Variations between studies ‘not enough to question value of mammography’

A re-examination of four large studies that evaluated the benefits and potential harms of mammography revealed the disparities in results were not as large as once thought.

In fact, once results from the studies were evaluated with the same standard population and screening scenario, they all showed screening contributed to a substantial reduction in breast cancer mortality, said Robert A. Smith, PhD, senior director of cancer screening at the American Cancer Society.

Robert A. Smith

“An enduring debate is whether there is significant value to mammography or very little value,” Smith said during a press conference. “Because most of these estimates of absolute benefit derive from the same data, we decided to determine how contextual this is, and whether [the variations were] just a matter of expressing the same data in a different way.”

Smith and colleagues re-examined the Nordic Cochrane review, the UK Independent Breast Screening Review, the US Preventive Services Task Force (USPSTF) review and the European Screening Network (EUROSCREEN) review. In those reviews, the estimates of the number of women who must be screened to prevent one breast cancer death ranged from 111 to 2,000, a nearly 20-fold difference.

Smith and colleagues identified several explanations for the disparities. For example, the estimates were based on different age groups undergoing screening, or different screening and follow-up periods. Also, some reviews calculated estimates based on the number of women invited for screening rather than the number actually screened.

To determine whether the disparities represented legitimate disagreements or were due primarily to differences in target populations, follow-up time or other factors, Smith and colleagues converted all four reviews to the same criteria.

They used the scenario outlined in the UK Independent Review, which examined the effect of screening for 20 years — from age 50 to 69 years — on breast cancer mortality among individuals aged 55 to 79.

After converting all four reviews to the same scenario, the variation in absolute benefits of breast cancer screening between the four reviews decreased from 20-fold to 2.5-fold, Smith said.

The disparity between reviews with regard to the number of women needed to screen to prevent one cancer death — which ranged from 111 to 2,000 prior to the re-examination of data — decreased to 96 to 257 after standardization to the same scenario.

“Once you standardize it to a common population, a common screening scenario and a common duration of follow-up, these differences really become not so significant — certainly not enough to question the value of mammography over a lifetime of screening,” Smith said. “The debate about the value of mammography screening is not likely to fade away, and there are real, reasonable differences of opinion about various aspects of screening. However, we hope these findings reassure clinicians and the public that there is little question about the effectiveness of mammography screening, which should continue to play a very important role in our efforts to prevent deaths from breast cancer.”

For more information:
Smith RA. Abstract #S1-10. Presented at: San Antonio Breast Cancer Symposium; Dec. 10-14, 2013; San Antonio.

Disclosure: Smith reports no relevant financial disclosures.