Recently, a new publication on the Canadian National Breast Screening Study (CNBSS) has generated significant press coverage with the unfortunate message that screening mammography is not useful. Critically important to note is that the CNBSS is an “outlier” as the only randomized controlled trial to not show a benefit of screening mammography. The analyses of why this was the case began over two decades ago, when this trial was widely discredited for concerns regarding its compromised execution.
Two major problems were uncovered: First, there were many more advanced cancers in the screening group than were statistically predicted, indicating that the randomization process had likely been compromised. Two studies pointed out that the CNBSS randomized some women after their required clinical examination, misallocating a proportion of them to the screening arm [Boyd et al., Radiology December 1993 and Kopans and Feig, American Journal of Roentgenology (AJR) October 1993].
Secondly, mammographic technique in the CNBSS has been shown to be below the standard of care [Baines et al. AJR October 1990], and even though the investigators documented substantial improvements in image quality during the course of the trial, five and a half years of the 8-year trial had elapsed, so the majority of mammography exams in this trial were still substandard. It therefore comes as no surprise that screening was unable to demonstrate a beneficial outcome. This was the case back in the 1990s and remains true today.
Previously, the screening debate revolved around who to screen and how often, with overall agreement that screening mammography does indeed save lives. Mammography is the only breast cancer screening modality proven to have survival benefit in multiple randomized controlled trials, even when this outlier Canadian trial is included in the systematic reviews, as reported by Smith et al in the September 2004 issue of the Radiologic Clinics of North America.
How many lives are we talking about? In the February 2011 edition of AJR, Hendrick and Helvie estimated that 100,000 more lives would be saved if all women in the U.S. between the ages of 30 and 39 began annual screening at age 40 (up to age 84) as compared to waiting until age 50, and then obtaining mammograms every other year as critics of mammographic screening would suggest. Screening may commit some women to additional testing and procedures, but without it many lives would be lost.
Read “Breast Imagers Defend Mammography in Wake of New Study” here.
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