Thursday, June 19, 2014

Valuing Lives: Breast Cancer Screening

Government agencies and healthcare professionals often issue guidelines that the layperson can find confusing and counterintuitive. In addition, different assemblies of experts can come to opposite conclusions. One area that has been in dispute for decades involves the efficacy of using mammograms as a tool to test for breast cancer. No one disagrees with the fact that mammograms are capable of detecting breast cancers before they can be detected in other ways—so what is the problem?

Paul Taylor has provided a summary of what is at issue in an article in the London Review of Books: Breast Cancer Screening. He provides this background:

"Mammography has been used as a tool for screening healthy women since the 1960s, but its use was stepped up dramatically in the 1980s. In the US, Ronald Reagan’s announcement in 1987 that his wife, Nancy, had been diagnosed with cancer following a screening mammogram made a significant impact; by 1991, most states had passed legislation requiring insurance companies to cover the cost of mammograms. In the UK, a national screening programme was set up in 1988. Even in the early days there were worries."

One of the issues associated with mammography is the difficulty in extracting an accurate assessment from a complicated image.

"All mammograms are read and reread in an effort to get accurate results, but mammography is not, as most radiologists would admit, a perfect test. Only around a third of the cancers detected in women over fifty are found at screening. Studies of cancers detected in women who had been given the all-clear at screening regularly find that 20 or 30 per cent of those cancers were missed on the screening mammogram. For now, however, we don’t have a better test."

Taylor indicates the controversy over screening is not due to the inaccuracy of the mammogram, or the monetary cost, or any unnecessary exposure to radiation that might be occurring. Rather, it is based on the fact that not all tumors that can be detected will ever evolve into a threat to life.

"One of the difficulties with breast cancer screening is that about 20 per cent of the cancers detected are ‘ductal carcinoma in situ’ or DCIS, a precursor of cancer rather than the thing itself. In some cases – it is hard to say exactly how many – these cells will not acquire the capacity to spread and do harm. A woman might benefit from having her DCIS detected before it has acquired the capacity to hurt her, or she might end up being subjected to harmful treatment for a disease she doesn’t yet have, and may never have."

If people with this class of tumor are treated aggressively for cancer their quality of life will be diminished and their lives may be threatened unnecessarily by the treatment. Taylor refers to the capturing of this class of tumor in screening as "overdiagnosis."

"When a woman has breast cancer but doesn’t need to know that she has it, that is over-diagnosis. Oncologists talk about a tumour’s ‘sojourn time’, the period of time between the point at which it becomes detectable at screening and the point at which it would be detected anyway. In some cases of breast cancer the sojourn time can be very long, so long that the cancer will never be detected. Any woman who dies of another cause during the sojourn time would have been better off not being screened. For such women, a positive mammogram means they are likely to receive harmful and mutilating treatment that can only reduce their life expectancy."

The issue then is to assess the number of women who will benefit from screening and compare that to the number of women who will be harmed by screening, and then—somehow—decide whether the gain exceeds the unnecessary pain. Unfortunately, that is not a simple task. Clinical trials to assess such things are very expensive and take decades to complete.

"Trials of mammographic screening are expensive: tens of thousands of participants are required (since only 7 per cent of them will develop cancer), and all of them must be monitored for ten or even twenty years. So far, there have been only eight trials. The first was in New York in 1963. There followed trials in Malmö, starting in 1976; the Two Counties trial, also in Sweden, in 1977; Edinburgh in 1978; Canada in 1980; Stockholm in 1981 and Göteborg in 1982; and the UK Age trial, which looked at younger women, in 1991."

Taylor discusses these early trials and the issues associated with them. They all seem to be compromised in one way or another. This has led some to dismiss them as unreliable while others have tried to extract information as best they can from whatever data they can find. Unfortunately they have arrived at quite different conclusions.

"The contrast is stark. Whereas Gøtzsche estimates that one cancer in three is over-diagnosed, Duffy suggests the figure is only one in 28."

The utility of mammography in screening is also a function of time. As progress is made in treating breast cancer, the importance of early detection by general screening is diminished. Also, as the accuracy of mammography increases, the risk of over-diagnosis increases as well. These developments may have rendered the old clinical trials obsolete. The ubiquity of screening today renders a modern clinical trial almost impossible.

It seems that each nation tries to come to its own conclusion on the screening issue.

"In February this year, the Swiss Medical Board published a report recommending that no new mammography screening programmes be introduced and that the existing ones be wound up."

The UK has come to a conclusion and has provided some numbers, based on their assessment, to try to explain the issues to its citizens.

"If you are told that screening reduces breast cancer deaths by 20 per cent you will probably feel that is a substantial reduction. But the same information, put differently, seems less impressive: the 20 per cent reduction means that the likelihood that you will die of breast cancer while in the screened age-group drops from 2.1 per cent to 1.7 per cent."

This assessment also concludes that about 20 percent of the cancers detected in screening are of the over-diagnosed variety.

An impressive chart is also provided to inform the public. It includes this information:

"If a group of 200 women all have breast screening every three years from the age of 50 to 70, then by the time they are 80:

"15 are diagnosed with breast cancer. 8 are diagnosed and survive. They would have survived without screening. 3 are diagnosed with cancer that would never have become life-threatening. 3 die of breast cancer even though they were screened. 1 has her life saved from breast cancer. She would have died without screening."

Faced with this assessment, how should policy be determined? Taylor suggests that the deciding factor will not be the conclusions of politicians or medical experts. The women who feel affected by the issue will decide what policy will be—and they are in favor of screening.

"In the end the question of whether or not breast screening should continue will be determined by politicians’ assessments of what the public wants. And the public seems to want screening. When asked about over-diagnosis, most women say that rates even as high as 30 per cent wouldn’t stop them having a mammogram."

On the men’s side, there is an analogous controversy about screening for prostate cancer. The numbers and methods are different, but the issues are the same.

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