In medical journals, doctors and scientists continue to debate the relative benefits and harms of breast cancer screening for women who are at average risk of developing breast cancer.

This debate is not always reflected in screening programs, most of which strongly recommend mammography to average risk women within a certain age bracket.

Some experts believe that many women are not being provided with complete information about both harms and benefits and cannot therefore make fully informed decisions.

One out of every nine women in Canada will develop breast cancer. This year in Canada, close to 24,000 new cases will be diagnosed and about 5100 women will die from the disease. Doctors can screen women for breast cancer using mammography – a particular type of x-ray image – to try to find cancers in the breast tissue before a lump can be felt by the patient or doctor during a routine breast exam.

In average risk women, who constitute about 90% of new breast cancer cases, the goal of screening is to identify aggressive cancer while it can still be cured. It may seem obvious that breast cancer screening works – after all, many breast cancer survivors have had their cancers detected on routine screening mammography.

But what if most of these cancers would have either done the women no harm, or been detected at a more advanced but curable stage anyway? And what about the women whose mammograms are reported as ‘suspicious’ and then undergo a biopsy, or even surgery and chemotherapy, when these interventions might not have been necessary?

Conflicting evidence, much controversy 

A comprehensive review published earlier this year by the Cochrane Collaboration concluded that for every 2000 women invited for breast cancer screening over ten years, just one woman’s life would be prolonged by early detection. At the same time, 10 out of the 2000 women would be ‘overdiagnosed’ and treated unnecessarily with surgery and possibly chemotherapy. More than 200 women would experience psychological distress because of a ‘false positive.’

Not everyone agrees with this interpretation of the data. Another group of researchers analyzed data from two large trials of mammography and concluded that if 1000 women were screened for 20 years, we could expect that 5 or 6 breast cancer deaths would be prevented at a cost of only about 2 cases of unnecessary treatment.

Has policy gotten too far ahead of the evidence?

The benefits and harms of mammography continue to be a subject of debate for researchers and doctors. However, many organizations that issue recommendations for cancer prevention strongly recommend regular screening for average risk women.

For example, Cancer Care Ontario’s website does not mention any possible harms on the main web page, although they do provide information about the benefits and risks of screening on a secondary web page.

Heather Bryant, Vice President, Cancer Control, at the Canadian Partnership Against Cancer says that currently “there is certainly a lot of information out there – the question is whether you are generating more questions than answers in the debate.” Bryant says that “all of the legitimate expert debate may have actually caused more confusion amongst the public” and the “question becomes what is the right amount of information to give to each individual, which will be different for each person. Linda Rabeneck, Vice President of Prevention and Cancer Control at Cancer Care Ontario notes that “part of our job is to make sure that doctors have the information at hand for the shared decision making conversation.”

Incorporating principles of ‘shared decision-making’ for public health initiatives like vaccination and screening may be more challenging than it is for interventions that doctors and patients consider after a disease has already been diagnosed. Within a shared decision making context, the doctor’s role is to interpret and apply the appropriate evidence, considering individual patient’s values, and then together with the patient come to a decision as to whether a particular treatment or test is worthwhile.

Incentives for breast cancer screening

Another reason why women may not end up being fully informed about breast cancer screening is that physicians in Ontario have incentives to increase mammography rates. Family doctors are eligible for bonuses that depend on the proportion of their patients who undergo mammography. These incentives range from $220 for a screening rate of 55% up to $2200 for a screening rate of 75%. In contrast, family doctors who provide complete information to their patients receive no payment if half of their patients choose not to be screened.

Gordon Guyatt, an expert on evidence based medicine and a doctor at McMaster University says “the desirable and undesirable consequences of breast cancer screening for average women are closely enough balanced that it is likely that many fully informed women would decline screening.” He argues that “the government’s current approach ignores this fact and implicitly undermines patient autonomy.”

Is it time to revisit our approach to mammography?

The discordance between the debate in the medical literature and the clear recommendations from some authorities is now the focus of significant attention in the United Kingdom. Mike Richards, the “cancer tsar” for the Department of Health in the UK is commissioning a review of both the evidence as well as how this information is distributed to the public. Richards says this work will “take account of current thinking on how to synthesize information on benefits and harms and how to present these so as to promote informed choice.”

The Canadian Task Force on Preventive Health Care, a federally funded group of experts, is expected to release guidelines later this year.* However, the task force has a relatively low profile, so its recommendations may or may not be viewed as authoritative by all relevant stakeholders.

Cornelia Baines, a doctor and researcher at the Dalla Lana School of Public Health who led one of the mammography trials and has herself had cancer, recently concluded that “it is reasonable for women to choose to be screened, but only if they are completely informed about the probability of benefit versus the probability of harm.” Baines adds however that “people in the field and the media are backing away from facing this controversy.”

Given the controversy about the benefits and harms of breast cancer screening, do you think that cancer agencies should

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* The Canadian Task Force on Preventive Health Care released their recommendations on screening for breast cancer in average-risk women aged 40-74 years on November 22, 2011. You can view the full recommendations here, and related commentary here.