By Damaris Christensen
Mammography Under the Microscope
Two reports zero in on screening's effects
By Damaris Christensen
The number of deaths each year due to breast cancer has dropped by 21 percent since 1975, a decrease that research groups attribute in about equal measure to wider use of mammograms and to advances in therapy, according to a recent study.
The study, published in the Oct. 27, 2005, New England Journal of Medicine, adds to a long-simmering debate over the benefits of mammograms. Organizations that offer recommendations—such as the American Cancer Society, the U.S. Preventive Services Task Force and the National Institutes of Health—have been nearly unanimous in recommending regular mammograms for all women beginning at age 40. But some researchers, including Donald A. Berry, a biostatistician at the University of Texas M. D. Anderson Cancer Center in Houston who led one of the groups participating in the new report, have questioned for many years whether the benefit of screening is as great as some earlier studies reported—and whether the benefit outweighs mammography's risks.
In the new study, researchers calculated that the occurrence of breast cancer, and thus the number of deaths attributable to breast cancer, would have continued to rise from 1975 to 2000 without the effects of screening and better treatment. The proportion of the benefits that was attributed to screening varied in seven research groups' models, from a low of 28 percent to a high of 65 percent, with the remaining benefit due to better therapy. In other words, screening accounted for anywhere from a 7 percent to a 23 percent reduction in deaths, while adjuvant therapy—such as the addition of hormonal or other therapy after surgery—accounted for a 12 percent to 21 percent drop. The decline in deaths can't be explained by either screening mammography or improvements in therapy alone, the researchers concluded.
The results show that the health benefits seen in carefully controlled trials do in fact translate to the real world. "There's pretty clear evidence that [for adjuvant therapy], the benefits we've seen in clinical trials translate almost perfectly into clinical practice," says Berry. While the magnitude of the benefit due to mammograms varied significantly between models, Berry says, "Some of the controversy is gone. There's more data [indicating that] there's a benefit." Nonetheless, Berry says he continues to question whether the benefit surpasses risks such as unnecessary treatment, biopsies or repeat mammograms.
The study also shows that "early detection [of breast cancer through screening] and advances in therapy have brought about important changes in mortality from breast cancer," says Robert Smith, director of cancer screening at the American Cancer Society (ACS) in Atlanta. The organization estimated that 211,240 new invasive cases of breast cancer would be diagnosed in women in the United States during 2005.
"What's interesting about this study is that they are following a general population and making an attempt to see [the effects of screening and adjuvant therapy] in the real world," says Carolina Hinestrosa, the executive vice president for programs and planning at the National Breast Cancer Coalition, a breast cancer advocacy group based in Washington, D.C. However, Hinestrosa says, screening is not the cure, and she suggests that there might be better ways to reduce breast cancer deaths, such as improving women's access to health care and treatments for metastatic disease.
Moreover, she adds, "It's disconcerting for consumers that the range of the benefit [for mammography screening] is so wide, because so much time and effort is put into public health messages about screening." Smith of the ACS, however, noted that the wide range of estimates in the report's results is to be expected, due to the diversity in the methods used by the seven research groups' models.
In the same journal issue, another team of researchers reported the results of a study comparing digital mammograms—electronic images stored on a computer—to regular mammograms, which are images made directly on film. About 43,000 women received both digital and film mammography and a total of 335 cases of breast cancer were diagnosed within 15 months after the baseline screening.
In the entire study population, film and digital mammograms were comparable in detecting cancers. However, digital images were more likely to detect cancers in women who were younger than 50, not past menopause, or who had very dense breasts.
"Given that screening with digital mammograms finds more cancer, it likely saves lives," says radiologist Etta D. Pisano of the Lineberger Comprehensive Cancer Center at the University of North Carolina at Chapel Hill. However, the study did not look at mortality from breast cancer, which would have required the researchers to follow many more women for at least a decade.
Digital mammography machines cost from one and a half to four times as much as conventional ones, but digital images are easier to store and retrieve, and to include in patients' electronic medical records, says Pisano.
There was no difference in detection between film and digital mammography for women who were postmenopausal, older than 50 or without dense breast tissue. That means that those women shouldn't fear that film mammography isn't up to par if they don't have access to digital mammography, Smith says.