in women in their 70s.
The guidelines are based on an earlier study that found radiation didn't reduce mastectomies or improve survival for women in their 70s who were taking the drug tamoxifen to lower their risk of getting breast cancer again.
"The message of this [new] study is that you maybe can't apply these guidelines to every patient," Smith says. "Especially for patients who are younger or who have high-grade tumors, there's still probably a benefit from radiation that's worth discussing with them."
Radiation After Surgery: Second Opinion
Other experts praised the study but said it shouldn't substantially change how most women are treated.
Kevin S. Hughes, MD, surgical director of breast screening at Massachusetts General Hospital and an associate professor at Harvard Medical School in Boston, called the study "nicely done."
Hughes conducted the study that influenced the current guidelines, but he was not involved in the current research.
He says it offers doctors and patients good information about which patients might be helped by radiation therapy after surgery.
But he points out that very few women -- just 3 for every 100 who get radiation therapy -- avoid a mastectomy later on. "That seems like more than you really need to do," he says.
Hughes also points out that breast cancer treatment has advanced since the study was done, so the findings may overestimate the benefit of radiation for women today.
Radiation is well worth it, Hughes says, if it will lower the chance that a cancer would return. But it also requires daily trips to a hospital for one to six weeks, and has side effects.
"The basic idea is that we don't want to do anything to a patient that doesn't help them in some substantial way," Hughes says. "Most women don't want radiation if they can avoid it."