Breast cancer screening for women of average risk in that age group results in a "relatively small" mortality benefit, which may be countered by an increased risk of overdiagnosis and unnecessary treatment.
In the final analysis, however, the decision to undergo screening mammography should remain with the individual patient and treating physician, who should thoroughly discuss the tradeoff between benefits and harms, as well as patient preferences, the guideline authors wrote in an article published online in CMAJ.
"We have recommended a screening interval of every two to three years for women 50 to 74 years of age using available evidence from randomized, controlled trials," Marcello Tonelli, MD, of the University of Alberta in Edmonton, and co-authors from the Canadian Task Force on Preventive Care wrote in conclusion.
"The concept of individualizing the interval for screening with mammography based on breast density or other risk factors is appealing but requires further study."
"Finally, given the importance of patient preferences for appropriate decision-making, further studies are needed to determine the best way to communicate information about the potential benefits and harms of mammography."
In an accompanying editorial, a Danish physician urged elimination of breast cancer screening altogether.
Citing evidence of overdiagnosis and unnecessary testing and treatment resulting from mammographic screening, Peter C. Gotzsche, MD, of the Nordic Cochrane Center in Copenhagen, wrote, "If screening had been a drug, it would have been withdrawn from the market."
The Canadian guidelines add another log to the opinion fires that have flared off and on since the U.S. Preventive Services Task Force (USPSTF) made similar recommendations in 2009. Debate over the need for screening mammography in women ages 40 to 49 continued through 2010, with multiple individuals and organizations taking sides.
The debate has shown no signs of losing steam. At various times during 2011, MedPage Today has reported studies showing that mammographic screening in women 40 to 49 offers a worthwhile mortality benefit and others that have found only a trivial benefit.
"As the Guideline on Breast Cancer Screening was last updated in 2001 and breast cancer screening has since become a subject for discussion amongst doctors and patients, the revitalized Canadian Task Force selected breast cancer screening as the topic for its first guideline.
We intend that this Guideline, which reflects the latest scientific evidence in breast cancer screening, be used to guide physicians and their patients regarding the optimum use of mammograms and breast examination."
The guideline says that a better balance is required between the harms and costs of overdiagnosis, overtreatment, and false positives and the outcomes of breast cancer screening with regard to tumor detection and mortality. Several recent studies have shown that routine breast cancer screening during middle age do not impact on mortality rates enough to outweigh the negative consequences of them.
Screenings that come up with a false-positive result can seriously impact on the patient's and her family's well-being, causing disruption and extra costs; they also use up resources of the health care system.
Dr. Tonelli said:
"Providing Canadians with guidelines that reflect the most current scientific evidence is our priority. We encourage every woman to discuss the risks and benefits of screening with their doctor before deciding on the best approach for them."
Below are the main recommendations:
As the risk of cancer is very low for women aged 40-49, and the risks of overdiagnosis, overtreatment and false-positives are relatively high, there should be no routine mammography for this age group
Women aged 50 to 69 years - routine screening should occur every two to three years
Women aged 70 to 74 years - routine screening should occur every two to three years
Average risk women should have no MRI screening
There should be no routine clinical breast exams by doctors
There should be no breast self-exams to screen for breast cancer
The guideline authors wrote:
"There was no evidence that screening with mammography reduces the risk of all-cause mortality. Although screening might permit surgery for breast cancer at an earlier stage than diagnosis of clinically evident cancer (thus permitting the use of less invasive procedures for some women), available trial data suggest that the overall risk of mastectomy is significantly increased among recipients of screening compared with women who have not undergone screening."
The Canadian Task Force on Preventive Health Care consists of 14 primary care and prevention experts - it is an independent body. It was established by the Public Health Agency of Canada to "develop clinical practice guidelines that support primary care providers in delivering preventive health care".
In a Commentary piece in the same journal, Dr. Peter Gøtzsche, Nordic Cochrane Centre, Copenhagen, Denmark, remarks:
"These guidelines are more balanced and more in accordance with the evidence than any previous recommendations."
Gøtzsche explains that there is no evidence supporting the use of routine mammography screening, he says it is ineffective at best, and also harmful because the "diagnosis of cancers that would otherwise be undetected lead to life-shortening treatments and mastectomies".
Dr. Gøtzsche wrote:
"The main effect of screening is to produce patients with breast cancer from among healthy women who would have remained free of breast disease for the rest of their lives had they not undergone screening.
(conclusion) The best method we have to reduce the risk of breast cancer is to stop the screening program," he concludes. "This could reduce the risk by one-third in the screened age group, as the level of overdiagnosis in countries with organized screening programs is about 50%."
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