Millions of Canadians happily sign up for breast cancer screening every year. After all, we’re told that it “saves lives” for women aged 50 to 74.
Yet, there is no evidence that it does. Most – but not all – studies conclude screening mammography reduces a woman’s chance of dying of breast cancer. But in randomized controlled trials on breast cancer screening tests, those who received the screening didn’t live longer than those who weren’t screened.
There could be many reasons for this. One is that there are many, many causes of death. It’s extremely difficult to prove that slightly reducing just one of those causes decreases mortality rates overall. “You’re into a substantial numbers problem,” says Anthony Miller, an epidemiologist and professor emeritus at the Dalla Lana School of Public Health in Toronto. It’s possible mammography screening does save lives but the life-saving benefit is so small and the trials so far haven’t followed enough patients to prove it.
There is, however, a more concerning theory as to why mammography screening doesn’t seem to extend lives: that the harms of breast cancer screening could offset the benefits.
According to an independent UK panel, for every one breast cancer death that annual mammography prevents, at least three women will undergo unnecessary treatment because cancers will be found that wouldn’t have caused problems if left alone. This treatment can involve surgery, chemotherapy and/or radiation. The latter two treatments can damage the heart or even cause other cancers, explains Peter Jüni, Director of the Institute of Primary Health Care at the University of Bern in Switzerland. One of the reasons breast screening doesn’t affect overall mortality in trials could be that “deaths through cardiovascular causes would probably be increased in those unnecessarily treated for breast cancer,” he says. Radiation exposure from breast cancer treatment could also lead to other cancers – interestingly, mammography screening doesn’t seem to reduce a woman’s risk of dying of cancer in general.
So why does Cancer Care Ontario’s website state that breast screening saves lives? The message is “intended to be understood by a lay audience,” explains Linda Rabeneck, vice president of prevention and cancer control at CCO. “Frankly, a more correct statement would say it reduces the risk of a woman dying from breast cancer.” Decreasing one’s risk of dying of breast cancer isn’t the only reason to get mammography screening, however. “If you’re diagnosed at an earlier stage, the cancer may be dealt with surgically and you may not need to have chemotherapy or radiation,” Rabeneck says.
Do the sicknesses and deaths prevented by breast cancer screening outweigh the sicknesses and deaths caused by unnecessary breast cancer treatment? Most think it does, but not all experts agree. And the controversy is not new, as Healthy Debate’s previous articles on mammography make clear.
Marcello Tonneli, chair of the Canadian Task Force on Preventative Health Care thinks the benefits of cervical, breast and colon cancer screenings outweigh the risks for the recommended age groups. Still, he says, “there is often naïve enthusiasm for screening” among health experts and patients alike.
Risk-benefit conversations still not happening
Rabeneck explains that CCO now encourages doctors to have more nuanced conversations around the positives and negatives of routine mammography with patients, based on “a new understanding of the evidence” whereby the benefit-to-harms ratio of mammography screening is lower than what was previously thought. In addition to the risk of unnecessary treatment, almost 30% of women above 50 who are screened over a decade will receive a false positive result, leading to further testing and psychological stress.
“[Health providers] inform a woman of the potential benefits and harms, and she gets to make her own decision, we don’t inflict [mammography] on anyone…she can opt out,” says Rabeneck.
But at least some doctors still aren’t talking to women about harms, according to experiences we’ve heard from Canadian women. Earlier this month, for example, Diane*, an Oakville, Ontario-based writer, went for her 50-year-old physical and was told it was time for her to have her first mammogram that would be covered by the province. “The doctor just said ‘Okay, you’re due for a mammogram and here’s your requisition.’ I didn’t question it.”
Two years ago, Leslie*, a mother of two living in Toronto, was told she should have mammogram screening at 52. “They certainly didn’t educate me on any risks for or against,” says Leslie. And when she presented her health provider with the evidence that the benefit of routine mammography is extremely small, “it just was like, ‘We’re not going to have this conversation.’” Leslie opted out of screening. “I had a cousin who ended up dying of leukemia, likely caused by the [radiation] that she received for breast cancer treatment. So that obviously swayed my vision as well.”
There are several other reasons these conversations aren’t always happening. One is that new understandings of risks and benefits can trickle down slowly, and doctors might simply be unaware of the numbers, says Miller.
Another is that, despite CCO’s expectation of doctors to fully inform patients, the organization’s patient information could send the message that it’s not important to talk about risk. CCO’s mammography brochure for doctor’s offices was updated in 2014 and now mentions the risk of unnecessary treatment, but no numbers are provided and false positives aren’t mentioned. In total, there are only two sentences about harm in a two-page, small-print brochure.
And the website still pushes routine mammography. Case in point: Several videos aimed at newcomers that don’t mention risk at all. ‘Vanita came to Canada from India…The doctor told her that if breast cancer is caught early, it is easier to treat. She gets tested regularly and encourages her friends to do the same,’ one video description reads. (The Canadian Task Force on Preventative Care’s website is much more objective.)
In a presentation last week, Dr. Cathy Risdon, associate chair of the department of family medicine at McMaster University and a practicing physician, said she feels “uncomfortable” with the fact “that there is not another side” in CCO’s letters to women on mammography screening. While noting there “are many, many reasons that women still choose to receive screening,” Dr. Ridson thinks women over 50 should also be told that “choosing not to screen is very sane and can be very consistent with caring for your health.” She supports either choice a patient makes.
For other physicians, financial incentives could tip the balance in favour of mammography screening. Most doctors in Canada are at least partly paid per service provided. And one long conversation on risks and benefits can take as much time as several quick screening referrals, while netting a much smaller payment. The Ontario government additionally pays doctors a $2,200 bonus per year if 75% of their eligible patients are referred for routine mammography. Alberta doesn’t have such an incentive but the government does have a mammography uptake target of 70% of eligible patients by 2020. (Currently, mammography screening participation rates in Ontario and Alberta are 60% and 56%, respectively, for women ages 50-74).
It’s more likely doctors have laudable reasons to oversell mammography, according to Doug Stich, program director of Toward Optimized Practice, the body that implements cancer screening recommendations in Alberta. “It’s a very real experience for a primary care physician to have a patient die of one of these diseases and to feel like they failed them and that they’re not going to let that happen again,” he explains.
Both doctors and patients are influenced by the stories we hear. People who develop organ problems or cancers often don’t trace it back to previous chemotherapy or radiation. But, explains Stitch, “patients who discover cancer through screening tend believe that the screening saved their lives.” Even if it didn’t.
Messaging around breast cancer screening should be more accurate
Some believe that given the changes in breast cancer treatment and advances in mammography, a new randomized controlled trial on screening mammography is crucial. Dr. Miller also points out it’s possible genetic research will reveal what types of breast cancers do not progress rapidly and it will therefore be clearer when treatment is necessary and when it’s not.
The evidence we have now, however, suggests that the benefit of mammography is small and the risks are not insignificant. Yet, poor information means most patients believe the benefit of mammography screening is much higher than it is. And if patients had more information, it’s quite likely fewer would opt for mammography. In a US study of 317 people, 51% of those polled said they wouldn’t want to be screened if screening leads to more than one unnecessarily treated person per one life saved.
Because comfort level with various risks differ by patient, in Alberta, Stich’s organization trains physicians that “a patient saying ‘I decided not to’ is equally as good a result as a patient saying ‘Yes, I will go ahead with the screen.’” But the Alberta government’s mammography target and the Ontario government’s mammography bonus send a message that patients should be sold on mammography screening.
Tonneli finds the incentives concerning. “We need to give people good information about the risks and benefits of screening, together with tools that help them to understand the information,” he says. “I worry that providing doctors with a financial incentive to screen more people is at odds with this goal.”
Peter Suter is chair of the Swiss Medical Board that recently recommended against routine mammography on the basis it “does not clearly produce more benefits than harms.” Suter thinks mammography screening should continue to be funded, but that it should be up to each woman to decide if she thinks the benefit outweighs the risk, or vice versa, in her particular case. (Someone with a family history of fatal breast cancer may be more likely to go for screening, for example.) “We need to stop saying mammography saves lives…this propaganda is borderline in terms of ethical issues,” he says.
*Names have been changed.