That is the hoped-for result in a study of a successful screening program. After the first round of screening, an excess of 324 cancers per 100,000 person–years was seen in the screened group. The comparison group was screened only once, between year 4 and year 6. In their observational study, one group of Swedish women was screened in year 1 and then annually for 4 years. The most compelling evidence for this observation comes from Zahl and colleagues 5 in work recently published in Lancet Oncology. That is, some cancers detectable only by mammography would never become clinically apparent (or life-threatening) 4, 5. The second observation is that overdiagnosis is observed in breast cancer screening trials. The data better fit a model in which the removal of estrogen and progesterone leads to the disappearance of an already established cancer 2. If hormone therapy increases risk by initiating new tumours, a gradual fall-off in risk would be expected because of the long latent period. In countries in which the use of hormone therapy dropped rapidly after publication of the results from the Women’s Health Initiative, a decline in breast cancer incidence was noticed immediately 3. The first relevant observation is that, after cessation of hormone replacement therapy, the excess risk of breast cancer attributable to exogenous hormones dissipates within 2 years 1, 2. Epidemiologic observations in two fields of study lead to the same conclusion: namely, that a proportion of breast cancers will go away without medical or surgical intervention.
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