EDITORIAL Lead Time and Overdiagnosis

In this issue of the Journal, Etzioni et al. (1) suggest a novel approach to estimating overdiagnosis in breast cancer. However, there is a key assumption in their method regarding the nature of overdiagnosis that is unlikely to hold and leads to an underesti-mate of the overdiagnosis rate. To understand this assumption, we need to go back to basic definitions. Lead time is the time from detection of preclinical cancer by screening to detection of clinical (symptomatic) cancer in the absence of screening. An overdiag-nosed cancer is a cancer detected by screening that would not have presented clinically during that person’s lifetime in the absence of screening. We define two types of preclinical cancer detected on screen-ing, progressive and nonprogressive, which are linked to two types of overdiagnosis described by Welch and Black (2). A pro-gressive preclinical cancer would present as clinical cancer in th