Low-certainty evidence indicates that screening for breast cancer with mammography results in a modest reduction in breast cancer mortality for women aged 40 to 74 years; the absolute benefit is lowest for women younger than 50 years.
Screening may lead to overdiagnosis, resulting in unnecessary treatment of cancer that would not have caused harm in a woman’s lifetime and false-positive results that can lead to both physical and psychological consequences; overdiagnosis and false-positives with related biopsies are more common in younger women.
The balance of benefits and harms from screening is less favourable for women aged 40 to 49 years than for older women.
Evidence on women’s values and preferences about screening suggests that a substantial proportion of women aged 40 to 49 years would not choose to be screened if they were aware of the outcomes for their age group, but more older women would choose screening, given the more favourable balance of benefits and harms.
In light of low-certainty evidence for benefits and harms from breast cancer screening, as well as variability in patient preferences, women should be supported to make an informed choice on screening that is congruent with their own values and preferences; women aged 50 to 74 years should be engaged in shared decision-making with their health care providers.
Breast cancer mortality rates among Canadian women have declined from 41.7 per 100 000 in 1988 to an estimated 23.2 per 100 000 in 2017, while age-standardized incidence has remained relatively stable, at around 130 per 100 000 since 2004.1 Declining mortality with stable incidence could reflect improvements in breast cancer treatment, timely detection of symptomatic cancer, screening programs, or all of these.1 Breast cancer screening programs have been in place in most regions of Canada since the early 1990s. In 2014, 54% of women aged 50 to 69 years had been screened in the previous 30 months via a screening program, and an unknown number of women were screened outside of programs.2 Screening may identify breast cancer earlier and lead to more effective and less invasive treatment; however, it may also lead to overdiagnosis and subsequent treatment of cancer that, left untreated, would not have become apparent or caused harm.3 Examples of adverse sequelae of treatments (e.g., surgical intervention, radiotherapy, chemotherapy) include pain, disfigurement and distress. In addition, false-positive screening results may have a psychological impact and can lead to adverse physical effects from further testing.3
The systematic review4 that informed task force recommendations in 20115 reported a reduction in breast cancer mortality with mammography screening for women aged 40 to 74 years. However, net benefit for women younger than 50 years was equivocal, given their lower absolute risk as well as their higher probability of being overdiagnosed and having false-positive screens compared with women aged 50 to 74 years.4 Current recommendations are informed by an updated evidence review of benefits and harms of breast cancer screening, as well as a new systematic review of women’s values and preferences about screening.
Scope
This guideline updates the task force’s previous recommendations for primary care providers on breast cancer screening for women aged 40 to 74 years who are not at increased risk of breast cancer.5 Those considered to be at increased risk include women with a personal or family history of breast cancer; women who are carriers of gene mutations such as BRCA1 or BRCA2, or who have a first-degree relative with these gene mutations; and women who had chest radiation therapy before age 30 years or within the past 8 years.6–13
Methods
The task force is an independent panel of clinicians and methodologists that makes recommendations on primary and secondary prevention in primary care (www.canadiantaskforce.ca). The current recommendations were developed by a working group of 6 task force members, with scientific support from Public Health Agency of Canada staff.14
Recommendations are based on 2 evidence reviews: the first on outcomes of breast cancer screening for women aged 40 years and older who are not at increased risk for breast cancer,15 and a second on women’s values and preferences about screening.16 Review protocols were registered on PROSPERO (CRD42017051498; CRD42017058476) and are available at www.canadiantaskforce.ca. The analytic framework is provided in Appendix 1 (available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.180463/-/DC2).
The evidence review on screening outcomes was an overview of reviews with an updated search for new evidence, conducted by the Evidence Review and Synthesis Centre at the Ottawa Hospital Research Institute.15 Outcomes included all-cause and breast cancer mortality, overdiagnosis and false-positive results with ensuing biopsies. Eighteen systematic reviews were considered for inclusion and assessed for quality, completeness of reporting and fit for purpose, according to eligibility and methodological criteria. Systematic reviews produced for guidelines by the United States Preventive Services Task Force,17 American Cancer Society18 and Canadian Task Force on Preventive Health Care4 were selected using these criteria. An initial search for additional primary studies published more recently was conducted using MEDLINE and the Cochrane Library from October 2014 to January 2017 for all screening modalities except breast self-examination, which was searched from October 2010, when the last review4 was conducted for the task force. Grey literature was also searched to January 2017.15
The University of Alberta’s Evidence Review and Synthesis Centre conducted a systematic review of women’s values and preferences on outcomes of breast cancer screening and how these valuations are used in decision-making.16 Studies of any design published from January 2000 to November 2016 in which women were asked about the relative importance of expected benefits and harms from breast cancer screening were identified from an initial search of MEDLINE, the Cochrane Library, CINAHL, PsycINFO and from the grey literature.16
The reviews15,16 and draft guideline were reviewed by content experts and stakeholders. The searches for both reviews were updated to December 2017 using the same keywords and databases, with evidence updated as required. A prepublication search for trial evidence on outcomes of screening was conducted in October 2018.
The Knowledge Translation Program at St. Michael’s Hospital (Toronto, Ont.) developed knowledge translation tools that accompany this guideline, for clinicians and patients; these are published on the task force website (www.canadiantaskforce.ca).
The task force used the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach to determine the quality or certainty of evidence and strength of recommendations (Box 1).19–21 Appendix 2 (available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.180463/-/DC2) provides the evidence-to-decision table for the recommendations.
Grading of recommendations
Recommendations are graded according to the Grading of Recommendations Assessment, Development and Evaluation system (GRADE).19 Whether a recommendation is strong or conditional* is based on considerations such as certainty in the effects of an intervention, including magnitude, as well as estimates of how patients value and prioritize outcomes, variability of these estimates and wise use of resources.
Strong recommendations
Strong recommendations are those for which the task force is confident that the desirable effects of an intervention outweigh its undesirable effects (strong recommendation for an intervention) or that the undesirable effects of an intervention outweigh its desirable effects (strong recommendation against an intervention). A strong recommendation implies that most people will be best served by the recommended course of action.
Strong recommendations are typically based on high-certainty evidence (i.e., high confidence in the estimate of the effect of an intervention). Strong recommendations may recommend in favour of an intervention (when there is high confidence of net benefit) or against an intervention (when there is high confidence of net harm). However, there are circumstances in which a strong recommendation may be considered based on low- or very low-certainty evidence or when there is absence of evidence.20
When there is an absence of evidence to provide confidence that there is benefit from implementing a new prevention service or when a conclusion of possible benefit requires a high level of speculation on linkages of uncertain evidence, but there is high certainty that some patients would be harmed or scarce health care resources expended, the task force may make a strong recommendation against service implementation. This is consistent with the GRADE approach, in which strong recommendations are sometimes made with low-certainty evidence combined with high certainty of harm or resource implications, and with the value that the task force places on using scarce primary care resources wisely.
Conditional recommendations
Conditional recommendations are those for which the desirable effects probably outweigh the undesirable effects (conditional recommendation in favour of an intervention) or undesirable effects probably outweigh the desirable effects (conditional recommendation against an intervention) but appreciable uncertainty exists. Conditional recommendations are made when the certainty of evidence is lower, when the margin between desirable and undesirable consequences is small and the balance depends on patient values and preferences, or when there is high variability in the values and preferences of patients. Conditional recommendations may also be applied when the balance of cost and benefits is ambiguous, key stakeholders differ about the acceptability or feasibility of the implementation, or the effects on health equity are unclear.
In certain cases where a conditional recommendation for an intervention is made, clinicians are encouraged to engage in shared decision-making, to recognize that different choices will be appropriate for individual patients, and to help each person arrive at a management decision consistent with their values and preferences.
Evidence is graded as high-, moderate-, low- or very low-certainty, based on how likely further research is to change the task force’s confidence in the estimate of effect.
↵* The task force previously used the term “weak recommendation,” but has replaced this with the term “conditional recommendation,” to improve understanding and facilitate implementation of guidance, based on feedback from clinician knowledge users. One reason for this change was the value that the task force places on shared decision-making, together with a need to clarify better when implementation of a recommendation depends on circumstances such as patient values, resource availability or other contextual considerations. Conditional recommendations based on patient values and preferences require clinicians to recognize that different choices will be appropriate for different patients and those decisions must be consistent with each patient’s values and preferences. Knowledge translation tools are available on the task force website (www.canadiantaskforce.ca) to facilitate decisions that are evidence informed and aligned with an individual’s priorities.
Management of competing interests
Funding for the Canadian Task Force on Preventive Health Care is provided by the Public Health Agency of Canada. The views of the funding body have not influenced the content of the guideline. All task force members are required to disclose financial and nonfinancial competing interests. These statements are made publicly available on the task force website. All task force members declared that they had no competing interests for this guideline.
Recommendations
These recommendations apply to women aged 40 to 74 years who are not at increased risk of breast cancer. See Box 2 for a summary of recommendations.
Recommendations
Recommendations apply to women aged 40 to 74 years who are not at increased risk of breast cancer.
Mammography
Screening women aged 40 to 49 years: For women aged 40 to 49 years, we recommend not screening with mammography; the decision to undergo screening is conditional on the relative value a woman places on possible benefits and harms from screening (conditional recommendation; low-certainty evidence).
Screening women aged 50 to 69 years: For women aged 50 to 69 years, we recommend screening with mammography every 2 to 3 years; the decision to undergo screening is conditional on the relative value that a woman places on possible benefits and harms from screening (conditional recommendation; very low-certainty evidence).
Screening women aged 70 to 74 years: For women aged 70 to 74 years, we recommend screening with mammography every 2 to 3 years; the decision to undergo screening is conditional on the relative value that a woman places on possible benefits and harms from screening (conditional recommendation; very low-certainty evidence).
Other screening modalities
We recommend not using magnetic resonance imaging, tomosynthesis or ultrasound to screen for breast cancer in women who are not at increased risk (strong recommendation; no evidence).
We recommend not performing clinical breast examinations to screen for breast cancer (conditional recommendation; no evidence).
We recommend not advising women to practise breast self-examination to screen for breast cancer (conditional recommendation; low-certainty evidence).
Screening using mammography
The overview of reviews identified 8 randomized controlled trials (RCTs) or quasi-RCTs (Appendix 3, available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.180463/-/DC2)22–30 with information on benefits of breast cancer screening using mammography with or without clinical breast examination.15 Trials were begun between 1963 and 1991 in Sweden, Canada, the United States and the United Kingdom. The number of women randomized ranged from 18 000 to 160 000, with mean follow-up from 18 to 30 years and screening intervals between 12 and 33 months. Duration of the screening period was from 3 to 12 years (median 7 yr) with participation rates of 65% to 88%.15
Low-certainty evidence from the 8 included trials found no statistically significant difference in overall all-cause mortality from screening using mammography, with 0.69 fewer deaths per 1000 women of all ages at moderate baseline risk (95% confidence interval [CI] 0.00 to 1.38 fewer).15 Baseline risk refers to rates of breast cancer mortality among women in the control group (not targeted for screening) with corresponding low (0.2%), moderate (0.4%) and high (0.6%) risk of breast cancer mortality, based on findings in the trials.15 Results for women aged 50 to 59 years are presented using the moderate baseline risk groups.
There were no statistically significant differences in relative risk of breast cancer mortality with screening between age subgroups (I2 = 0%, p = 0.44).15 Thus, the all-ages relative risk has been applied to each of the age subgroups to calculate corresponding risk of breast cancer mortality31,32 (Table 1) (for calculations, see Appendix 4, available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.180463/-/DC2).
In relation to harms of breast cancer screening using mammography, 3 trials38–40 and 6 cohort studies41–46 were identified that reported on overdiagnosis. However, only the Canadian RCT39 estimated rates of overdiagnosis by age in a trial judged to be at moderate risk of bias; these findings are presented in Table 2.15 Risk of bias may include such issues as failure to conceal allocation to group, failure to blind, loss to follow-up and not considering intention-to-treat, among others.
Rates of false-positive screens with resulting biopsies were calculated using data from the Canadian Partnership Against Cancer’s2 2011/12 cycle report.15,47 Table 3 presents estimated rates of false-positive screens and resulting biopsies for a cohort of 1000 women screened over 7 years (the median duration of screening in included trials), as well as the number of women to be screened to prevent a death from breast cancer.
No evidence was identified on the benefits and harms of screening women aged 75 years and older.
Women aged 40 to 49 years
For women aged 40 to 49 years, we recommend not screening with mammography; the decision to undergo screening is conditional on the relative value a woman places on possible benefits and harms from screening (conditional recommendation; low-certainty evidence).
As shown in Table 1, there is low-certainty evidence that the absolute benefit of screening on breast cancer mortality for women aged 40 to 49 years is 0.58 fewer deaths per 1000 women screened for a median of 7 years (95% CI 0.27 to 0.85 fewer). The number needed to screen (NNS) to prevent a death from breast cancer is 1724 (95% CI 1176 to 3704). Among women aged 40 to 49 years, 5 years after screening, 41% of identified invasive and in situ cancers are estimated to be overdiagnosed, and 32% of identified invasive cancers are estimated to be overdiagnosed (Table 2). Among 1000 women screened over 7 years, 294 will receive a false-positive result and 43 will undergo a biopsy (Table 3).
Women aged 50 to 69 years
For women aged 50 to 69 years, we recommend screening with mammography every 2 to 3 years; the decision to undergo screening is conditional on the relative value that a woman places on possible benefits and harms from screening (conditional recommendation; very low-certainty evidence).
Table 1 reports very low-certainty evidence that the absolute benefit of screening on breast cancer mortality for women aged 50 to 59 years is 0.75 fewer deaths per 1000 women who have been screened for a median of 7 years (95% CI 0.35 to 1.10 fewer). The NNS to prevent a death from breast cancer for women aged 50 to 59 years is 1333 (95% CI 909 to 2857). Five years after screening, 25% of identified invasive and in situ cancers are estimated to be overdiagnosed among women aged 50 to 59 years, and 16% of identified invasive cancers are estimated to be overdiagnosed (Table 2). Among 1000 women aged 50 to 59 years screened over 7 years, 294 will receive a false-positive result and 37 will undergo a biopsy (Table 3).
As indicated in Table 1, there is low-certainty evidence that for women aged 60 to 69 years, the absolute benefit of screening on mortality is 0.92 fewer deaths per 1000 women screened over 7 years (95% CI 0.43 to 1.35 fewer). The NNS to prevent a death from breast cancer for women aged 60 to 69 years is 1087 (95% CI 741 to 2326). No information is available on rates of overdiagnosis for women of this age. Among 1000 women aged 60 to 69 years screened over 7 years, 256 will receive a false-positive result and 35 will undergo a biopsy (Table 3).
Women aged 70 to 74 years
For women aged 70 to 74 years, we recommend screening with mammography every 2 to 3 years; the decision to undergo screening is conditional on the relative value that a woman places on possible benefits and harms from screening (conditional recommendation; very low-certainty evidence).
Table 1 shows very low-certainty evidence that the absolute benefit of screening on breast cancer mortality for women aged 70 to 74 years is 1.55 fewer deaths per 1000 women screened for a median of 7 years (95% CI 0.72 to 2.27 fewer). The NNS to prevent a death from breast cancer in women aged 70 to 74 years is 645 (95% CI 441 to 1389). No information is available on rates of overdiagnosis for women of this age. Among 1000 women screened over 7 years, 219 will receive a false-positive result and 30 of them will undergo a biopsy (Table 3).
Screening using modalities other than mammography
We recommend not using magnetic resonance imaging (MRI), tomosynthesis or ultrasound to screen for breast cancer in women who are not at increased risk (strong recommendation; no evidence).
No evidence was identified on the effect of breast cancer screening using MRI, tomosynthesis or ultrasound on outcomes important to patients.15 The recommendation is strong because these modalities would require the use of substantial and scarce health care resources when used for screening without evidence of benefit from their use.
We recommend not performing clinical breast examinations to screen for breast cancer (conditional recommendation; no evidence).
No evidence was identified on effectiveness of breast cancer screening using clinical breast examination.15
We recommend not advising women to practise breast self-examination to screen for breast cancer (conditional recommendation; low-certainty evidence).
The evidence review15 identified 2 cluster randomized trials48,49 comparing breast self-examination to usual care. Low-certainty evidence from these trials did not show that breast self-examination as a screening modality reduced all-cause mortality (1.79 fewer deaths per 1000 women screened; 95% CI 5.36 more to 8.48 fewer).15 One of these trials49 also reported low-certainty evidence that self-examination likely does not reduce breast cancer mortality (0.03 more deaths per 1000 women screened: 95% CI 0.19 fewer to 0.31 more).15
Values and preferences
Twenty-nine studies assessed the relative importance women placed on anticipated benefits and harms from breast cancer screening and how these valuations may have influenced screening decisions.16 The studies, most published after 2010 (range 2000–2017), were conducted in 11 different countries (1 in Canada) and had sample sizes from 6 to 16 000. They included 5 qualitative studies,50–54 9 RCTs,55–63 1 single-arm trial,64 8 cross-sectional surveys,65–72 3 uncontrolled pre–post studies,73–75 2 stated preference studies76,77 and a single deliberative jury.78 Studies varied widely in how information on benefits and harms was presented but tended to provide high benefit-to-harm ratios.16
There were 13 studies that directly addressed how women weigh benefits and harms (Appendix 5, available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.180463/-/DC2) and 16 that used screening attendance or intention to be screened to imply relative weights of outcomes based on data presented in decision aids or by other means (Appendix 6, available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.180463/-/DC2). Studies in both sets provided information on how other factors influenced outcome valuations and screening decisions.16
Of women aged 40 to 49 years, a substantial proportion appear to reduce or reverse their initial perceptions of a high benefit-to-harm ratio for breast cancer screening when provided with accurate estimates on the absolute benefits and harms for their age group.16
For most women aged 50–69 years, reduction in breast cancer mortality from screening appears to outweigh concerns about potential harms. When provided with a brief description of overdiagnosis, women showed acceptance of relatively high rates of overdiagnosis with screening. On the other hand, when more detailed information on overdiagnosis was provided, some women said that screening was no longer attractive. Those who had been screened regularly maintained high intentions to continue to be screened regardless of how benefit-to-harm ratios were presented.16
For healthy women in their 70s, it appears that acceptance of screening may be quite high, perhaps because they have been screened for many years.16
For some women of all ages, other outcomes, including consequences of a false-negative screen, reassurance and the possibility of milder treatment courses, held importance. Beliefs about the severity and harms of breast cancer screening, such as viewing overdiagnosis as a consequence of treatment rather than screening, also influenced decisions about screening. In general, studies found that most women, regardless of age, value the opportunity to make informed decisions about screening that are consistent with their values and preferences.16
Resource use
A systematic review on cost-effectiveness of breast cancer screening was not conducted. Estimates from Statistics Canada (2015) on resources used by screening programs for breast cancer suggest that costs for screening alone vary by screening interval and age of women at initiation of screening.79
Feasibility, acceptability, cost and equity
Screening programs for breast cancer have been in place across Canada since the early 1990s.1 In the judgment of the task force, current recommendations are both feasible and acceptable to women and clinicians and are not expected to have an increased negative effect on health equity or to pose additional costs to the health care system.
Rationale
The task force judged that no change was required in the direction of its previous recommendations on breast cancer screening with mammography, although the certainty of evidence (previously referred to as “quality of evidence”)21 was downgraded upon reappraisal because of serious concerns regarding risk of bias in the original studies.15 As mentioned previously, no evidence was identified on the benefits and harms of screening women aged 75 years and older.
The absolute benefit of breast cancer screening with mammography remains lowest for younger women, with the balance of benefits and harms becoming more favourable with increasing age. Women aged 40 to 74 years should be supported to make an informed choice on breast cancer screening that is congruent with their own values and preferences. Knowledge translation tools are provided on the task force website to support this process.
Recommendations on screening mammography are conditional because of very low- to low-certainty evidence, uncertain balance of benefits and harms and variation in women’s values and preferences, underlining the need for shared decision-making between women and their health care providers. Shared decision-making supports evidence-based and values-informed screening decisions.80–82
An earlier task force recommendation against MRI was updated to include tomosynthesis and ultrasound. As previously with MRI, no evidence was identified that assessed effectiveness of these modalities for screening purposes. No additional evidence was identified to inform previous recommendations on breast self-examination or clinical breast examination.
Recommendations for clinical and self-breast examination are conditional against screening based on the absence of evidence of benefit and the potential risk of harm to some women. There is a lack of evidence on the effectiveness of other screening modalities, and as their use in screening would consume scarce resources, the task force made a strong recommendation against their use.
Women aged 40 to 49 years
There is low-certainty evidence that women aged 40 to 49 years who are not at increased risk have a modest absolute reduction in breast cancer mortality from screening. Compared with women aged 50 years and older, they have higher rates of overdiagnosis with resulting harms from treatment for a cancer that would not have caused harm in their lifetime, and they are more likely to see consequences from false-positive screens, including biopsies.15 Evidence on women’s preferences suggests that a substantial proportion of women would not choose to be screened if they knew their age group’s absolute risk reduction for breast cancer mortality and increased potential for harm from screening.16
After balancing the overall benefits and harms of screening, and considering the values and preferences of these women, in the judgment of the task force, the undesirable effects of overdiagnosis and consequences of false-positive results outweigh potential benefits; therefore, the recommendation is against screening women of this age. This recommendation is conditional, as some women in this age group may wish to be screened, based on their values and preferences; in this circumstance, care providers should engage in shared decision-making with women who express an interest in being screened.
Women aged 50 to 69 years
There is very low-certainty evidence that women aged 50 to 69 years who are not at increased risk have a modest absolute reduction in breast cancer mortality from screening.15 Rates of overdiagnosis and consequences of false-positive screening results, although lower than for younger women, remain a concern. Women in this age group generally weigh even modest reductions in breast cancer mortality as more important than harms in their decision to be screened.16
In balancing the overall benefits and harms of screening for women aged 50 to 69 years, and considering their values and preferences, the task force places greater weight on women’s preferences for screening to reduce breast cancer mortality by a modest amount than on harms, and therefore recommends in favour of screening using mammography every 2 to 3 years. Care providers should engage women of this age in shared decision-making because those who place a higher value on avoiding harms, compared with a modest absolute reduction in breast cancer mortality, may choose to not undergo screening.
Women aged 70 to 74 years
There is very low-certainty evidence of a modest absolute reduction in breast cancer mortality for women aged 70 to 74 years who are not at increased risk, and the consequences of false-positive screening results remain a concern. No evidence was identified on the risk of overdiagnosis for these women.15 Healthy women of this age generally accept screening, perhaps owing to familiarity.16
In balancing the overall benefits and harms, the task force places relatively more weight on women’s acceptance of screening programs in the context of a modest absolute reduction in breast cancer mortality, and lower weight on the risk of harms, and thus recommends in favour of screening these women using mammography every 2 to 3 years. Care providers should engage in shared decision-making with women of this age, because those who place a higher value on avoiding harms, compared with a modest absolute reduction in breast cancer mortality, may choose to not undergo screening.
Considerations for implementation
The recommendations focus on breast cancer screening using mammography because of the absence of evidence on patient-important outcomes of screening with other modalities and evidence that performing self-breast examination for screening has no impact on breast cancer mortality.15 The recommendation is to screen women every 2 to 3 years, because screening intervals in the trials ranged from 12 to 33 months with a pooled analysis indicating similar benefits across intervals.4
There is some evidence that women with dense breast tissue are more likely to develop breast cancer than women without dense breasts.83–85 Increased breast density is more common among women aged 40 to 49 years than in those aged 50 years and older.86 However, classification of breast density status has poor reliability, and density status is reclassified on subsequent examination for 1 in 5 women by the same radiologist and for 1 in 3 by a different radiologist.87 Further, supplemental screening may increase the number of women experiencing overdiagnosis with unnecessary treatment and negative effects from false-positive results.87 A recent review conducted for the US Preventive Services Task Force did not identify evidence on patient-important outcomes from supplemental screening of women with dense breast tissue following a negative mammogram, and as such, its benefit is unknown.87
Performance measures for implementation
The task force suggests that quality assurance indicators for screening programs for breast cancer continue to be monitored through the Canadian Partnership Against Cancer, including false-positive rates and resulting biopsies, as well as stage of cancer at diagnosis. The recommendations on screening for breast cancer are conditional on women’s values and preferences, which means that not all women aged 50 to 74 years should be screened. Quality improvement programs should focus on ensuring that decision-making is shared between women aged 50 to 74 years and their health care providers, as the ideal proportion of women to be screened is unknown. Strategies that promote increasing the proportion of women screened (such as through financial inducements) instead of the shared decision-making process are not aligned with these recommendations.
Other guidelines
Current task force recommendations for breast cancer screening are consistent with previous task force guidance and with major national guideline developers. As indicated in Table 4, some stakeholders specializing in cancer care have differing recommendations for screening.
Gaps in knowledge
More and better-quality evidence is needed on the impact of breast cancer screening for women of all ages, and particularly for women younger than 50 years and older than 70. These populations are currently the focus of the AgeX Trial, a cluster randomized trial underway in the United Kingdom with results expected in 2026 (ClinicalTrial.gov, no. NCT01081288).
Although the task force did not specifically review evidence on supplemental screening for women with dense breast tissue, a recent review for the US Preventive Services Task Force concluded that studies are needed to clarify approaches to classification and called for rigorous comparative studies to determine patient-important outcomes of supplemental screening.87
Hanley and colleagues have proposed alternative approaches to interpreting screening outcomes because of diminishing returns from screening after its cessation. These authors recommend calculating breast cancer mortality rates by year of screening rather than estimating cumulative mortality.92
Greater understanding of the risk of overdiagnosis from screening requires a common definition and agreed-upon denominator, as well as more rigorous study, particularly for women aged 60 to 74 years.3 Additional studies on values and preferences of Canadian women regarding screening, conducted using accurate estimates of both benefits and harms, would assist in guiding future recommendations. Finally, better estimates of the costs of breast cancer screening would also support guidance on screening.
Conclusion
Based on 2 evidence reviews, the task force has determined that recommendations for screening women for breast cancer should remain similar to those in the 2011 task force guideline.5 Although breast cancer screening has the potential to reduce breast cancer mortality, it increases risk of harms, particularly because of the risk of overdiagnosis, leading to unnecessary treatment and consequent adverse sequelae. Women should be supported to make an informed decision on screening that fits with their values and preferences. Future challenges include reducing uncertainty in estimates of benefits and harms from screening and gaining a greater understanding of values and preferences of Canadian women about breast cancer screening.
Acknowledgements
The authors would like to thank the Ottawa Hospital Research Institute and University of Alberta Evidence Review and Synthesis Centre teams (Pauline Barbeau, Adrienne Stevens, Andrew Beck, Becky Skidmore, Angel Arnaout, Muriel Brackstone, Alexandra Ginty, Amanda Hey, Brian Hutton, Beverley Shea, David Moher and Julian Little of the Ottawa Hospital Research Institute, and Jennifer Pillay, Tara MacGregor, Lisa Hartling and Robin Featherstone of the University of Alberta) for the evidence reviews that supported this guideline. The authors also thank the Knowledge Translation Program, Li Ka Shing Knowledge Institute, St. Michael’s Hospital for their contributions to patient engagement and knowledge translation work related to this guideline. Additionally, the authors thank peer reviewers and organizational stakeholders who provided feedback on the draft guideline.
Footnotes
This guideline is available in French at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.180463/-/DC1
CMAJ Podcasts: interview in English at https://soundcloud.com/cmajpodcasts/180463-guide-eng; entrevue en français au https://soundcloud.com/cmajpodcasts/180463-guide-fre
Competing interests: None declared.
This article has been peer reviewed.
Guideline Writing Group: Scott Klarenbach (voting Task Force member), Nicki Sims-Jones (non-voting member of Public Health Agency of Canada [PHAC] scientific support team), Gabriela Lewin (voting Task Force member), Harminder Singh (voting Task Force member), Guylène Thériault (voting Task Force member), Marcello Tonelli (voting Task Force member), Marion Doull (non-voting member of PHAC scientific support team), Susan Courage (nonvoting member of PHAC scientific support team), Alejandra Jaramillo Garcia (non-voting member of PHAC scientific support team), Brett D. Thombs (voting Task Force member).
Contributors: Scott Klarenbach, Nicki Sims-Jones, Gabriela Lewin, Harminder Singh, Guylène Thériault, Marcello Tonelli, Marion Doull, Susan Courage, Alejandra Jaramillo Garcia and Brett Thombs contributed substantially to the study concept and design, interpretation of the evidence and critical revision of the guideline draft. Nicki Sims-Jones, Susan Courage, Marion Doull and Alejandra Jaramillo Garcia provided scientific and logistical support to the writing group. Scott Klarenbach, Heather Colquhoun, James Dickinson, Roland Grad, Stéphane Groulx, Gabriela Lewin, Eddy Lang, John Leblanc, Ainsley Moore, Donna Reynolds, Harminder Singh, Guylène Thériault, Brett Thombs, Marcello Tonelli and Brenda Wilson drafted the recommendations. Scott Klarenbach, Nicki Sims-Jones, Susan Courage, Marion Doull and Alejandra Jaramillo Garcia drafted the guideline statement. All of the named authors gave final approval of the version of the guideline to be published and agreed to be accountable for all aspects of the work. All collaborating members of the Canadian Task Force on Preventive Health Care also made contributions to the development of the guideline and all gave final approval of the version of the guideline to be published.
Funding: Funding for the Canadian Task Force on Preventive Health Care is provided by the Public Health Agency of Canada. The views of the funding body have not influenced the content of the guideline. The views expressed in this article are those of the Task Force and do not necessarily represent those of the Public Health Agency of Canada.
Collaborating members of the Canadian Task Force for Preventive Health Care: Heather Colquhoun, James Dickinson, Roland Grad, Stéphane Groulx, Eddy Lang, John Leblanc, Ainsley Moore, Donna Reynolds, Brenda Wilson. The complete list of all current members of the Canadian Task Force on Preventive Health Care is available at http://canadiantaskforce.ca/about/members.
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