Another Look at Mammography
Over-diagnosis Not Reason to Abandon It
An article in last week’s Journal of the American Medical Association is once again causing controversy because it calls into question screening for breast cancer with mammography. Researchers in Denmark claim that mammography screening programs “over-diagnose” as many as 35 percent of invasive cancers, and 52 percent of all breast cancers (British Medical Journal, 2009;339:b2587). Over-diagnosis is defined as the detection of cancers that would not cause death or symptoms if left untreated. They postulate that mammography finds primarily the slow-growing, non-aggressive cancers, while missing the more lethal fast-growing cancers. The end result of this type of screening would be to subject thousands of women to unnecessary treatment, without substantially reducing deaths from breast cancer.
Those of us who specialize in breast cancer know there is some truth to these findings. We have always been aware, from the very first screening trials in the 1960s, that mammography does not find all cancers in time, and some of the cancers mammography does detect are not life-threatening. So what should we do? Should we go back to the “good old days” before mammography when the average size of breast cancers at the time of diagnosis was 3 centimeters, or about the size of a lime? I don’t think so, and here’s why.
There is another study, presented at the recent American Society of Clinical Oncology, which women should be aware of. Researchers looked at almost 7,000 women diagnosed with invasive breast cancer between 1990 and 1999 in Massachusetts. After follow-up through 2007, 461 deaths from breast cancer occurred in these women. Only 25 percent of these deaths occurred in the 80 percent of women who were getting mammograms at least every two years. By contrast, 70 percent of the breast cancer deaths occurred in women who had never had a mammogram, and another 5 percent in women whose prior mammograms were more than two years before diagnosis. Altogether, three quarters of breast cancer deaths occurred in the 20 percent of women not regularly screened with mammography. Less than 10 percent of deaths occurred from interval cancers discovered between regular mammograms. So the real question is not whether mammography is good, but whether it is good enough.
We know mammography does a poor job of finding early cancer in women with dense breast tissue, and studies have validated the use of ultrasound in addition to mammography for these women. A recently published trial of automated whole-breast ultrasound screening showed that cancer detections were doubled, and invasive cancers detected at 10 millimeters or less in size were tripled, compared to mammography alone. Ten millimeters is about the size of an M & M candy.
Ongoing research has made tremendous headway in defining which cancers need aggressive treatment and which don’t, using hormone receptors tests and tumor markers. Biopsy and surgery have become less invasive through image-guided needle procedures, and sentinel lymph node mapping, so that treatment for early breast cancer is more successful and less invasive than ever. Now is not the time to take a giant step backwards by abandoning screening.-Judy C. Dean, MD, Santa Barbara