Behaviour within a Clinical Trial and Implications for Mammography Guidelines

Mammography guidelines have weakened in response to evidence that mammograms diagnose breast cancers that would never eventually cause symptoms, a phenomenon called “overdiagnosis.” Given concerns about overdiagnosis, instead of recommending mammograms, US guidelines encourage women aged 40–49 to get them as they see fit. To assess whether these guidelines target women effectively, I propose an approach that examines mammography behaviour within an influential clinical trial that followed participants long enough to find overdiagnosis. I find that women who are more likely to receive mammograms are healthier and have higher socioeconomic status. More importantly, I find that the 20-year level of overdiagnosis is at least 3.5 times higher among women who are most likely to receive mammograms. At least 36 of their cancers are overdiagnosed. These findings imply that US guidelines encourage mammograms among healthier women who are more likely to be overdiagnosed by them. Guidelines in other countries do not.