Earlier screening for breast cancer: Benefits and harms

The US Preventive Services Task Force (USPSTF), an independent government agency that makes recommendations on screening and other preventive care guidelines, recently released a new draft recommendation for breast cancer screening. The new guideline lowers the starting age of biennial mammograms from 50 to 40 for women at average risk.

Why did the USPSTF depart from their previous recommendation and what are the potential harms and benefits of this change? Let’s take a closer look.

Author’s note: The USPSTF recommendations apply to anyone assigned female at birth, which includes not only women but also trans men and some nonbinary individuals. For the purposes of readability I use the term “women” throughout the blog in reference to all people for which this recommendation applies.

Why did the USPSTF lower the screening age?

The USPSTF guideline change came as a surprise to many health experts, because there have been no new clinical trials of breast cancer screening that would provide a reason for adjusting the guidelines. Rather, the USPSTF noted that the incidence of breast cancer in young women has been steadily rising, which changes the risk-benefit profile of screening. The more likely it is that someone has cancer, the more beneficial screening them for cancer will be.

Yet some healthcare researchers noted that even adjusting predictive models for higher rates of cancer in younger women, the risk-benefit profile still is not very different from the agency’s previous results from 2016.

“Change always happens over time, obviously, as the evidence evolves. At the same time, there needs to be a compelling reason and in the materials here, I don’t see a compelling reason yet. When I looked back at the 2016 modeling studies, the harm-benefit analysis was very similar,” said Ruth Etzioni of the Fred Hutchinson Cancer Center, in STAT.

The USPSTF also wanted to acknowledge that Black women are diagnosed with breast cancer at later stages and face a higher rate of mortality from breast cancer than other racial groups; thus, an earlier start date of screening for these patients could save lives and reduce racial disparities in breast cancer outcomes. But although the USPSTF ran new models exploring the benefits and harms of screening in Black women, they stopped short of recommending earlier screening for Black women specifically.

Benefits and harms of earlier screening

What are the actual numbers that drove the USPSTF’s new recommendation? The USPSTF modeling report presents a myriad of scenarios that estimate the rates at which breast cancer screening starting at different ages result in certain benefits and harms.

At a baseline, with no screening, an estimated 28 women out of 1000 will die from breast cancer over their lifetime. For every 1000 women screened every two years from age 40-74, about eight will avoid death from breast cancer over their lifetime. However, there will also be 1540 false positives, 210 unnecessary biopsies, and 12 cases of overdiagnosis (when someone is diagnosed and treated for a cancer that never would have harmed them). And among the 1000 women screened, 20 will still die from breast cancer. It’s worth mentioning that although screening prevents death from breast cancer, there is no evidence that screening mammograms reduce overall mortality (death from any cause).

Interval & Age groupBreast cancer deaths not avertedBreast cancer deaths avertedFalse positivesBenign biopsiesOverdiagnosed cases
No screening28.30000
Biennial screening age 50-7421.46.9102114810
Biennial screening age 40-7419.98.4154021012
Source: USPSTF modeling report, pgs 33,35

Essentially this means that an individual at average risk getting screened is more likely to be treated for a cancer that never would have harmed them than they are to avoid death from breast cancer. They are more than twice as likely to die of breast cancer anyway than to catch and successfully treat a dangerous cancer. And they are far more likely to have undergo a biopsy unnecessarily or receive a false positive result than avoid death from breast cancer.

What’s the difference from the previous recommendation? Lowering the starting screening age from 50 to 40 avoids one more death from breast cancer out of 1000 women screened, but results in 519 more false positives, 62 more unnecessary biopsies, and two more cases of overdiagnosis.

It’s interesting to look at scenarios that the USPSTF did not adopt in their draft recommendation — for example, screening every year instead of every other year. Screening 1000 women every year from age 40-74 would avoid three more deaths from breast cancer compared screening every other year. However, it would result in 883 more false positive results, 71 more unnecessary biopsies, and seven more cases of overdiagnosis (see table below).

Similarly, continuing annual screening until age 79 would avoid four more breast cancer deaths per 1000 screened, compared to screening every other year until age 74. But it would increase the number of false positives by more than 1000 and the number of overdiagnosed cases by more than ten per 1000 screened.

Interval & Age groupBreast cancer deaths not avertedBreast cancer deaths avertedFalse positivesBenign biopsiesOverdiagnosed cases
No screening28.30000
Biennial screening, age 40-7419.98.4154021012
Annual screening, age 40-7417.311242328119
Annual screening, age 40-7916.112.2259530123
Source: USPSTF modeling report, pgs 33, 35

Examining the tradeoffs

These comparisons bring up important questions of what tradeoffs we are willing to make to catch cancer early. Many would say that avoiding one more death from breast cancer is worth the harms to hundreds more from overtreatment, but it’s important to recognize that these downsides exist and to acknowledge that there is a line at which the harms outweigh the benefits.

For example, in a 2019 tongue-in-cheek article, doctors and researchers Myung Kim, Go Nishikawa, and Vinay Prasad pointed out that if we only care about the benefits of preventive care and not the harms, we would give everyone a double mastectomy to prevent all cases of breast cancer before they start. The ridiculousness of this proposal shows how we must take into account the harms of screening alongside the benefits.

It’s also important to understand that a decision made for the purposes of population health may not be acceptable to every individual. With this recommendation, the USPSTF is essentially saying that the tradeoffs of 500 more false positives, 60 biopsies, and 2 overdiagnosis cases are worth it to avoid an additional death per 1000 people from breast cancer. Across a population of millions, that’s hundreds of thousands of deaths that could be avoided. Many patients will find the risks of screening acceptable on an individual level as well. But members of the USPSTF task force also say that decisions about when to start screening are personal, and that there is always room for a discussion with your doctor. Women at average risk of breast cancer considering screening should be informed to make the best decision for them.

The public health impact of earlier screening

While models can help us understand the theoretical benefits and harms of screening, things are very different in the real world. The guideline assumes that everyone has equal access to screening and to healthcare in general, which is simply not true. About a quarter of women age 50-64 have not had a mammogram in the past two years, and 55% of uninsured women in this age group haven’t received a mammogram in the past two years. About 5% of women age 50-74 have never had a mammogram.

It’s unclear how lowering the screening age to 40 will increase access among women who don’t currently have access to care despite already being eligible for screening. In fact, as we pointed out in a previous blog on colon cancer screening, lowering the screening age could increase disparities in access if more healthy young people rush to get screened and take up the available slots.

The USPSTF is right in calling for more research to evaluate the benefits and harms of screening particularly for Black women, who are at greater risk of breast cancer mortality. However, we need far more action to reduce racial disparities than just lowering the screening age. Disparities in breast cancer deaths are a result of structural racism, which means we should be dedicating more energy to ensuring regular access to medical services, improving guideline-concordant cancer care, and addressing the social drivers of health in communities of color that may contribute to cancer like environmental conditions.