Doctors and patients want and need tests that detect disease while that disease is still treatable — ideally, while curable. Even better, we want screening tests that can accurately detect “pre-disease,” not just detect it once present. An example is colonoscopy, which can detect certain types of polyps that degenerate into colon cancer. And of course, we want tests that actually improve longevity or quality of life.

On the other side, we don’t want too many false alarms (or “false positive” results) — these are results that suggest disease when, in fact, no disease is present. This is especially true if those false alarms lead to unneeded surgery or other risky procedures.

Even when risky treatments are avoided, there’s another downside to false alarms: worry. If you’re told your screening test for cancer is abnormal, the time it takes to figure out that it’s a false alarm can be terribly frightening. And it can seem like forever.

Assessing the value of the screening mammogram

In a previous blog, I wrote about how we have relatively few time-tested and reliable medical screening tests that deliver on their promise. A 2016 study questions the value of one of those tests: the mammogram.

This study analyzed data from women over 40 and compared the size of breast cancers at the time of diagnosis detected in the 1970s (before mammography became common) with the size of tumors detected between 2000 and 2002, when screening mammography was routine. Treatments and rates of death due to breast cancer 10 years after the diagnosis were also analyzed. The study found that:

• As more women underwent routine screening mammograms, more small breast cancers were detected. Many of these tumors were restricted to the ducts within the breast (called ductal carcinoma in situ), and even without treatment would never threaten the health of the woman.
• The detection of larger, more aggressive breast cancers was unchanged in frequency between the pre-mammogram and more recent time periods. This is important since, if screening mammograms caught these cancers earlier, in theory, the frequency of detecting these more dangerous tumors should be falling.

Are there benefits of a screening mammogram?

The study’s authors did not suggest that mammography is useless. They estimated that about 20% of women with small tumors that could only be detected with a mammogram received therapy that might be lifesaving. But the other 80% of women did not benefit. Similarly, estimates suggest that about two-thirds of the reduction in breast cancer deaths in recent years is due to better treatments, not better detection. Some additional reduction may be related to falling rates of post-menopausal hormone replacement; but the contribution of mammography to this trend may be relatively small.

The authors suggest that the benefits of mammography have been overestimated and, as a result, many women may be receiving medical and surgical treatments that are unnecessary. It’s not surprising that this new study is controversial. A number of experts have criticized its conclusions and worry that such research will discourage women from having mammograms that could save their lives.

Debate on the value of screening mammograms has happened before

This isn’t the first time questions have been raised about the usefulness of mammography. A 2012 study also raised the concern that unimportant tumors were being detected, leading to unnecessary treatment for large numbers of women. There has also been controversy about when women should start having them and how often they should be repeated. For example, the US Preventive Services Task Force suggests that routine screening mammograms should begin at age 50 and be repeated every two years until age 74. But the American Cancer Society says that women should begin at age 45, have them yearly until age 55, and then every two years.

So, what’s a woman to do?

While the message of this new study is clear — mammography may not be as good as we thought — it isn’t at all clear what women should do with this information. It’s likely that in light of these concerns, recommendations will change in the future. But how?

Optimal recommendations regarding mammography will depend on:

• Developing better ways to distinguish breast cancers that pose a threat to health and warrant aggressive therapy from those that don’t
• Understanding the individual factors that affect the usefulness of mammography. For example, family history, genetic factors, use of hormonal therapy, breast density, obesity, and reproductive history all can affect the risk of breast cancer or the usefulness of mammography.
• Reaching a consensus regarding an acceptable level of accuracy for screening tests. As screening tests for breast cancer evolve — including better mammograms, ultrasound, and MRI — the accuracy of diagnosis should improve. But since no test is perfect, we have to accept that some breast cancers will be missed and false alarms will happen, even with the best available screening methods.

For now, it’s important to talk to your doctor about this important study. My guess is that he or she will continue to recommend mammograms as before.

Still, it’s worth a conversation. There is a real possibility that screening tests such as mammography can cause harm — and that’s worth understanding before you have the test.

Follow me on Twitter @RobShmerling

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