New study reports screening mammograms have not reduced breast cancer deaths
According to a new study, screening mammograms have doubled the number of early-stage breast cancers detected in the United States. However, this has not made much of a difference in the number of breast cancer deaths. Furthermore, up to one-third of all new breast cancer cases are overdiagnosed as a result of screening mammograms. Overdiagnosis refers to a diagnosis that is worse than the actual disease condition that is present. The findings were published on November 22 in the New England Journal of Medicine.
The study authors noted that, in order to reduce mortality, screening must detect life-threatening disease at an earlier, more curable stage. Therefore, effective cancer-screening programs both increase the incidence of cancer detected at an early stage and decrease the incidence of cancer presenting at a late stage. The researchers reviewed breast cancer screening in the US by accessing data from the Surveillance, Epidemiology, and End Results data to examine trends from 1976 through 2008 in the incidence of early-stage breast cancer (ductal carcinoma in situ and localized disease) and late-stage breast cancer (regional and distant disease) among women 40 years of age or older.
The investigators found that the introduction of screening mammography in the United States has been associated with a doubling in the number of cases of early-stage breast cancer that are detected each year, from 112 to 234 cases per 100,000 women; this marks an absolute increase of 122 cases per 100,000 women. During the same period, the rate at which women present with late-stage cancer has decreased by 8%, from 102 to 94 cases per 100,000 women; this marks an absolute decrease of 8 cases per 100,000 women. The researchers explained that, with the assumption of a constant underlying disease burden (actual number of new cases occurring), only 8 of the 122 additional early-stage cancers diagnosed were expected to progress to advanced disease.
The researchers excluded cases associated with hormone replacement therapy (HRT) and adjusted for trends in the incidence of breast cancer among women younger than 40 years of age. They estimated that breast cancer was overdiagnosed (i.e., tumors were detected on screening that would never have led to clinical symptoms) in 1.3 million US women over the past 30 years. They estimated that in 2008, breast cancer was overdiagnosed in more than 70,000 women; this accounted for 31% of all breast cancers diagnosed.
The authors concluded: “Despite substantial increases in the number of cases of early-stage breast cancer detected, screening mammography has only marginally reduced the rate at which women present with advanced cancer. Although it is not certain which women have been affected, the imbalance suggests that there is substantial overdiagnosis, accounting for nearly a third of all newly diagnosed breast cancers, and that screening is having, at best, only a small effect on the rate of death from breast cancer.”