Estimating Breast Cancer Overdiagnosis After Screening Mammography Among Older Women in the United States.

BACKGROUND: Overdiagnosis is increasingly recognized as a harm of breast cancer screening, particularly for older women. OBJECTIVE: To estimate overdiagnosis associated with breast cancer screening among older women by age. DESIGN: Retrospective cohort study comparing the cumulative incidence of breast cancer among older women who continued screening in the next interval with those who did not. Analyses used competing risk models, stratified by age. SETTING: Fee-for-service Medicare claims, linked to the SEER (Surveillance, Epidemiology, and End Results) program. PATIENTS: Women 70 years and older who had been recently screened. MEASUREMENTS: Breast cancer diagnoses and breast cancer death for up to 15 years of follow-up. RESULTS: This study included 54 635 women. Among women aged 70 to 74 years, the adjusted cumulative incidence of breast cancer was 6.1 cases (95% CI, 5.7 to 6.4) per 100 screened women versus 4.2 cases (CI, 3.5 to 5.0) per 100 unscreened women. An estimated 31% of breast cancer among screened women were potentially overdiagnosed. For women aged 75 to 84 years, cumulative incidence was 4.9 (CI, 4.6 to 5.2) per 100 screened women versus 2.6 (CI, 2.2 to 3.0) per 100 unscreened women, with 47% of cases potentially overdiagnosed. For women aged 85 and older, the cumulative incidence was 2.8 (CI, 2.3 to 3.4) among screened women versus 1.3 (CI, 0.9 to 1.9) among those not, with up to 54% overdiagnosis. We did not see statistically significant reductions in breast cancer-specific death associated with screening. LIMITATIONS: This study was designed to estimate overdiagnosis, limiting our ability to draw conclusions on all benefits and harms of screening. Unmeasured differences in risk for breast cancer and differential competing mortality between screened and unscreened women may confound results. Results were sensitive to model specifications and definition of a screening mammogram. CONCLUSION: Continued breast cancer screening was associated with greater incidence of breast cancer, suggesting overdiagnosis may be common among older women who are diagnosed with breast cancer after screening. Whether harms of overdiagnosis are balanced by benefits and for whom remains an important question. PRIMARY FUNDING SOURCE: National Cancer Institute.