N ow that we’ve spent the month of October scaring women about breast cancer, isn’t it time for a month scaring them about mammography? If not a month, a week? A day?

It’s not right to scare people, but it’s also not right to leave them in the dark about an important debate going on in the medical community. While all agree that a mammogram is an important diagnostic test for women with new breast lumps, its use as a routine screening test is more contentious. That’s because screening mammography is a double-edged sword: It lowers the breast cancer death rate but it also leads some women to be treated for cancer unnecessarily.

The U.S. Preventive Services Task Force says women who get mammograms every one to two years probably have a 16 percent to 19 percent lower death rate from breast cancer. That’s why they recommend it.

But they are also clear that it is by no means the most important thing we do in medicine. In fact, they estimate that about 1,000 women have to be screened for 14 years to avert one death from breast cancer. The other 999 don’t benefit.

Now the bad news. While screening mammography probably reduces a woman’s chances of dying from breast cancer, it definitely increases her chances of getting diagnosed with breast cancer.

The problem is that screening mammography finds too many cancers. Among women in a specific age group, medical researchers know about how many will develop breast cancers that will ever grow to cause symptoms or death. But mammography finds more than this number – published studies estimate between 5 percent to 30 percent more. Doctors call this problem “over-diagnosis.”

Because doctors don’t know which cancers will be harmful, we treat all of them. That means some women are needlessly given the diagnosis of “cancer” (itself terrifying) and needlessly undergo disfiguring surgery and the nausea, fatigue and hair loss associated with chemotherapy.

Over the past decade, medical researchers have begun to recognize that the problem of over-diagnosis is real. The uncertainty is no longer about whether it happens; instead, it’s about how often it happens.

Although there was no single starting date for mammography in the U.S., there was about a 50 percent increase in the number of American women diagnosed with breast cancers associated with mammography’s introduction during the early 1970s to mid-1980s.

One of the major randomized clinical trials of mammography reported 25 years of follow-up data in 2006 – showing that about 1 in 4 cancers detected by mammography represent over-diagnosis.

But here’s some good news about the bad news: The harm of over-diagnosis is not a fixed attribute of screening mammography. Instead, it is related to how hard we look for breast cancer. Some women harbor small, innocuous breast cancers that will never cause symptoms or death (just as some men harbor small, innocuous prostate cancers). The harder we look, the more likely we are to find these cancers.

News reports focus on which approach finds more cancer. Conventional versus digital mammograms? Digital is better because it finds more cancer. Mammograms versus MRI? MRI is better because it finds more cancer. But the problem of over-diagnosis means that finding more cancer is not better – it’s the wrong way to measure progress. Real progress would be to find only the cancers that matter.

Women should be aware that looking harder might not be in their interest. And that doctors who recommend less-aggressive mammography (less frequently, waiting until you are age 50 or stopping it when you are older) or are less quick to biopsy might not be bad doctors but good ones.

And women should demand (and participate in) research that looks less hard, finds less cancer – but finds the cancers that matter.