Breast Cancer Screening Questioned Yet Again

Zosia Chustecka

January 25, 2013

The independent review of breast cancer screening in the United Kingdom, ordered by the national cancer director after a fierce debate over the benefits and harms of mammography, was to have laid the controversy to rest.

However, the conclusions of that review are being questioned by Michael Baum, MB ChB, FRCS, ChM, an eminent professor emeritus of surgery at the University College London, United Kingdom. He outlines his arguments in a personal view published online January 23 in BMJ. A footnote indicates that Dr. Baum has been an outspoken critic of mammographic screening for some years.

Although Dr. Baum was involved in setting up the breast cancer screening program in the United Kingdom 20 years ago, he recently discussed his change of heart in a radio interview on BBC4. "As the evidence changes, one should, as a clinical scientist, be prepared to change one's position," he said.

"I have watched with increasing alarm as evidence has accumulated that suggests the initial estimates of benefit were exaggerated and the initial estimate of harm was, frankly, ignored. What has gone wrong is that we would never have predicted how many of these cancers detected at screening lack the potential to threaten the woman's life,' Dr. Baum said.

Challenge to Conclusions

The independent review was conducted by an independent panel of experts, headed by Sir Michael Marmot, MBBS, MPH, PhD, professor of epidemiology and public health and director of the Institute of Health Equity at University College London (Lancet. 2012;380:1778-1786).

On the basis of clinical trials, the panel estimated that 43 breast cancer deaths can be prevented for every 10,000 women 50 years of age who are invited to screening for the next 20 years. However, they also estimated that overdiagnosis (i.e., the diagnosis of a cancer that would never cause symptoms or death during the person's life) would occur at a rate of 129 per 10,000 women.

The review concluded that the breast screening program in the United Kingdom extends lives and that, overall, the benefits outweigh the harms.

Dr. Baum challenges this conclusion. If death caused by treatment for breast cancer is included in the calculations, the harms from breast cancer screening outweigh the benefits, he says.

Dr. Baum focuses on the harms to the women who are overdiagnosed by the screening process. Four fifths of these women would receive radiotherapy, which increases their risk of dying from ischemic heart disease (by 1.3%) and lung cancer (by 2.0%). "Adding this to the all-cause mortality, I crudely estimate that an addition of 1 to 3 deaths might be expected from other causes for every breast cancer death avoided."

In addition, Dr. Baum criticizes the clinical trial data that were used in the review — these come from trials that were conducted 20 to 25 years ago and are "irrelevant to the modern practice of medicine," he says.

Improvements in the treatment of breast cancer in recent years will have the effect of reducing the benefits of screening, he notes. In addition, recent data from observational studies suggest that the overdiagnosis rate is higher (around 50%) than that seen in clinical trials (estimated to be 19% in the independent review).

This is important because it could alter how women 50 to 70 years of age feel about going for regular (every 3 years) mammograms, which are covered by the National Health Service in the United Kingdom.

Also published online January 23 in BMJ was a qualitative study of women's views on overdiagnosis in breast screening. It found that an overdiagnosis rate of 50% would make women think twice about going, whereas lower estimates (up to 30%) left many women committed to screening.

In an accompanying editorial, Dr. Cliona Kirwan, a clinical scientist in surgical oncology at the National Institute for Health Research, notes that the harms from overdiagnosis last "a life time."

She explains that the 19% rate of overdiagnosis estimated in the independent review is "higher than has been hitherto communicated to women."

Nonetheless, Dr. Kirwan says that the experts "correctly conclude that breast screening is worthwhile."

"As professionals, we need to help women weigh the benefits and harms of screening by providing them with up to date and transparent information, including information on the uncertainties," she writes.

BJM. Published online January 23, 2013. Personal View, Abstract, Editorial