Does a New Mammogram Study Affect Screening Guidelines?

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In women’s ongoing dilemma over when to start routine mammogram screening for breast cancer, a large new, longitudinal study may add a wrinkle.

The Swedish Two-County Trial, which began in the late 1970s, followed 133,000 women who were divided into two groups: half were invited to have routine mammograms for seven years and the other half received “usual care,” which did not include mammograms. After 29 years of follow-up, the study’s authors found that routine screening was associated with a 30% lower risk of death from breast cancer.

The data suggested that one death was prevented for every 1,300 mammograms; put another way, one breast-cancer death would be prevented by screening 300 women every two to three years for 10 years.

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In the study, women aged 40 to 49 in the screening group received a mammogram every two years; those aged 50 to 74 were screened about every three years.

The findings suggest that mammograms can save lives. Of course, that fact was never in question. In 2009, when the United States Preventive Services Task Force (USPSTF) reversed its longstanding guidelines and began recommending routine breast-cancer screening for women starting at age 50, instead of age 40. The issue was not that mammograms didn’t save lives. Rather, it was that before age 50, the task force found, the risks and potential harms of routine screening outweighed its potential benefits.

Mammograms can result in false-positive results, which can further lead to unnecessary tests and invasive procedures like biopsies. False positives also lead to tremendous anxiety. What’s more, mammograms often pick up cancers that are so slow-growing, they would never even have been noticed in a woman’s lifetime. And often by the time they catch deadly fast-growing cancers, the tumors have already spread to other parts of the body and become incurable.

In November 2009, the USPSTF recommended that most women (those without gene mutations that are associated with breast cancer or close relatives with the disease) begin getting mammograms every two years starting at age 50. At the time, the New York Times reported:

Over all, the report says, the modest benefit of mammograms — reducing the breast cancer death rate by 15 percent — must be weighed against the harms. And those harms loom larger for women in their 40s, who are 60 percent more likely to experience them than women 50 and older but are less likely to have breast cancer, skewing the risk-benefit equation. The task force concluded that one cancer death is prevented for every 1,904 women age 40 to 49 who are screened for 10 years, compared with one death for every 1,339 women age 50 to 59, and one death for every 377 women age 60 to 69.

Many American women who had begun getting yearly mammograms starting at age 40 — as per the task force’s previous recommendations from 2002 — were dismayed by the new rules. And other national groups including the National Cancer Institute and the American Cancer Society held firm to their previous guidelines despite the USPSTF’s about-face. Both organizations still recommend routine screening every one or two years for women age 40 and older.

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But while mammograms catch many cancers, they miss up to 20% of them, according to the National Cancer Institute. And some 6% to 8% of mammograms may result in false positives.

“It’s not a perfect examination by any means but we have quite an accumulation of data that shows mammography is associated with a reduced risk of dying from breast cancer and the opportunity to be treated less aggressively,” Robert A. Smith, director of cancer screening at the American Cancer Society, told CNN.

It’s worth noting that the Swedish study’s findings did not separate out women by age, so they don’t shed any light on the risk-benefit ratio of screening for women in their 40s versus women in their 50s or 60s. They also don’t much impact the USPSTF’s 2009 recommendations.

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As with many complicated health issues, it remains largely up to individual women and their doctors to decide when to begin screening.

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