Effect of Three Decades of Screening Mammography on Breast-Cancer Incidence

The primary aim of cancer screening programs is to reduce the rate of death from cancer. To be effective, screening must detect potential life-threatening cancers at an early curable stage and reduce the incidence of late-stage cancer. It is clear that misdiagnosis (overdiagnosis) of tumors unlikely to cause illness will subject women to unnecessary harmful treatment (various combinations of surgery, radiation therapy, hormonal therapy, and chemotherapy). This study investigated trends over 3 decades of screening mammography in the incidence of early-stage breast cancer (ductal carcinoma in situ DCIS or localized disease) and late-stage breast cancer (regional or distant disease) among women 40 years or older. Surveillance, Epidemiology, and End Results (SEER) data for 1976 through 2008 were used to determine whether increased detection of early-stage cancers would result in reduction in the incidence of late-stage cancers. Data were adjusted for the transient excess incidence associated with hormone replacement therapy and for trends in the incidence of breast cancer among women younger than 40 years. The data show that use of screening mammography in the United States over the past 3 decades has doubled the number of cases of early-stage breast cancer detected each year (from 112 to 234 cases per 100,000 women—representing an absolute increase of 122 cases per 100,000 women). During this same study period, there was only an 8% decrease in the rate at which women present with late-stage cancer (from 102 to 94 cases per 100,000 women—an absolute decrease of 8 cases per 100,000 women). Assuming a constant underlying disease burden in these women, only 8 of the 122 additional early-stage cancers diagnosed were destined to progress to advanced disease, which implies a detection of 114 excess cases per 100,000 women. During the study period, screening detected tumors in more than 1 million US women that would have not resulted in clinical symptoms. In 2008, it is estimated that breast cancer was overdiagnosed in more than 70,000 women, accounting for 31% of all breast cancers detected. These findings indicate that screening is associated with substantial over diagnosis, accounting for nearly a third of all newly diagnosed breast cancers, while having only a small effect on the rate of death from breast cancer. These data raise serious concerns over the value of screening mammography.