Benefits of Screening Mammography More Consistent Than Previously Thought

Photo Courtesy © SABCS/Todd Buchanan 2013

Robert A. Smith, PhD

According to an analysis of key screening studies presented at the 2013 San Antonio Breast Cancer Symposium, the benefit of screening mammography is more consistent across studies than has previously been thought.

Robert A. Smith, PhD, senior director of cancer screening at the American Cancer Society, Atlanta, concluded from this closer look that, “The so-called controversy over the benefit of mammography screening as estimated from the trials is largely contrived.”

It is generally accepted that mammography screening prevents deaths from breast cancer, but the absolute size of the mortality benefit and the concomitant risks associated with screening have varied among the studies.

Smith and colleagues re-examined data from four large studies of the benefits and harms of mammography screening, reviewing the apparent disparities between the reviews of the effects of screening on breast cancer mortality. The four studies were the US Preventive Services Task Force (USPSTF) review, the UK Independent Breast Screening Review, the Nordic Cochrane review, and the EUROSCREEN review. In these studies, the estimated number needed to screen to prevent one death from breast cancer ranged widely, from 90 to 2000.

“What we found was that the estimates are all based on different situations, with different age groups being screened, different screening and follow-up periods, and differences in whether they refer to the number of women invited for screening or the number of women actually screened,” Smith said. “The differences between the reviews with respect to the absolute breast cancer mortality reduction are almost entirely due to expressing the same basic result relative to different denominators, choice of population mortality rates, and so forth.”

For example, studies that conducted intent-to-treat analyses on all women invited for screening would contain bias and would underestimate the benefit. “This is a poor measure of effectiveness, since about 30% of women never show up for their mammograms,” he noted.

Follow-up time is also critical, he added, noting that some studies capped follow-up at 10 years, which could miss about half the deaths that will occur among hormone receptor-positive patients beyond this time point. Younger age at screening (ie, 40-50) is also a diluting factor since fewer breast cancers occur in this group.

Substantial Reduction in Breast Cancer Mortality Shown

The investigators standardized all estimates to a common scenario—conforming to the UK Independent Breast Screening Review, which they determined to represent the most rational screening scenario. The UK review investigated the effect of screening women in the United Kingdom aged 50 to 70 over 20 years, and it estimated that 180 women would be needed to be screened to prevent one breast cancer death.

After standardizing the studies to this scenario, the variability among the studies then dropped from 20-fold to approximately 2.5-fold. Smith reported.

The number needed to screen, in the adjusted analyses confirming to the UK Review standard, was now 180 for the UK Review, 193 for the USPSTF, 257 for the Nordic Cochrane Review, and 96 for the EUROSCREEN.

“In short, the absolute benefit, once you standardize to a common population, a common screening scenario, and a common follow-up, becomes not so significant, and certainly not enough to question the value of mammography over a lifetime of screening,” he suggested at a press briefing Wednesday.

Kent Osborne, MD, director of the Dan L. Duncan Cancer Center at Baylor College of Medicine, Houston, who moderated the press briefing, said the new analysis “makes intuitive sense.”

He said one could “easily understand that including women who don’t show up for their mammogram [ie, invited but not screened] will underestimate the true benefit of screening.” Secondly, he said, “we are learning that breast cancer is even more indolent than we thought, especially estrogen receptor-positive disease, and many patients don’t recur for a decade or more. If we don’t follow patients long enough to capture these recurrences and deaths, we don’t get a full assessment of the value of screening. Long-term follow-up is extremely important.”

Plenary Lecture

H. Gilbert Welch, MD, MPH, professor of Medicine at The Dartmouth Institute for Health Policy & Clinical Practice in New Hampshire, addressed the issue of screening mammography and overdiagnosis in a Plenary Lecture Wednesday. He maintained that for the early detection of breast cancer, “There is no single ‘right’ number to describe the magnitude of either the benefits or harms of screening mammography, and there is no single ‘right’ value to assign to the various outcomes. Consequently, screening mammography is a choice, not a public health imperative.”

While the benefit of screening has been exhaustively studied, he noted that many of the trials are decades old, and current treatments allow women detected with breast cancer to live much longer. As the efficacy of treatment improves, the benefits of early detection diminish but the associated harms remain, he suggested.

Before investigators identify the ideal balance between risk and benefit, Welch suggested that mammography adherence be rescinded as a quality indicator, which would relieve clinicians of the pressure to maximize screening adherence and would allow women who choose to be screened—or not—to feel equally at peace with their choices.

Reference

Smith RA, Duffy S, Chen TH-H, et al. Disparities in the estimates of benefits and harms from mammography: Are the numbers really different? Presented at: 2013 San Antonio Breast Cancer Symposium; December 10-14, 2013; San Antonio, TX. Abstract S1-10.

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