Did I stir up a hornet’s nest when I tried to examine the benefits and harms of mammography? I’m glad I did. I might have been less than generous in my remarks about mammography, but that does not diminish my unqualified admiration of those who believe that it works. I am happy that some of you have stood up for what you feel is best. Not that I admire science less, but I respect individual standpoints too. I also know that numbers alone may not tell the full story.
But should we let our anecdotal perceptions and biases decide how good—or how bad—a screening test is? Should our decision-making process be coloured by the worst cases that keep on haunting us or the best cases that boost our self-worth?
We learnt in our medical schools that benefits and harms of a breast cancer screening test depend on an individual’s risk of developing and dying of breast cancer, how good the screening test is, how effective and tolerable the treatment is, and the woman’s own personal values.
How best can we understand these trade-offs between benefits and harms of breast cancer screening? By expressing them in absolute numbers. These numbers come from large systematic reviews aimed at finding the true worth of a mammogram. They do not represent an individual opinion, nor are they based on anecdotal evidence.
To screen or not to screen— that is not the question. For, there is no right or wrong answer about whether to have breast screening. Like all screening tests, mammograms are not white and black. We need to tell a 50-year-old woman that a mammogram also generates images that have several shades of grey.
“If you periodically screen 1000 women like me, and follow us for the next twenty years, what would happen?” This precisely is a question that we need to answer. How many breast cancer deaths does a mammogram avoid? How often does a mammogram trigger a false-alarm: it finds something that looks like cancer, but turns out to be benign (not cancer). And how often does it detect cancer that would not have been found in a woman’s lifetime – overdiagnosis.
How do we inform our patients who are too eager to undergo a test but are too afraid to ask questions? Here is a sample breast cancer screening decision aid used in a study (JAMA Intern Med. 2014; 174(3):417-24). The data come from Australia; rates of false-positive mammograms are higher in the United States.