Mammography has traditionally been presented to women as a bit of a ‘no-brainer’ – essentially you’d have to be lacking a brain not to submit to regular breast cancer screening. Over the last few years, though, there has been the publication of several scientific articles which highlight the fact that mammography simply does not deliver on its promise, and the decision regarding whether to undergo mammography is by no mean clear cut. I’ve written about these issues more than once, but here’s a short summary:
- You need to screen lots and lots of women to save one from dying from breast cancer (see below).
- For each woman who will be saved of dying from breast cancer, many more will be subjected to ‘false alarms’ and/or unnecessary further investigation or treatment. Some of these women will turn out not to have a problem at all. Some, will turn out to have forms of breast cancers that would not have bothered them of the natural course of their lives. This latter scenario is described as ‘overdiagnosis’.
In a recent article in the British Medical Journal, Professors Steve Woloshin and Lisa chwartz take to task the world’s largest breast cancer charity – Susan G Komen for the Cure – for what they see as its one-sided and biased promotion of mammography.
The authors use this advertisement to illustrate their point.
The first point the authors make is that screening does not guarantee a woman will survive breast cancer, as is suggested. They draw our attention to data which shows that a 50-year-old woman’s chances of dying from breast cancer over 10 years are 0.53 per cent. With regular mammography, that falls to 0.46 per cent. The drop is therefore 0.07 per cent.
We can calculate the number of women who would have to be screened in order to save one life by dividing that figure into 100. The number is 1,429. I think many people, including doctors, would be shocked to know just how ineffective mammography really is.
As you can see, the authors also take issue with the charity’s use of ‘5-year mortality’ data. 5-year mortality is the percentage of people alive five years after diagnosis. However, by virtue of the fact that mammography detects cancer earlier, 5-year survival will be better here than if tumours were detected some years later, even if overall survival is not improved (or is worsened). Importantly, though, 5-year survival statistics tell us nothing about mammography’s ability to prevent death. As we know, on this count, it performs very badly.
To illustrate just how misleading 5-year survival statistics can be, the authors use this example:
“…imagine a group of 100 women who received diagnoses of breast cancer because they felt a breast lump at age 67, all of whom die at age 70. Five year survival for this group is 0%. Now imagine the women were screened, given their diagnosis three years earlier, at age 64, but still die at age 70. Five year survival is now 100%, even though no one lived a second longer.”
The authors go on to make the point that overdiagnosis inflates survival statistics. Essentially, mammography will pick up a lot of people who have cancer who then survive it. But many of these people will have survived it anyway, even in the absence of mammography. This fact makes mammography look more effective than it is in reality.
The authors then turn their attention to the harms of mammography, which the Susan G Komen for a Cure charity does not even mention. According to the authors, 20-50 per cent of screened women will experience at least one ‘false alarm’ the requires at least a biopsy. That’s 286-714 women to balance the one woman who did not die from breast cancer because of screening. And we also get to learn that for each woman who’s life is saved, because of overdiagnosis anywhere from 2 to 10 other women will undergo needless surgery, radiotherapy or chemotherapy for cancers which would not have harmed them.
The authors close with the following damning (but I think, accurate) paragraph:
“Women need much more than marketing slogans about screening: they need—and deserve—the facts. The Komen advertisement campaign failed to provide the facts. Worse, it undermined decision making by misusing statistics to generate false hope about the benefit of mammography screening. That kind of behaviour is not very charitable.”
References:
1. Woloshin S, et al. How a charity oversells mammography. BMJ 2012;345:e5132 (Published 2 August 2012)
