Yet another study suggests that mammograms are not terribly effective in decreasing breast cancer mortality. For earlier studies suggesting the same, see here. For the definition of false positives in mammography, see here.
The newest study is available here. The study is an ecological one, comparing mammography use and breast cancer mortality between different US counties. Because of its nature, the study is open to certain criticisms which the authors address at the end of the article. None of them seem to me to be strong enough to support other explanations over the main finding of the study, this:
When analyzed at the county level, the clearest result of mammography screening is the diagnosis of additional small cancers. Furthermore, there is no concomitant decline in the detection of larger cancers, which might explain the absence of any significant difference in the overall rate of death from the disease. Together, these findings suggest widespread overdiagnosis.
By "overdiagnosis" the study means that mammograms may be
identifying small, indolent, or regressive breast tumors that would not otherwise become clinically apparent
Overdiagnosis is undesirable. It leads to further medical diagnosis and treatment, some of it may have high risks, all of it costing money and causing some pain. And the anxiety caused by possible false positives is something we should be concerned about.
What causes this? The obvious answer is that we don't understand the natural histories of various types of tumors well enough*. Perhaps many small tumors would not grow at all, even if they were not found in a mammogram. Perhaps some tumors would disappear on their own. Perhaps some types of tumors, those which tend to be fatal, lack effective treatments, so that an early diagnosis doesn't ultimately affect mortality rates.
But an early diagnosis in those latter cases would look like a possible increase in life expectancy, simply because the disease was found earlier but couldn't ultimately be effectively treated.
What's a woman to do**, with all this data coming in? The authors of the article:
Nonetheless, we do not believe that the right rate of screening mammography is zero. As is the case with screening in general, the balance of benefits and harms is likely to be most favorable when screening is directed to those at high risk, provided neither too frequently nor too rarely, and sometimes followed by watchful waiting instead of immediate active treatment.37Which leaves most of us with the advice of talking to our health care providers and hoping that they are up to date about this information and about our own medical histories.
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*In other words, what would happen in the absence of the diagnosis and treatment, how would various tumors develop (grow, metastize, shrink, vanish, stay the same) without any treatment.
Given the current state of knowledge in the field, it's not really possible to say that a woman whose cancer was found in a mammogram, who was treated for it and is cancer-free ten years after the original mammogram had her life saved by that diagnostic tool. That may be the case, of course. But it's also possible that her particular tumor might never have developed into a fatal cancer.
What we really need in the case of breast cancer (as in the case of many other cancers) are tremendous scientific leaps in the understanding and treatment of the disease.
** Or a man. It's important to note that some types of preventive screening are very efficient and effective. This is true of the PAP smear for cervical cancer and of colonoscopy for the cancer of the colon, though not so much for widespread prostate screening. Thus, the best approach is to look at each test, discuss its desirability with one's health care provider, get as educated as possible and then act based on all that. I wish there were more clear-cut options.