Conventional wisdom has long dictated that routine mammography can detect early breast cancers and save lives. A recent report, however, from Canada§ ignited a torrent of controversy about the validity of this premise. In summary, researchers followed almost 90,000 women, aged 40-59. Half were randomly assigned to receive regular mammograms plus physical breast exams, and the other half received only breast exams by trained medical personnel abetted by self examination. Both groups were followed for up to 25 years. Overall, 7.1% of all subjects developed cancer, and 1.1% succumbed from this disease. But importantly, mortality was the same in both groups. Of those who received mammography, 1 in 424 (representing 22% of those diagnosed with cancer) was considered “over diagnosed”. Over-diagnosis was estimated in this type of screening trial when they failed to progress after a sufficiently long period has elapsed from the cessation of screening—that is, when all cancers should have become clinically apparent. Over diagnosis generally leads to unnecessary cancer treatment, including surgery, chemotherapy and radiation. This latter figure is probably an underestimation, because earlier studies had estimated that approximately 25% to 30% of small cancers detectable by X-ray only (without a palpable lump) will not progress to invasive, or life threatening, disease, at any time during one’s lifetime.
Let’s look a bit further into this problem. If a woman tests positive on a mammogram, the likelihood she has cancer is clearly higher than those testing negative, but, within that group there are numerous “false positives”. This means that although these individuals are harboring “densities” in X-ray images, only a minority will be found to actually have cancer. On the other hand, a negative mammogram provides more reassurance, for this reduces the likelihood of not having cancer to nearly 100%, virtually eliminating one’s chance of having this disease. Given these underlying numbers, a negative result is far more useful and reassuring than is a positive result.
These numbers help to explain why so much controversy had previously surrounded the issue whether widespread screening of women at any age with mammography should be undertaken. Since the vast majority of positive responders will have no disease, this often leads to considerable anxiety and further testing (additional imaging and/or biopsies) that entails significant expense and sometimes even risk.
Previous research had already provided important data that, at least in women under the age of 50, there was insufficient evidence to support a mortality benefit stemming from regular mammography screening. Even in those beyond this age, there was little convincing evidence to indicate that very early detection of subtle abnormalities—when found in individuals without palpable breast “lumps”—added any benefit in lowering ultimate mortality. But finding palpable lump(s), especially with a positive mammogram, dramatically increases the odds that invasive, life threatening, cancer will be found. This latter group would more likely be improved—with reduced mortality—when receiving active treatment. Other factors such as a strong family history or results of genetic testing will help to increase the odds of invasive breast cancer, and this would also justify a more aggressive approach to diagnosis and treatment.
What can the individual woman conclude from this apparently conflicting information? In general the results of one study generally cannot be used to alter universal management, but, in this case, they add to an already growing body of information that questions the value of routine mammography screening. No matter how one elects to respond, however, careful breast examinations by one’s self and/or by trained medical personnel, becomes more important than ever! Mammography may eventually be relegated to a secondary role in screening.
§ BMJ 2014;348:g366, Feb 11, 2014