Screening Mammography Benefits and Harms in Spotlight Again

Nick Mulcahy

April 02, 2009

April 2, 2009 — A new study adds to the debate about how the benefits and harms of screening mammography are presented to the public and provides new estimates of the absolute numbers of breast cancer deaths prevented by the screening.

The authors argue that the public is not currently presented with a balanced view of the screening, with potential benefits overemphasized and potential harms rarely discussed.

The study was published April 2 in the open-access online journal BMC Medical Informatics and Decision-Making.

Repeated screening mammography starting at age 50 saves about 1.8 lives over 15 years for every 1000 women screened, according the study authors.

They calculated that the absolute death risk from breast cancer without any screening is about 1% over those 15 years. "This means the survival percentage among women aged 50 to 60 who are not screened is 99%," said lead author John D. Keen, MD, senior attending radiologist at the John H. Stroger Hospital of Cook County, in Chicago, Illinois.

The survival percentage among women aged 50 to 60 who are not screened is 99%.

These statistical facts about mammography are in sharp contrast with what is most publicized about this screening, which is "mammography saves lives," suggested Dr. Keen.

"From a consumer perspective, the screening-mammography discussion in the United States is rather paternalistic and 1-sided," said Dr. Keen in an interview with Medscape Oncology.

What's needed is a balanced presentation of the facts to patients, which should include mention of the absolute benefits associated with screening and a discussion of potential harms, such as false positives, anxiety, unnecessary biopsies, and overdiagnosis, emphasized Dr. Keen. "This is not occurring now," he added.

However, a critic of the study says that its statistical methods are questionable and its findings debatable. Furthermore, the paper "labors an obvious point" about preventive medicine, says the critic.

"One has to apply an intervention to large numbers of healthy subjects in order to benefit the few who are unlucky enough to develop the disease," writes Stephen W. Duffy, MD, from the Cancer Research Center, in London, the United Kingdom, in an editorial that accompanies the study.

Dr. Keen's response to that criticism was that the "public has a right to know what the statistics are regarding mammography screening so that individuals can make informed decisions about participating."

Another cancer-screening expert approached by Medscape Oncology joined the study authors in calling for a better presentation of the facts about screening mammography. "We all have to do a better job to best inform the public about the benefits and harms of screening mammography," said Bob Smith, PhD, director of cancer screening at the American Cancer Society, in Atlanta, Georgia.

We all have to do a better job to best inform the public about the benefits and harms of screening mammography.

The new study appears at a time of increased public scrutiny of screening mammography. The New York Times recently covered the story of a public-health leaflet on screening mammography in the United Kingdom and the public outcry that resulted because the leaflet failed to mention the potential harms of screening. The controversy had been reported a month earlier by Medscape Oncology.

Findings Disputed and Supported

In their new paper, Dr. John Keen and his brother and coauthor James E. Keen, DVM, PhD, associate professor of epidemiology at the University of Nebraska, in Lincoln, analyzed the claim that "mammography saves lives" by calculating the life-saving absolute benefit of screening mammography in reducing breast cancer mortality in women aged 40 to 65.

To make this estimate, the authors used a variety of existing data sources, including randomized trials and SEER data on breast cancer mortality.

Dr. Duffy points out in his editorial that this study is one of modeled results and not empirical observation. "The accuracy of the figures arrived at is questionable," he writes.

Dr. Duffy notes that empirical data from 2 different randomized trials indicate that 3 or 2.1 breast cancer deaths, respectively, are prevented by repeatedly screening 1000 women. So the figure of 1.8 prevented deaths used in the current study is low, he says.

However, in a second editorial that accompanies the new study, Michael Retsky, PhD, from Harvard Medical School, in Boston, Massachusetts, says that the 1.8 deaths prevented by screening are consistent with other results.

"The crux of the problem is that early detection from screening only benefits approximately 1 in 1000 participants. That ratio appears in many trials and in the Keen and Keen paper," he told Medscape Oncology. The numerical results of the new study "are consistent with my knowledge and experience," he writes.

The crux of the problem is that early detection from screening only benefits approximately 1 in 1000 participants.

Dr. Duffy also disputes the new study's estimate that only 4.3% of screen-detected breast cancer cases have their lives saved as a result of screening. "This is clearly at odds with the experimental evidence. In the Swedish Two-County Trial, 141 breast cancer deaths were prevented — 15% of the 928 screen-detected cancers," he writes.

However, Dr. Keen countered this criticism by pointing out that a recent review of Swedish screening trials had turned up "multiple problems" and that his study findings predicted the results from the most recent AGE trial in England. He also said that the new study's data are derived from American sources and are "therefore more likely accurate for women in the United States."

Challenging a Popular Perception; A Question About Patient Knowledge

The finding that so few (only 4.3%) screen-detected breast cancer cases have their lives saved as a result of screening seems counterintuitive, write the authors. There is a widely held view that breast cancer is a uniformly progressive disease and that it is rarely curable unless caught early, they write. But in reality, breast cancer is a heterogenous disease, and it may be systemic from the start or it may never metastasize, they say.

In effect, this finding about lives saved by mammography challenges "the popular perception that earlier detection through mammography helps most patients with screen-detected breast cancer," write the authors.

In his editorial, Dr. Retsky agrees that the new study findings "defy" some "well-established viewpoints." He also believes that screening has considerable limitations.

"Personally, I am doubtful that imaging-based early detection will ever make dramatic improvements in breast cancer mortality. Rather, I think improved therapies are needed," Dr. Retsky commented to Medscape Oncology.

The limitations of mammography screening make the discussion of the harms all the more important, suggested Dr. Keen.

"Authorities acknowledge that screening-mammography harms include a 30% increase in overdiagnosis and overtreatment, delayed diagnosis, and radiation-induced cancers," write the authors. They also cite a survey that showed that physicians discuss these potential harms only 7% of the time when talking to women before a baseline mammogram (Health Expect. 2008;11:366-375).

However, the American Cancer Society's Dr. Smith believes that women are not uninformed about most of the potential harms of screening mammography.

"The authors imply that women are naive about these harms. They aren't. A study by Schwartz [et al. BMJ. 2000;320:1635-1640] observed that 90% of women said they were fully aware of false-positive mammograms and biopsies, and this awareness remained just as high when women who had experienced a false positive were compared with those who had not," he told Medscape Oncology.

The new study authors suggest that substantial business concerns influence the likelihood that consumers will receive "professional even-handed advice" about the harms and benefits of mammography. "Fundamentally, cancer testing is a business," they write.

Breast radiologists, equipment manufacturers, and advocacy groups all have conflicts of interest arising from financial incentives. "These incentives create the temptation to exaggerate the benefits and dismiss the harms of screening for breast cancer when advertising to the public," the authors write.

The study authors have disclosed no relevant financial relationships. Dr. Retsky disclosed that he has a patent application for a therapy for early-stage breast cancer.

BMC Med Inform Decis Mak. 2009;9:18, 19, 20.

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