Breast cancer screening was not associated with any reduction in the incidence of advanced cancer, and overdiagnosis of invasive tumors and ductal carcinoma in situ (DCIS) is a common problem, according to a new study conducted in Denmark.
“Effective breast cancer screening should reduce the incidence of advanced tumors,” wrote study authors led by Karsten Juhl Jørgensen, MD, of the Nordic Cochrane Centre in Copenhagen. “Screening mammography detects many small tumors that would not have become clinically evident in the remaining lifetime without screening.”
Denmark provides an ideal setting for testing overdiagnosis and screening efficacy, because only 20% of the population aged 50 to 69 years was invited to participate in a mammography screening program over a 17-year period. The researchers compared breast cancer rates and estimated overdiagnosis in this large population; the results were published in Annals of Oncology.
Overall, they found that the incidence of nonadvanced tumors increased in screening periods compared with prescreening periods, with an incidence rate ratio of 1.49 (95% CI, 1.43–1.54). Instituting screening was not, however, associated with any decrease in the incidence of advanced tumors.
They estimated that in 2010, 271 invasive breast tumors and 180 DCIS cases were overdiagnosed each year. When DCIS was included, they found an overdiagnosis rate of 24.4%, compared with the incidence observed in 50- to 69-year-old women in non–screening areas of the country; without DCIS, the rate was 14.7%. In other words, approximately one in every five women in that age group diagnosed with breast cancer was overdiagnosed in areas where screening was used.
A secondary estimation approach that accounted for regional differences in women younger than screening age found an overdiagnosis rate of 48.3% including DCIS, and 38.6% excluding DCIS.
“Seventeen years of organized breast screening in Denmark has not measurably reduced the incidence of advanced tumors, but has markedly increased the incidence of nonadvanced tumors and DCIS,” the authors wrote. “These findings support that screening has not accomplished the promise of a reduction in invasive therapy or disease-specific mortality.”
In an accompanying editorial, Otis W. Brawley, MD, of the American Cancer Society in Atlanta, wrote that “acknowledging the existence of breast cancer overdiagnosis challenges the value of screening: it means that the benefits of breast screening have been overstated, and that some women who have been ‘cured’ were harmed because they received unnecessary treatment.” He noted that routine mammography should be advocated for women at significant risk, and that in the future it may become easier to separate those at very low or very high risk in order to stratify for screening purposes.