October is Breast Cancer Awareness Month, which means we’re experiencing an onslaught of pink ribbons and breast-cancer-related marketing. Among the most visible campaigns is the NFL’s pink-themed “Crucial Catch” program, which counsels that “screening saves lives” and that “everyone can help make a crucial catch.” But the science is a lot more complicated than that.

A study published online today by the The New England Journal of Medicine added to a growing body of evidence that for every woman who has been helped by a mammogram screening for breast cancer, many more have been harmed.

This latest study examined data from 1975 to 2012 on women ages 40 and older taken from a national program that tracks cancer cases. The researchers’ goal was to determine whether this type of screening, which became widespread in the 1980s, was really catching cancer earlier and thus reducing the number of big tumors that were being diagnosed later.

The results were sobering, as H. Gilbert Welch, a physician at Dartmouth and the study’s lead author, explains in this short video. The idea behind cancer screening is that it saves lives by identifying cancers when they are more treatable — i.e., earlier, when they are smaller. If it works, then we should see a rise in the number of small tumors being detected and a commensurate drop in the number of large cancers, as treating the small cancers would eliminate them before they become big ones, in the same way that picking small zucchini from your garden prevents you from developing humongous ones. But the new study found that although the incidence of cancers smaller than 2 centimeters rose quite dramatically after widespread mammography was introduced, by 162 cases per 100,000 women, the incidence of larger tumors fell by a much smaller amount — only 30 cases of cancer per 100,000 women.

This chart shows how the incidence of tumors smaller than 2 centimeters (the blue lines) and 2 centimeters or larger (reddish lines) changed after the introduction of mammography:

Most importantly, the incidence of metastatic cancer, which is the type that causes most deaths, was flat. Welch said that means that screening finds a lot of small cancers that would never have killed anyone.

Screening did result in more cancers being detected, he said, but the data suggests that only about 30 of the 162 additional small tumors per 100,000 women that screening mammograms found would ever have progressed to a dangerous stage. That means that 132, or 81 percent, of the 162 extra tumors detected represented “overdiagnosis” — the discovery and treatment of tumors that were never destined to harm.

None of this is news — this study is simply the latest evidence that for every life saved by mammography, many other women are unnecessarily diagnosed and treated for a cancer that wouldn’t have hurt them. (For a primer on overdiagnosis and the biology of cancer, see my 2014 article, “The Case Against Early Cancer Detection.”) Welch’s group has published other studies with similar results. One of those categorized cancers according to stage, rather than size, however, and some critics countered that the way that cancers are categorized has evolved, which could make comparisons difficult. So in this latest study, the researchers looked at size — a measure that is objective and hasn’t changed over time — and came to the same conclusion: Mammography produces an increase in the number of women diagnosed and treated for breast cancer and a non-invasive cancer called ductal carcinoma in situ (DCIS), but it doesn’t do much to prevent the deadliest cases.

The chart below shows invasive cancers only, which doesn’t include DCIS, a condition whose rates rose steeply after the introduction of mammography.

That’s the bad news. Here’s the good: Since 1990, mortality rates from breast cancer have fallen by about a third. That’s true for women older than 40, who most commonly have mammograms, as well as for women younger than 40, who aren’t routinely screened. That means that screening can’t explain all of the drop, Welch said. Instead, the analysis found that most of this mortality reduction came from improved treatments. And as treatments improve, early detection becomes less important. “No matter what the tumor’s size, treatment has improved,” he said.

Despite the findings that mammograms may lead to overdiagnosis, there are likely a small number of women who avert a breast cancer death by getting screened.

Put this all together, and what it says is that whether to have a mammogram isn’t a life-or-death decision, Welch said. As I wrote last year, science can’t settle the mammogram debate — choices about whether to get screened are value judgments with no one correct answer. Women who want to get screened should, but women who opt out should not be demonized for their choice, Welch said.