“This is wonderful news,” Dr. Sylvia Adams, a breast cancer specialist and researcher at NYU Langone Health’s Perlmutter Cancer Center, said in an email. She was not part of the study, which she called “a very important milestone.”

She added, “It’s a trial worth highlighting, as there have been very few studies in advanced breast cancer in general showing a clear survival benefit.”

Younger patients have been a particular concern, because “breast cancer is known to be more aggressive and to be associated with a poorer prognosis in younger women than in older women,” the researchers wrote in The New England Journal of Medicine, which will publish the article online Tuesday, when the results are to be presented at the annual meeting of the American Society of Clinical Oncology, in Chicago.

“I think there’s a lot of optimism now that we have pushed the survival boundary, that we can go farther,” said Dr. Debu Tripathy, an author of the study and chairman of breast medical oncology at the M.D. Anderson Cancer Center in Houston. “Once you break a boundary, you learn how to break more boundaries.”

About 25 percent of breast cancers in the United States are diagnosed in premenopausal women, Dr. Tripathy said. In poorer countries, he said, the proportion of young women is higher, because the risk does not keep rising with age the way it does in richer areas, where it appears linked to weight gain.