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Demystifying breast cancer screeningTibor Kolley/The Globe and Mail

How is it possible? For decades, regular mammograms were touted as a woman's best defence against the scourge of breast cancer. Last week, women were told that, according to new guidelines from the Canadian Task Force on Preventive Health Care, mammograms are useless for women in their 40s and only somewhat helpful for older women.

We've been through this before. Until about 10 years ago, women were told that female hormones taken after menopause would delay aging, and prevent heart disease and stroke. Then new research studies showed that doctors had it all wrong. Hormones didn't prevent these diseases and definitely increased the risk of breast cancer.

It would be logical to assume that the new mammography recommendations are the result of new studies. But they're not. The task force simply took nine studies done 20 to 50 years ago and drew different conclusions from the old results. Cynics are screaming that although money will be saved, millions of Canadian women will be denied a potentially life-saving service.

So let's look at the results of the old studies. For women over 50, there was about a one-in-four reduction in breast cancer deaths with mammography. For women in their 40s, a smaller benefit was seen, and only when the data from all the studies was combined. There are several reasons why mammography wasn't more effective. It couldn't detect 20 per cent to 50 per cent of the cancers. It found many cancers that were already incurable. But most of the cancers were cured, regardless of how they were detected.

Nothing magical happens at the age of 50, but, as women get older, mammography is more likely to save lives with fewer downsides. Breast cancer becomes more common, the composition of the breast changes, making cancers more visible, and there are fewer hormone fluctuations to cause false positives that lead to unnecessary tests. One only has to screen 400 women in their 60s for 10 years to save one life, compared with 2,000 women in their 40s. On the other hand, the younger that woman, the greater the number of productive years added to her life.

The previous guidelines recommended mammography for all women between 50 and 69. For a woman 40 to 49, the decision was to be based on her own preferences after discussion with her doctor. In fact, the new guidelines are not that different. Mammography is not only still recommended for women 50 to 69, but the age limit has been extended to 74. While the previous recommended frequency was every year or two, it's now every two to three years (most studies show every year is unnecessary, and a few show that every three years is safe). For women in their 40s, there's a weak recommendation for not screening routinely, but this doesn't mean that a woman can't have mammograms.

As for monthly breast self-examination, we've known for years that it doesn't save lives. Unlike mammography, which may detect a cancer years before it causes symptoms, monthly self-examination probably doesn't find a lump more than a few weeks earlier than it would otherwise have been found. No study completed to date has looked at whether annual physical examination of the breast by a physician saves lives, but it's doubtful it would if monthly self-examination doesn't work.

So why the flurry of media coverage? Why the outcry from experts and the public?

One argument is that these new recommendations are based on data from outdated mammography. Today's mammography, particularly digital mammography (think digital camera), finds half the cancers missed by film mammography in women in their 40s and in older women with "dense breasts." Had the old studies used today's techniques, many more lives would have been saved.

In the past 20 years, however, there's also been a marked improvement in breast cancer cure rates due to treatment advances. It can then be argued that aggressive, fast-growing cancers, which still can't be cured with today's treatments, also happen to be the cancers least likely to be detected by mammography because they grow large enough to cause symptoms between mammograms (even if done every year), whereas the slower-growing cancers picked up by mammography (with any technique) would be cured with today's treatments even if detected years later without screening.

Unfortunately, there is evidence to support both the "more mammography" and "less mammography" camps and, given today's costs and the rate of change of technology, a new mammography study that could settle the issue will never be done. Given this uncertainty, it's imperative that high-quality mammography be accessible to any woman 40 or older who chooses to have it. I also believe there's sufficient evidence to support digital over conventional mammography.

Every woman should see her doctor to have a breast cancer risk assessment, as the new guidelines apply only to women facing average risk. Women with a family history of breast cancer, or other important risk factors, may need earlier, more frequent or additional screening. And every woman should be "breast aware," and report any change in the appearance or feel of her breasts to her doctor.

Ellen Warner is an oncologist at Sunnybrook Health Sciences Centre and a professor of medicine at the University of Toronto.