You might think that screening for breast cancer by mammography is a slam-dunk. It was not the last time I wrote about it; and it is not now.
In a YouTube video sent to me recently, the head of the Nordic Cochrane Center makes an emphatic case against breast cancer screening. This is important, because Cochrane is among the leading sources of medical evidence assessment and synthesis in the world, and those who run their centers are highly qualified to judge such evidence. On the other hand, this particular expert is a rather inveterate contrarian, opposing much of what “Big Medicine” does. That doesn’t make him wrong, but it does highlight his penchant for staking out and defending extreme positions.
The support for this position on mammography is clear enough: some large population studies show no mortality benefit of screening for breast cancer versus no screening at all. Cancer screening, of every kind, certainly has potential to do harm related to treatment that may be unnecessary, or follow-up testing that may be invasive and dangerous, and needed to determine whether or not cancer is truly present. The harms of such testing are particularly hard to condone when false positives (i.e., the test suggests cancer but there really isn’t any) significantly outnumber true positives (i.e., follow-up testing confirms the presence of cancer), as is true for mammography.
Yet, I support breast cancer screening, as does the U.S. Preventive Services Task Force, which assigns a “B” grade in support of mammography for women age 50 to 74. A “B” grade on their scale means: “there is high certainty that the net benefit is moderate or there is moderate certainty that the net benefit is moderate to substantial.” By way of comparison, colorectal cancer screening gets an “A” grade, indicating clear evidence of benefit, while prostate cancer screening gets a “C,” indicating uncertainty about the balance of benefit and harms. For those of us in Preventive Medicine, the USPSTF recommendations are something of a bible; you can access them yourself online.
There are a number of reasons why the benefits of mammography might actually be, or seem, less than we might hope.
Mammography is far from 100 percent accurate in general. This is true of most screening tests and simply something we must tolerate in a world where “perfect is the enemy of good.” The implications are that the technology should be improved or replaced to produce better images, and radiologists (and pathologists) should be ever more highly trained to interpret subtleties. Both of those are in fact on-going processes, so the images and their interpretation improve over time.
Breast tissue is not all created equal. I fully support the campaign to raise awareness about dense breast tissue, and the need for alternatives to mammography when those images are unreliable. Every time a woman with breast tissue too dense for reliable mammography is screened with a mammogram, it adds to the “evidence” that mammography is not helpful. The reality is that mammograms can work well for some women, while others are much better off with alternative screening tests.
Cancer treatment is improving rapidly, and mortality is a very blunt measure. This combination is really the clincher in my view. Cancer mortality rates are declining overall because of advances in treatment. This means that even breast cancer found later, without screening, may be treatable in ways that forestall death. But as noted, death is a very blunt measure, and often the one used to judge the merits of mammography. But the treatment for late stage, metastatic cancer is apt to be much, much tougher and more toxic to the body than the localized treatment for an early stage cancer found through screening. Perhaps the “death” rate would be comparable between them, but the “duress” rate certainly would not be. Paradoxical though it may seem, advances in cancer treatment, especially treatment for advanced cancers, may obscure the advantages of screening because lives can be “saved” with or without early detection. But that does not rule out the desirability of early detection, and less extensive treatment.
Breast cancer screening approaches are not perfect, but I — along with the U.S. Preventive Services Task Force — remain convinced they confer net benefit, and do good. Let’s pursue better methods, but not make perfect the enemy of the good our current methods can do.