Overdiagnosis in breast cancer chemoprevention trials.

the reduction in the use of hormone replacement therapy, and the immediacy of the effect suggests the disappearance of established subclinical cancers rather than the prevention of new cancers 23–25. The complementary observations concerning the immediate effects of tamoxifen chemoprevention and hormone replacement withdrawal suggest that a slowly growing er-positive cancer might be induced to regress based on the sudden removal of an estro-genic signal. We question whether that paradigm is relevant for evaluating antiestrogen-based chemoprevention studies—in particular if the cohorts under study were being followed for incident cancers with annual mammography. If the paradigm were to hold true for patients assigned to an antiestrogen arm in the screening trials, then removal of the estrogen signal might provoke the regression of some er-positive nonpalpable tumours, while similar cancers in the placebo arm might remain detectable—but would not threaten survival (and might eventually regress). If all incident breast cancers in the chemoprevention trials were to have been diagnosed by mammography, overdiagnosis would be a concern. In contrast, if the great majority of cancers were detected by clinical breast exam or self-exam (being therefore palpable), overdiagnosis would be less of an issue. However, when the relevant studies were initiated, overdiag-nosis was not a recognized phenomenon. We therefore reviewed the incidence and mortality data from the breast cancer prevention trials, seeking to determine whether breast cancers were detected only by mammography or were palpable. A preventive effect restricted to women with cancers detected only by mammography would raise the question of whether the observed incidence benefit could be expected to translate into a mortality benefit. If so, then an excess of nonpalpable mammography-detected er-positive breast cancers would be expected in the placebo arm compared with the treatment arm, and restriction of the analysis to palpable cancers would attenuate or eliminate the Several randomized controlled trials have demonstrated that the preventive use of an antiestrogen agent such as tamoxifen 1–4 , raloxifene 5–7 , anastro-zole 8 , or exemestane 9 will reduce the incidence of estrogen receptor (er)–positive breast cancers by 50% or more. The reduction in risk becomes apparent shortly after tamoxifen initiation 10. However, no mortality benefit has yet been demonstrated with tamoxifen or any other agent, an effect that might be statistical: that is, the statistical power to detect a difference in mortality could be lacking because deaths from breast cancer are far fewer in number than cases of breast cancer, …