Breast cancer screening: understanding the benefits and harms

Breast cancer is a terrible disease and we all want to find ways to lessen its burden and improve outcomes. That’s why the family physicians, nurse practitioners, researchers and specialists on the Canadian Task Force on Preventive Health Care (Task Force) conducted a comprehensive review of evidence on breast cancer screening. This review included more than 155 studies, with recent observational trials, randomized controlled trials and modelling informing the draft breast cancer screening recommendations released May 30, 3024.

The key message is that breast cancer screening is a personal choice, and that women need the full picture so that they can decide what is right for them. 

Evidence shows benefits and harms

What we found was surprising. The evidence did not show a clear benefit of screening for breast cancer in women, especially those aged 40 to 49, and it also showed there are harms. Most people know that there are potential benefits, but many are not aware of the size of these benefits and how these balance with harms. For example, the evidence review, using all the study data, showed that screening people aged 40-49 over 10 years resulted in (per 1,000):

• 1 or fewer deaths avoided from breast cancer
• 368 people having additional tests (formerly called false positives) of whom 55 would undergo biopsies
• 2 people would be diagnosed and treated for breast cancer that would not have caused problems (overdiagnosis) 

For people aged 50-74, the potential benefits tended to increase in older age groups while the harms tended to decrease. For example, the evidence review showed that screening people aged 60-69 over 10 years resulted in (per 1,000):

• 2 or fewer deaths avoided from breast cancer
• 257 people having additional tests (formerly called false positives) of whom 33 would undergo biopsies
• 1-2 people would be diagnosed and treated for breast cancer that would not have caused problems (called overdiagnosis) 

The Task Force also reviewed 82 studies on women’s values and preferences on screening. When given a scenario with similar numbers, the evidence showed that for women 40-49, the majority may weigh the harms as greater than the benefits. For women 50-74, a large majority probably think the benefits outweigh the harms. There was variability in patient values and preferences however, and a lack of data from racial and ethnically diverse populations.

Informed choice

That’s why the Task Force recommends informed choice for women 40-74 years. It is important that everyone have information on both the benefits AND harms of screening to make a personal decision about whether or not to screen. This principle is consistent with current healthcare practices. 

If, after being informed, a woman wants to be screened, she should be offered a mammogram every 2 to 3 years. 

“We all want to find ways to reduce the burden of breast cancer and improve outcomes,” said Dr. Guylène Thériault, a family physician, teacher of evidence-based medicine and chair of the Task Force and breast cancer working group. “People may find that information about breast cancer screening is surprising – there are potential benefits to screening, but there are also harms. We want women to have all the information they need to make the decision that’s right for them.” 

The Task Force also found that there is a lack of evidence on screening in Black women and other diverse races/ethnicities, and in those with dense breasts. That’s why it’s calling for more research to better understand breast cancer in these populations and the best ways to reduce the burden of disease and deaths.

“We echo the frustration and surprise that there is not more evidence on the cause of disparities in breast cancer rates and increases in breast cancer,” said Dr. Thériault. “That’s why we have called for more research in these areas. 

It’s important to clarify that the recommendations are for people who do not have symptoms (such as a breast lump), and are not for those at high risk of breast cancer. If someone has a breast lump or other concerning symptom, they should see a health care provider. 

The draft recommendations, evidence reviews and patient tools can be found here. 

The Task Force thanks everyone who provided feedback during the public comment period. The final guideline will be published in 2025, with all feedback summarized and posted online. 
 

About the Canadian Task Force on Preventive Health Care

The Canadian Task Force on Preventive Health Care is an independent group of family physicians, nurse practitioners, specialists and experts in preventive health care and guideline methodology who volunteer their time to develop evidence-based national clinical practice guidelines for primary care on a range of topics.