But the survival advantage with lumpectomy held up even when researchers accounted for age, tumor stage and type, race, economic status and other factors. Among women younger than 50 with hormone-sensitive cancers, for instance, those who had lumpectomy had a 7 percent lower chance of death than those who had mastectomy.


Hwang said the survival difference between recipients of lumpectomy and mastectomy might be partly explained by the fact that women who got a mastectomy tended to be in worse health to begin with.


Her team also looked at causes of death in the short term, three years after treatment, to gauge whether other serious health conditions like heart or respiratory disease might have influenced a woman's choice to go with mastectomy and might also have skewed survival rates.


With other diseases taken into account, the survival advantage for lumpectomy was much smaller but still held.


The study cannot prove that lumpectomy is the factor responsible for the apparent survival benefit. And the researchers did not have access to some specific details about the women's tumors, or whether some women carried gene mutations that could affect their susceptibility to cancer or influence their treatment choices.


In 2013, The American Cancer Society estimates that 232,340 U.S. women will be diagnosed with breast cancer, and 39,620 will die from it.


In the U.S. a lumpectomy can cost over $7,000 while a mastectomy can cost more than $10,000. According to the American Cancer Society (ACS) each surgery comes with risks, including pain, swelling and scar tissue. Radiation, which usually accompanies lumpectomy, also comes with side effects, including complications during breast reconstruction.


Aside from surgery, ACS says women may also choose to be treated with chemotherapy, targeted genetic treatments and hormone-blocking therapy.


Hwang also cautions that the survival differences are relative and the actual benefit associated with lumpectomy in this study is small for an individual woman.


"I wouldn't overstate these results, because the survival difference can come from other things," said Dr. Dawn Hershman, co-leader of the Breast Cancer Program at the Columbia University Medical Center in New York.


But Hershman, who was not involved with the new study, told Reuters Health that the findings are reassuring.


"Sometimes patients in practice can be very different than patients in randomized trials. It's reassuring that patients who get breast-conserving therapy do at least as well as those with mastectomy," she said, adding that not all women are candidates for a lumpectomy.