Breast cancer screening in Denmark was associated with a substantial increase in the incidence of nonadvanced tumors and DCIS but not with a reduction in the incidence of advanced tumors. The rate of overdiagnosis was also substantial, according to findings of a cohort study recently published in Annals of Internal Medicine (doi: 10.7326/M16-0270).
Effective breast cancer screening should detect early-stage cancer and prevent advanced disease. Overdiagnosis occurs when mammography detects small tumors that may never affect the patient's health during a lifetime. The problem with overdiagnosis is that it exposes patients to the potential harms of treatment, such as surgery, chemotherapy, and radiation, without a clinical benefit. Whether screening reduces the incidence of advanced tumors has important treatment implications.
"Effective breast cancer screening should reduce the incidence of advanced tumors," study authors wrote. "Screening mammography detects many small tumors that would not have become clinically evident in the remaining lifetime without screening (overdiagnosis). Whether screening reduces the incidence of advanced tumors has important therapeutic implications. Overdiagnosed lesions may be unnecessarily treated with surgery, chemotherapy, and radiation, which subjects women to the harms of therapy without benefit."
Using data from two comprehensive Danish cancer registries, researchers sought to examine the association of screening with a reduction in the incidence of advanced cancer and estimate the level of overdiagnosis in the country's breast screening program, which offered biennial mammography for women aged 50 - 69 years beginning in different regions at different times.
Women in Denmark who lived in areas covered by Danish breast cancer screening programs from 1991 to 2010 were compared with those who lived in areas of Denmark that did not offer mammography screening.
The authors measured the incidence of advanced (>20mm) and nonadvanced (<20 mm) breast cancer tumors in screened and unscreened women. To examine trends in overdiagnosis, the authors compared the incidence of advanced tumors in women aged 50 to 84 in screening and nonscreening areas and compared the incidence for nonadvanced tumors among women aged 35 to 49, 50 to 69, and 70 to 84 years in both screening and nonscreening areas.
They concluded that screening was not associated with lower incidence of advanced tumors and approximately 1 in 3 invasive tumors and cases of DCIS diagnosed in screened women represent overdiagnosis.
In an accompanying editorial, Otis Brawley, MD, MACP, Chief Medical Officer of the American Cancer Society said that it's time to accept that overdiagnosis is real and that the benefits of breast screening have been overstated (doi: 10.7326/M16-2850). He writes that "considering all small breast cancer lesions to be deadly aggressive cancer is the "pathology equivalent of racial profiling."
This does not mean that screening should be abandoned, but we should try to recognize its limitations, use it in the most effective way possible, and try to improve it.
"We must carefully examine screening, realize its limitations, maximize its effectiveness, and try to improve it," Brawley wrote. "In addition, we must examine all elements of breast cancer control (to include prevention) and evaluate how they are best used."
Brawley suggests that more emphasis should be focused on preventing breast cancer through diet, weight control, and exercise.