According to the U.S. Preventive Services Task Force, if 1,000 women in their 40s have mammograms every two years for a decade, two of them will die from breast cancer in the next 10 years.Chicago Tribune/Getty Images
Should routine breast cancer screening begin at age 40 or 50? What may seem like a straightforward question is at the centre of a highly charged debate that’s been playing out in recent months as a national task force prepares a long-awaited update to Canada’s screening guidelines.
Some patients, advocates and breast cancer specialists say the evidence is clear: moving routine screening for average-risk individuals to 40, from the current recommendation of 50, will save lives. Earlier this year, a U.S. task force made that change, prompting Canadian advocacy groups and some experts to call for a similar move here.
The federal government subsequently gave $500,000 to the Canadian Task Force on Preventive Health Care, led by a group of medical experts with varied backgrounds, to expedite its planned update to the guidelines. The finished product is expected to be published this fall.
Martin Yaffe, senior scientist at Sunnybrook Health Sciences Centre, says the existing guidelines, released in 2018, were based largely on studies that were “too old” and didn’t take advances in screening technology into account. These advances, he and others say, makes the mammograms of today much more accurate and able to detect cancer earlier in women of all ages, including those in their 40s.
(The screening guidelines don’t apply to those who face an increased risk of breast cancer. And some provinces do offer routine screening to average-risk women, despite the recommendation to start at 50.)
But other experts point to data that suggests lowering the screening age may not improve outcomes. In fact, evidence shows that regular mammograms have no impact on breast cancer mortality rates for women in their 40s and may do more harm than good.
According to the U.S. Preventive Services Task Force, if 1,000 women in their 40s have mammograms every two years for a decade, two of them will die from breast cancer in the next 10 years. Among those who forgo mammography, three will die, which means screening resulted in one less death. At the same time, of those who undergo regular screening, the task force says 36 per cent will experience a false alarm; nearly 6 per cent will have an unnecessary biopsy; and two women will be overdiagnosed with a cancer that never would have posed a health risk.
Some experts say they are concerned the benefits of mammograms for people in their 40s who are at average risk are being overstated.
“All screening is much more complicated than people would like,” said Michael Pollak, professor of medicine and oncology at McGill University and an oncologist at the Jewish General Hospital in Montreal.
Renée Pellerin, a journalist who authored a 2018 book questioning the benefits of mammography, said messages about screening are oversimplified to the point of being harmful. Women in their 40s often hear that mammograms save lives without understanding how rare breast cancer mortality is at that age or about the chance of unnecessary follow-ups or invasive procedures.
“Above all, women need to understand how very complex all of this is,” Ms. Pellerin said.
One of the main reasons the U.S. task force cited for its decision to lower the screening age was a rise in the number of women diagnosed with breast cancer in their 40s, particularly from 2015-2019.
But incidence data is a misleading metric to base screening assumptions on because of what’s known as “lead time bias.” Simply put, if one woman is diagnosed with breast cancer after a mammogram and lives for 10 more years, that would be considered a screening win – it saved her life. But if that same woman chose not to get screened and instead was only diagnosed years later after developing symptoms, she could have had the same outcome. Experts say it’s impossible to know if screening actually “saves” anyone based on incidence alone.
For that reason, many experts say mortality data is more reliable. And on that count, it’s clear that among women in their 40s in Canada, breast cancer deaths have steadily declined in the past two decades.
Given that earlier screening can lead to more anxiety, unnecessary biopsies, or overdiagnosis of cancer, some say the best course of action is for people in their 40s to weigh the pros and cons with their health care professional considering their risk factors and preferences.
Last month, a paper published in the New England Journal of Medicine looked at breast cancer mortality data from four countries: the U.K., Denmark and Switzerland, which don’t offer screening to women in their 40s; and the U.S., where around 60 per cent of women in that age group get routine mammograms. The mortality rate was lowest in Switzerland, followed by Denmark, then the U.S. and the U.K.
The findings suggest that other factors, including availability of treatment, have a greater effect than screening mammograms, said Steven Woloshin, who authored the paper and is a professor at the Dartmouth Institute for Health Policy and Clinical Practice in New Hampshire.
Dr. Woloshin noted that for women in their 40s, the risk of dying from breast cancer is small and screening doesn’t appear to make a big difference.
The Decibel: The controversy over lowering the age of breast cancer screening
Some experts have said starting screening at age 40 will help address the worse health outcomes experienced by Black women in the U.S. Dr. Woloshin questioned this, pointing to data that show Black women already have comparable breast screening rates to white women. But Black women are far more likely to suffer worse outcomes because they have a higher chance of being diagnosed with triple-negative breast cancer, which is highly aggressive and has few treatment options. This, Dr. Woloshin suggests, means the focus should be on finding better treatments for that subtype of cancer, not increasing the already high screening rates.
Dr. Pollak said he’d like more effort to be directed at helping women prevent cancer from developing in the first place. For instance, not enough people are aware that alcohol consumption increases a woman’s risk of developing breast cancer.
“Much of cancer is just bad luck, but if you’re drinking two bottles of wine a week, you’re making a lifestyle choice that has lifestyle implications,” he said.
Others, like Dr. Yaffe, say that screening women in their 40s may reduce the chance of missing an aggressive cancer, which outweighs the harms.
The Canadian Task Force on Preventive Health Care is including newer mammography studies in its evaluation of new guidelines.
“Our goal is to ensure that Canadians and their physicians have the best information they can have to make decisions about screening,” said task force co-chair Guylène Thériault.