Breast Cancer Revisited

A few people have asked me to explain this to them again after reading my other note on the accuracy of medical tests.

Here is a nice summary: If your mammogram is positive, how likely is it that you actually have breast cancer? They’ve done surveys asking this question, and most laypeople and many doctors guess 90%. Actually it’s only 10%.

Mammograms are 90% accurate in spotting those who have cancer (this is called the sensitivity of the test). They are 93% accurate in spotting those who don’t have cancer (this is called the specificity of the test). 0.8% of women getting routine mammograms have cancer (this is the prevalence of the disease). This means that of every 1000 women getting mammograms, 8 of them have cancer. Of those 8 women with cancer, 7 of them will have true positive results, and one will have a false negative result and be falsely reassured that she does not have cancer; 992 of the 1000 women do not have cancer. Of those, 70 will have false positive results and 922 will have true negative results. So in all, there will be 77 positive test results, and only 7 of those will actually have cancer – roughly 10%.

It gets worse. How many lives are saved by mammography? If 1000 women are screened for 10 years starting at age 50, one life will be saved. 2-10 women will be over diagnosed and treated needlessly. Ten to fifteen women will be told that they have breast cancer earlier than they would otherwise have been told, but this will not affect their prognosis. 100-500 women will have at least one false alarm, and about half of them will undergo a biopsy they didn’t really need.