Details of the study
Using the SEER registry to identify breast cancers linked to a 5% sample of Medicare beneficiaries, Richman and colleagues (funded by the National Cancer Institute and based at Yale University) conducted a retrospectivecohort study to estimate the likelihood of overdiagnosis associated with screening mammography among older women over 15 years of follow-up. Specifically, they assessed the difference in cumulative incidence of in situ and invasive breast cancer among women aged 70 years and older without a history of breast cancer when screened in 2002. During the subsequent 3 years, participants either continued screening (screened group) or did not (unscreened group). Women were followed through 2017.
Among almost 55,000 women followed, 88% were White, 6% were Black, and 3% were Hispanic. Mean follow-up was 13.7 years among women aged 70 to 74 years at baseline. For those aged 75 to 84 at baseline, mean follow-up was 10 years, and for those aged 85 years and older, mean follow-up was 5.7 years.
Estimated rates of overdiagnosis. Overall, among women aged 70 to 74 at baseline who were eventually diagnosed with breast cancer, the investigators estimated that 31% of these cancers were overdiagnosed. The corresponding percentage of breast cancers estimated to represent overdiagnosis climbed to 47% for those aged 75 to 84 years at baseline and to 54% for those aged 85 years and older at baseline.
The investigators assessed the impact of greater screening among women with a first-degree relative with a diagnosis of breast cancer and determined that this did not explain their results. With respect to cancer stage, the investigators noted that overdiagnosis was more prevalent among in situ and localized invasive cancers compared with those with regional or distant spread. Of note, the incidence of cancer with regional or distant spread was neither higher nor lower among those who were screened. Finally, the investigators did not observe significant differences in breast cancer–specific mortality by screening status.
The proportion of cancers that were overdiagnosed was particularly high among women with in situ as well as those with localized invasive disease. The investigators pointed out that as many as 90% of women aged 80 and older diagnosed with localized cancer undergo surgery, and almost two-thirds of those older than 70 years have radiation therapy for early-stage disease. In addition to the burdens associated with these treatments for overdiagnosed cancers in older women, simply being diagnosed with breast cancer profoundly affects the health and well-being of women, resulting in anxiety and substantial reductions in quality of life.
The authors also noted that some studies suggest that, among breast cancers diagnosed with screening, chemotherapy is less likely to be employed among older women, a screening benefit that must be weighed against the high likelihood of overdiagnosis. However, this benefit is unlikely to be meaningful for the majority of patients in this study who presented with in situ or early invasive lesions since chemotherapy often is not recommended for such women.
Study strengths and limitations
If screening mammography is effective, the incidence of advanced-stage tumors and breast cancer–specific mortality should be reduced in screened populations. Accordingly, in this large, long-term study using reliable sources of data, the findings that the incidence of advanced-stage disease as well as breast cancer–specific mortality were similar in the screened and unscreened cohorts provides powerful evidence that screening mammography is not effective in older women.3
As the authors pointed out, their findings regarding a high prevalence of overdiagnosis associated with screening mammography in older women are consistent with findings of other studies, some of which used different methodology.
The authors acknowledged that some women in their Medicare cohort who initially continued screening likely stopped screening subsequently, while some who initially did not continue screening might have been screened subsequently. They went on to indicate that if patients were completely adherent with subsequent screening (or not getting screened) the likelihood that cancers among screened women were overdiagnosed would be even higher.
Lead-time bias occurs when screening finds a cancer earlier than that cancer would have been diagnosed because of symptoms. This study followed the cohorts over a long timeframe to reduce the possibility that lead time was inappropriately identified as overdiagnosis. They also observed that, among women aged 85 and older, most cohort members had died by the end of study follow-up; accordingly, lead time is not likely to have explained their findings.
Limitations. The authors acknowledged that miscoding the mammogram type (screening vs diagnostic) could result in higher estimates of overdiagnosis. In their most conservative sensitivity analysis, the overdiagnosis rates could be as low as 15% for women aged 70 to 74, 36% for those aged 75 to 84, and 44% for people aged 85 and older.
Because this was an observational cohort study, unmeasured differences in breast cancer risk and underlying health factors may have been confounders. Specifically, people with severe life-threatening conditions that limited their expected life span may have chosen not to undergo regular screening. Although the authors did attempt to adjust for these factors, there may have been unrecognized confounders. This study was designed to estimate overdiagnosis, and therefore the specific benefits and harms of screening could not be addressed based on the data collected. ●
WHAT THIS EVIDENCE MEANS FOR PRACTICE
The high prevalence of overdiagnosis and lack of a breast cancer–specific mortality benefit among older women who undergo screening mammography is sobering. Clinician recommendations and shared decision making with our patients regarding screening mammography should take into consideration overdiagnosis and the considerable harms associated with overtreatment. Although we may recognize that overdiagnosed cancers are often indolent tumors with a long presymptomatic phase, in older women, even finding a biologically aggressive cancer may represent overdiagnosis if life expectancy is limited.
BARBARA LEVY, MD, MSCP; ANDREW M. KAUNITZ, MD, MSCP.