Use of the shorter course of radiation increased in both groups of women from 2008 to 2013, but still only a minority received this treatment. In the group that should have received the shorter therapy under the guidelines, 10.6 percent received it in 2008 and 34.5 percent in 2013. In the group that received no recommendation for or against the shorter treatment, the percentage who used it rose from 8.1 percent to 21.2 percent over that time.

In Canada and Britain, the statistics were far different. At least two-thirds of women in both groups received the shorter therapy.

In the United States, total medical expenses for the shorter therapy in women for whom it was endorsed were $28,747. For comparable women receiving the longer course of treatment, the cost was $31,641. For the second group of women — for whom the shorter therapy was neither endorsed nor discouraged — medical costs were $64,723, compared with $72,860 for conventional therapy. Health insurers pay for radiation in a piecemeal fashion, and the shorter course involves about 16 doses, compared with about 33 with the conventional therapy.

Dr. Harold J. Burstein, a medical oncologist at Dana-Farber Cancer Institute in Boston, said that when the initial results of a Canadian trial on the shorter therapy were published in 2002, “there was real ambivalence about changing practice based on one study.” Doctors wanted to see what would happen as the women were followed over a longer time, and they wanted to see the result confirmed.

The results of that study contradicted years of practice in the field, said Dr. Gary M. Freedman, a breast radiation oncologist at the University of Pennsylvania and an author of the new study. In the 1970s and 1980s, when equipment was much less sophisticated, radiation oncologists found that shorter and more intense therapy burned women’s skin and scarred their breasts, making them shrivel and shrink over the ensuing decade.

“I started my residency in 1993,” Dr. Freedman said. “That was drilled into us”: Shorter and more intense radiation therapy “was a bad idea and would have a bad cosmetic result.”

But with improved equipment and methodology, he said, the clinical trials found that cosmetic results were just as good with the shorter treatment. “They did not just publish that the cure rates were the same, but they published very in-depth cosmetic assessments, particularly the British, who took pictures that were graded by blinded observers,” he said.

“That’s when the tide started turning.”