Detecting and treating an early form of breast cancer lowers the number of invasive breast cancers found over the next three years, according to a study of 5.2 million U.K. women.

The study, published on Friday in the journal Lancet Oncology, provides reassurance that early cancer detection is beneficial amid growing concern that it also leads to unnecessary treatment, the researchers said.

Ductal carcinoma in situ, or DCIS, involves clusters of abnormal cells confined to the milk ducts of the breast. It was seldom seen before mammograms came into widespread use in the 1980s but now accounts for nearly one-quarter of all breast cancers detected by screening—some 60,000 cases a year in the U.S.

Some experts argue that DCIS shouldn’t be labeled cancer at all, since it’s unclear whether it will spread outside the milk ducts. Yet virtually all DCIS cases are treated as if they were cancer, with lumpectomies, with or without radiation, or mastectomies. Since DCIS is seldom left untreated, there is little data on how many cases would become invasive cancers, although some estimates suggest that as few as 20% progress.

In the latest study, researchers at Queen Mary University of London analyzed records of 5.2 million women ages 50 to 64 in England, Wales and Northern Ireland who had mammograms between 2003 and 2007. In 90% of the local screening areas, for every three cases of DCIS detected and treated, one fewer case of invasive cancer was diagnosed in the following three years than would have been without early intervention, the study found.

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The study analyzed overall statistics and couldn’t track how individual patients fared. Still, the findings “suggest that a substantial proportion of DCIS will become invasive if untreated, and it is therefore worth detecting and treating early,” said Stephen Duffy, a professor of cancer screening and the study’s lead author.

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Asked if the study showed that treating two out of every three DCIS cases was unnecessary, Dr. Duffy said “we can’t be sure they would not have progressed. It may have taken longer than three years.”

He and other cancer experts said the U.K. findings underscore the need for better ways to predict which DCIS cases are likely to advance.

“This study tells us that diagnosing a lot of DCIS seems to lead to the prevention of some cancers,” said Otis Brawley, chief medical officer of the American Cancer Society. “That does not mean that all DCIS needs to be diagnosed or treated.”

For now, he said he advises patients to treat their DCIS, but he cautions that even opting for a double mastectomy—as 12% of women with early-stage breast cancers do—doesn’t completely eliminate their risk of recurrence, since some breast tissue is left behind.

Efforts are under way to assess whether low-grade cases of DCIS can be treated with hormone-blocking drugs such as tamoxifen rather than surgery or radiation. One study at Duke University is treating nearly 100 DCIS patients with hormone therapy for six months before surgery in hopes of eliminating the lesions. Another study in the works would give patients with low-grade DCIS, in which cells look less abnormal than in higher grades, a choice of lumpectomy, mastectomy or active surveillance.

Shelley Hwang, chief of breast-cancer surgery at Duke, who is spearheading those studies, said, “Women should be aware of the trade-offs of all we are doing to try to prevent deaths from cancer. There’s never going to be a magic bullet. Everything we do comes at some cost.”

Write to Melinda Beck at HealthJournal@wsj.com

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