Does screening mammography save lives?

When 25-year follow-up data from the Canadian National Breast Screening Study—published earlier this year—showed no benefit for annual mammography in women aged 40 to 59 years, the findings generated renewed debate about whether screening mammography actually saves lives.¹

In that study, Miller and colleagues continued their follow-up of almost 90,000 women who had been randomly assigned to mammography (five annual screens) or no mammography from 1980 to 1985. Women aged 40 to 49 in the mammography arm and all women aged 50 to 69 underwent annual clinical breast examination (CBE). Women aged 40 to 49 in the control arm had a single CBE and continued usual care in the community. The main outcome measure was death from breast cancer.¹

During the entire 25-year study, 3,250 women in the mammography arm were given a diagnosis of breast cancer, and 3,133 in the control arm received the same diagnosis. Of these, 500 and 505 women, respectively, died of the malignancy.

The overall hazard ratio for death from breast cancer in the mammography and control arms was 0.99 (95% confidence interval, 0.88–1.12). After 15 years of follow-up, 106 residual excess cancers (106/484; or 22%) were identified in the mammography arm and were attributed to “overdiagnosis.”¹

During the screening period the mean size of breast cancers identified was 1.91 cm and 2.10 cm in the mammography and control arms, respectively (P = .01), and 30.6% and 32.4% of tumors, respectively, were associated with positive lymph nodes (P = .53).

PROFESSIONAL SOCIETIES STICK BY THEIR GUIDELINES
Following publication of the Canadian findings, the American College of Obstetricians and Gynecologists (ACOG) reaffirmed its recommendation for women at average risk for breast cancer to initiate annual screening at age 40. In an announcement issued February 14, 2014, ACOG noted that it had “a number of concerns” with the Canadian study.²

Similarly, the American Cancer Society reiterated its own recommendation that women aged 40 and older undergo annual mammography and CBE for as long as they remain healthy.³

The American College of Radiology went a few steps further, calling the Canadian study “incredibly flawed and misleading.”⁴ Its guidelines call for annual mammography beginning at age 40.

The US Preventive Services Task Force (USPSTF) 2009 guidelines on breast cancer screening also stand, with biennial mammography beginning at age 50 for women at average risk for breast cancer.⁵

The Canadian Cancer Society also reaffirmed its recommendations for breast cancer screening following publication of the Canadian trial 25-year follow-up data—although its recommendations call for screening to begin at age 50 and to be repeated thereafter at 2- to 3-year intervals.^6,7

In short, nothing has changed…yet. But the Canadian trial raises a number of questions about breast cancer screening—and the answers aren’t as clear-cut as you might imagine.

IS THE CANADIAN TRIAL CREDIBLE?
Results from earlier randomized, controlled trials have indicated that screening mammography reduces death from breast cancer.

“The Canadian study is an outlier,” says Barbara Monsees, MD, Ronald and Hanna Evens Professor of Women’s Health in the department of radiology at Washington University in St. Louis, Missouri.

“There is an overwhelming amount of evidence that tells us that screening mammography saves lives,” says Dr. Monsees. “This evidence includes other randomized trials, case-control studies, results of organized screening programs, and downward trends in breast cancer deaths where screening is used.”

Mark D. Pearlman, MD, also believes the body of evidence shows that screening mammography is effective. Dr. Pearlman is vice chair and service chief in the division of obstetrics and gynecology and professor of surgery and director of the breast fellowship in obstetrics and gynecology at the University of Michigan Health System in Ann Arbor, Michigan. He has been on the surgical staff of the Breast Care Center there since 1990, with expertise in the management of women with breast disease and increased genetic risks for breast and ovarian cancer.

The Canadian trial is “a reasonably done study,” he says, “but there are some concerns. First, it’s not a new study—it was initially published 22 years ago. This latest publication is just a continuation of following these women.”

“This study, along with seven other randomized, controlled trials, was considered by the USPSTF in formulating its 2009 recommendations. In that meta-analysis, which included women in their 40s, screening mammography had benefit in every decade of life of interest.⁸ That is the basis on which ACOG made its recommendation for women at average risk to start annual screening at age 40 and continue at least until age 70,” Dr. Pearlman says. “When the USPSTF considered this negative study, it realized that there is benefit for mammography despite this single trial.”