Barron H. Lerner, M.D., professor of medicine and public health at Columbia University Medical Center, is the author of "The Breast Cancer Wars: Hope, Fear and the Pursuit of a Cure in Twentieth-Century America."
How can some breast cancer survivors be absolutely sure that having a mammogram saved their lives and statisticians be equally positive that this cannot be the case?
Imagine you are a woman in your 40s with no family history of breast cancer. Based on the old guidelines - as they existed before new recommendations this month by the U.S. Preventive Services Task Force - you go for a mammogram. A nodule is found. The biopsy reveals breast cancer which, fortunately, seems localized to the breast. You receive surgery and radiation as well as chemotherapy to kill any cancer cells that may be elsewhere in your body.
Fast-forward. You are a five-year survivor. Then a 10-year survivor. At some point, you are considered cured.
When you tell your story, it's natural to attribute your survival to the mammogram that detected the cancer. After all, the reason you had the screening in the first place was that you understood the importance of early detection. You are living proof that mammography saves lives.
"I received a diagnosis of stage 2 breast cancer at 45," wrote one woman recently, in response to the new guidelines, which advised against mammograms before age 50. "If I had waited until 50, I would be dead."
But is this truly proof that the mammogram saved her life? The answer has to be no. It is impossible to conclude on the basis of an individual case whether finding the cancer when you did was a contributing factor to her survival.
This concept - which is counterintuitive to our standard understandings of early detection - makes sense physiologically. In the 1920s, when the American Society for the Control of Cancer (later the American Cancer Society) first urged patients to see their doctors if they saw abnormal lumps or bleeding, physicians believed that cancers grew very slowly and spread very late in their course. Finding them early and removing all cancer cells made sense.
But we now know that breast and other cancers often spread early on. Even women with seemingly localized disease found on mammograms may have cancer beyond the breast. One study, for example, found that more than 30 percent of such women had invisible cancer cells in their bone marrow. When cures occur in such cases, they result from chemotherapy and the body's natural defenses killing these cells - regardless of how early the original cancer was discovered.
In other words, aggressive cancers kill and the less aggressive ones, whenever they are found, seem not to. At what point you detect either type makes less difference than we used to think. This is why chemotherapy has become almost routine for all breast cancers, regardless of their apparent spread.
Studies analyzed by the task force compared thousands of women who received mammograms in their 40s with a matched group who did not and found that the scans rarely save lives in this age group. They miss many cancers due to dense breast tissue, while also leading to so-called false positives, nodules that look like cancer but are not. In such cases, women may undergo unnecessary biopsies and other invasive testing and experience anxiety.
If aggressive cancers in younger women either spread between mammograms or evade detection altogether, and if slower-growing cancers have the same prognosis regardless of when you find them, it follows that doing mammograms in this age group may not make a difference. This is the proof provided by sophisticated statistical studies.
Using similar reasoning, the task force gave a thumbs-down to breast self-examination, although it still recommends mammograms for women aged 50 to 74 as potentially lifesaving. Mammograms probably work better in this range because older women's breasts are less dense and the pictures are better.
Biostatisticians would be the first to admit that randomized controlled trials are not perfect. Conditions in a clinical trial differ from those in the real world, and subjective factors are always present in the interpretation of data.
But such statistics are far more reliable than the information obtained from the stories of individual women - no matter how powerful and moving they are. If physicians simply used individual cases to guide screening and treatment decisions, it would be impossible to know which of the interventions actually worked. Large groups of patients must be studied and compared.
So we must rationally consider the possibility that the task force decision is right, even if it goes against our emotional and cultural beliefs about breast cancer detection and treatment. This won't be easy to do, especially because there is nothing to replace mammography and breast self-examination for women in their 40s. And there are legitimate concerns that insurance companies may stop paying for screening mammograms in younger women based on the recommendations, meaning that only wealthy women, who can pay out of pocket, would have them done. Of course, if the test indeed causes more harm (unnecessary biopsies and anxiety) than good, it's not clear that this is any type of advantage.
The task force's report is understandably jarring to breast cancer survivors whose cancers were found on a mammogram. But we cannot disregard the fact that the best data we have call into question the value of the test. Saying that a mammogram saved one's life makes for a good story, but it isn't always a true one.