The conclusions were based on the most extensive collection of data ever analyzed on ductal carcinoma in situ: 100,000 women followed for 20 years.

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Up to 60,000 U.S. women each year are told they have an early stage of breast cancer — Stage 0, as it is commonly known — a possible precursor to what could be a deadly tumor. And almost every one of the women has either a lumpectomy or a mastectomy, and often a double mastectomy, removing a healthy breast as well.

Yet it appears treatment may make no difference in their outcomes. Patients with this condition had close to the same likelihood of dying of breast cancer as women in the general population, and the few who died did so despite treatment, not for lack of it, researchers reported Thursday in JAMA Oncology.

Their conclusions were based on the most extensive collection of data ever analyzed on the condition, ductal carcinoma in situ, or DCIS: 100,000 women followed for 20 years. The findings are likely to fan debate about whether tens of thousands of patients are undergoing unnecessary and sometimes disfiguring treatments for conditions unlikely to develop into life-threatening cancers.

Diagnoses of DCIS, involving abnormal cells confined to the milk ducts of the breast, have soared in recent decades. They account for up to one-quarter of cancer diagnoses made with mammography, as radiologists find smaller and smaller lesions.

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But the new data on outcomes raise provocative questions: Is DCIS cancer, a precursor to the disease or just a risk factor for some women? Is there any reason for most patients with the diagnosis to receive brutal therapies? If treatment does not make a difference, should women even be told they have the condition?

Such questions are unlikely to be resolved by the new information.

The new analysis is helpful, said Dr. Barnett Kramer, director of the division of cancer prevention at the National Cancer Institute, and is consistent with other data pointing in the same direction. The new analysis, he added, provides, “the type of evidence that builds the justification for less morbid treatment.”

Dr. Otis Brawley, chief medical officer at the American Cancer Society, said he was not ready to abandon treatment until a large clinical trial is done that randomly assigns women to receive mastectomies, lumpectomies or no treatment for DCIS, and that shows treatment is unnecessary for most patients. But Brawley, who was not involved in the study, also said he had no doubt that treatment had been excessive.

A majority of the 100,000 patients in the database the researchers used, from a national cancer registry, had lumpectomies, and nearly all the rest had mastectomies, the analysis found.

Their chance of dying of breast cancer in the two decades after treatment was 3.3 percent, no matter which procedure they had, about the same as an average woman’s chance of dying of breast cancer, said Dr. Laura Esserman, a breast-cancer surgeon and researcher at the University of California, San Francisco, who wrote an accompanying editorial.

The data showed that some patients were at higher risk for developing cancer: those younger than 40, black women and those whose abnormal cells had molecular markers found in advanced cancers with poorer prognoses.

DCIS has long been regarded as a precursor to potentially deadly invasive cancers, analogous to colon polyps that can turn into colon cancer, said Dr. Steven Narod, lead author of the paper and a researcher at Women’s College Research Institute in Toronto. The treatment has been to get rid of the tiny specks of abnormal breast cells, just as doctors get rid of colon polyps when they see them in a colonoscopy.

But if that understanding of the condition had played out as expected, women who had an entire breast removed or both breasts should have been protected from invasive breast cancer. Instead, the findings showed, they had the same risk as those who had a lumpectomy. Almost no women went untreated, so it is not clear if as a group they did worse.

But some women who died of breast cancer ended up with the disease throughout their body without ever having it recur in their breast; many, in fact, had no breast because they had had a mastectomy. Those very rare fatal cases of DCIS followed by fatal breast cancer, Narod concluded, had most likely already spread at the time DCIS was detected.

Esserman said that if deadly breast cancers started as DCIS, the incidence of invasive breast cancers should have plummeted with rising detection rates. That has not happened, even though in the pre-mammography era, before about 1980, the number of women found to have DCIS was only in the hundreds. Nearly 240,000 women receive diagnoses of invasive breast cancer each year.

Those facts lead Narod to a blunt view. After a surgeon has removed the aberrant cells for the biopsy, he said: “I think the best way to treat DCIS is to do nothing.”

Others did not concur. Dr. Monica Morrow, chief breast-cancer surgeon at Memorial Sloan Kettering Cancer Center, said it made more sense to view DCIS as a cancer precursor that should be treated the way it is now, with a lumpectomy or mastectomy. She questioned whether those women who were treated and ended up dying of breast cancer anyway had been misdiagnosed.

In some cases, pathologists look at only a small amount of tumor, Morrow said, and could have missed invasive cancer. Even the best mastectomy leaves cells behind, she added, which could explain why a small number of women with DCIS who had mastectomies, even double mastectomies, died of breast cancer.

The notion that most women with DCIS might not need mastectomies or lumpectomies can be agonizing for those, like Therese Taylor of Mississauga, Ontario, who have had such treatment. Four years ago, when she was 51, a doctor sent her for a mammogram. It turned out she had DCIS in her left breast. A surgeon told her “it was consistent with cancer” and she should have a mastectomy.

She had the surgery and regrets it. “It takes away your feeling of attractiveness,” she said. “Compared to women who really have cancer, it is nothing. But the mastectomy was for no reason, and that’s why it bothers me.”