Do Some Breast Cancers Disappear?

We know little about the natural history of untreated breast cancer. In a prospective cohort study, investigators took advantage of the introduction of biennial mammography screening in four Norwegian counties and compared 6-year cumulative incidences of invasive breast cancer before and after regular screening became available. The screened cohort consisted of women (age range when screening began, 50–64) who underwent biennial screening mammograms. Women in the age-matched control cohort (for which follow-up began 4 years before the screening program was initiated) each underwent a single prevalence screening mammogram at the end of their 6-year follow-up. The primary outcome, invasive breast cancer, was ascertained through the Norwegian …