The Daily Telegraph reported yesterday that “Breast cancer screening ‘works and we should move on” – “Women should undergo breast cancer screening because it halves the chance of them dying of the disease, according to a new study that claims to draw a line under the controversy”.
So simple? No!
Here’s the paper. It’s a case control study. So you look back at women who have died of breast cancer, and take a comparable group of women, but who didn’t die of breast cancer. Then you find out if one group went for breast screening more than the other. The researchers found that there was “an average 49% reduction in breast cancer mortality for women who are screened.”
I don’t think this means that we have proven whether breast screening works or not. The best way to do that is through a randomised controlled trial. In this type of trial, a comparable group of women are sorted into two random groups, one group is given breast screening, the other are not, and the end results are compared.
I think this is better, mainly because it reduces bias best. Bias – skewing of your results – can happen when, for example, the healthiest people (and least likely to die of breast cancer) attend most for screening; it will look as though screening made them live longer, but in fact, they would have lived longer anyway.
Many case control studies will try and account for this by controlling mathematically for these types of bias. But it may not be easy to do this fairly.
We can see this with this example, which I have taken from Karsten Jorgensen’s work on this. The Malmo trial (in Sweden) contained over 21,000 women in each of the screening and non screening groups, aged over 45, and followed for a mean of 8.8 years. The data were assessed as a RCT, and the difference in mortality between the two groups was negligible. But the same data was then assessed as though it was a look-back, case control study, and found an odds ratio of 0.42.
The Malmö mammographic screening trial assessed as a randomised trial (Janzon and Andersson, 1991)
|Invited (n=21088)||Controls (n=21195)|
|Breast cancer deaths||63 (31 were non- participants)||66|
|Relative risk||0.96 (95% CI 0.68–1.35)|
The Malmö trial assessed using a case-control design Janzon and Andersson, 1991)
|Living controls (random sample)||Women dead from breast cancer||Total|
|Participation in screening|
|Crude odds ratio||0.46|
|Adjusted (matching for age) odds ratio||0.42 (95% CI 0.22–0.78)|