Thirty Years of Screening Mammography and Breast Cancer Incidence

In order for screening to reduce the cancer mortality rate, the screening must advance the time of diagnosis of cancers that are destined to cause death, and early treatment of those cancers must create some advantage over treatment at clinical presentation. Screenings that advance the time of diagnosis will have a predictable effect on the stage-specific incidence of cancer. As the time of diagnosis is advanced, more cancers will be detected at an early stage and the incidence of early-stage cancer will increase; if the time of diagnosis of cancers that will progress to a late stage is advanced, fewer cancers will be present at a late stage and the incidence of late-stage cancer will decrease.

Routine screening mammography in women ≥40 years of age has been conducted for >3 decades. Researchers recently conducted a study to assess the temporal effects of mammography on the stage-specific incidence of breast cancer. They quantified the expected increase in the incidence of early-stage breast cancer and determined the extent to which this has led to a corresponding decrease in the incidence of late-stage cancer. Results of the study were reported in the New England Journal of Medicine [2012;367(21):1998-2005].

The researchers utilized Surveillance, Epidemiology, and End Results data to identify trends from 1976 through 2008 in the incidence of early-stage breast cancer (ductal carcinoma in situ [DCIS] and localized disease) and late-stage breast cancer (regional and distant disease) among women ≥40 years of age.

Since widespread screening mammography was introduced in the 1980s, the number of cases of early-stage breast cancer detected each year has doubled, from 112 cases per 100,000 women to 234 cases per 100,000 women, an absolute increase of 122 case per 100,000 women. This large increase reflects both detection of more cases of localized disease and the advent of the detection of DCIS, which was not detected prior to the availability of mammography.

In the same time period, the rate at which women present with late-stage cancer has decreased by 8%, from 102 cases per 100,000 women to 94 cases per 100,000 women, an absolute decrease of 8 cases per 100,000 women. The smaller decrease in cases of late-stage disease reflects detection of fewer cases of regional disease.

Assuming constant underlying disease burden, only 8 of the additional 122 early diagnoses were destined to progress to advanced disease, implying a detection of 114 excess cases per 100,00 women.

After excluding the transient excess incidence associated with hormone-replacement therapy and adjusting for trends in the incidence of breast cancer among women <40 years of age, the researchers estimated that breast cancer was overdiagnosed in 1.3 million women in the past 30 years. They defined overdiagnosis as detection of tumors on screening that would never have led to clinical symptoms. They further estimated that in 2008, breast cancer was overdiagnosed in >70,000 women; this accounted for 31% of all breast cancers diagnosed.

In conclusion, the researchers stated, “Despite substantial increases in the number of cases of early-stage breast cancer detected, screening mammography has 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.”