Breast cancer screening: Efficacy and harms
The earliest studies of breast cancer screening with mammography were randomized controlled trials (RCTs) that compared screened and unscreened patients aged 40 to 74. Nearly all the RCTs and numerous well-designed incidence-based and case-control studies have demonstrated that SM results in a clinically and statistically significant reduction in breast cancer mortality (TABLE 4).4,6,8 Since the mid-1980s and continuing to the current day, SM programs are routinely recommended in the United States. In addition to the mortality benefit outlined in TABLE 4, SM also is associated with a need for less invasive treatments if breast cancer is diagnosed.9,10
With several decades of experience, SM programs have demonstrated that multiple harms are associated with SM, including callbacks, false-positive mammograms that result in a benign biopsy, and overdiagnosis of breast cancer (TABLE 4). Overdiagnosis is a mammographic detection of a breast cancer that would not have harmed that woman in her lifetime. Overdiagnosis leads to overtreatment of breast cancers with its attendant side effects, the emotional harms of a breast cancer diagnosis, and the substantial financial cost of cancer treatment. Estimates of overdiagnosis range from 0% to 50%, with the most likely estimate of invasive breast cancer overdiagnosis from SM between 5% and 15%.11-13 Some of these overdiagnosed cancers are due to very slow growing cancers or breast cancers that may even regress. However, the higher rates of overdiagnosis occur in older persons who are screened and in whom competing causes of mortality become more prevalent. It is estimated that overdiagnosis of invasive breast cancer in patients younger than age 60 is less than 1%, but it exceeds 14% in those older than age 80 (TABLE 4).14
A structured approach is needed to counsel patients about SM so that they understand both the substantial benefit (earlier-stage diagnosis, reduced need for treatment, reduced breast cancer and all-cause mortality) and the potential harms (callback, false-positive results, and overdiagnosis). Moreover, the relative balance of the benefits and harms are influenced throughout their lifetime by both aging and changes in their personal and family medical history.
Counseling should consider factors beyond just the performance of mammography (sensitivity and specificity), such as the patient’s current health and age (competing causes of mortality), likelihood of developing breast cancer based on risk assessment (more benefit in higher-risk persons), and the individual patient’s values on the importance of the benefits and harms. The differing emphases on mammography performance and the relative value of the benefits and harms have led experts to produce disparate national guideline recommendations (TABLE 5).
Should SM start at age 40, 45, or 50 in average-risk persons?
There is not clear consensus about the age at which to begin to recommend routine SM in patients at average risk. The National Comprehensive Cancer Network (NCCN),7 American Cancer Society (ACS),4 and the US Preventive Services Task Force (USPSTF)5 recommend that those at average risk start SM at age 40, 45, and 50, respectively (TABLE 5). While the guideline groups listed in TABLE 5 agree that there is level 1 evidence that SM reduces breast cancer mortality in the general population for persons starting at age 40, because the incidence of breast cancer is lower in younger persons (TABLE 6),4 the net population-based screening benefit is lower in this group, and the number needed to invite to screening to save a single life due to breast cancer varies.
For patients in their 40s, it is estimated that 1,904 individuals need to be invited to SM to save 1 life, whereas for patients in their 50s, it is 1,339.15 However, for patients in their 40s, the number needed to screen to save 1 life due to breast cancer decreases from 1 in 1,904 if invited to be screened to 1 in 588 if they are actually screened.16 Furthermore, if a patient is diagnosed with breast cancer at age 40–50, the likelihood of dying is reduced at least 22% and perhaps as high as 48% if her cancer was diagnosed on SM compared with an unscreened individual with a symptomatic presentation (for example, palpable mass).4,15,17,18 Another benefit of SM in the fifth decade of life (40s) is the decreased need for more extensive treatment, including a higher risk of need for chemotherapy (odds ratio [OR], 2.81; 95% confidence interval [CI], 1.16–6.84); need for mastectomy (OR, 3.41; 95% CI, 1.36–8.52); and need for axillary lymph node dissection (OR, 5.76; 95% CI, 2.40–13.82) in unscreened (compared with screened) patients diagnosed with breast cancer.10
The harms associated with SM are not inconsequential and include callbacks (approximately 1 in 10), false-positive biopsy (approximately 1 in 100), and overdiagnosis (likely <1% of all breast cancers in persons younger than age 50). Because most patients in their 40s will not develop breast cancer (TABLE 6), the benefit of reduced breast cancer mortality will not be experienced by most in this decade of life, but they are still just as likely to experience a callback, false-positive biopsy, or the possibility of overdiagnosis. Interpretation of this balance on a population level is the crux of the various guideline groups’ development of differing recommendations as to when screening should start. Despite this seeming disagreement, all the guideline groups listed in TABLE 5 concur that persons at average risk for breast cancer should be offered SM if they desire starting at age 40 after a shared decision-making conversation that incorporates the patient’s view on the relative value of the benefits and risks.
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