The results of research on screening for breast cancer with mammography can be presented in ways that make the benefits seem larger or smaller
Similarly, the benefits can be described as avoiding deaths from breast cancer or avoiding deaths from any cause
Part of the debate about the benefits of screening mammography may be related to the differences in the way that the benefits of screening are presented.
The recent recommendations on breast cancer screening released by the Canadian Task Force on Preventive Health Care have created a storm of controversy about the benefits and harms of mammography, particularly for women aged 40 to 49. The recommendations suggest routinely screening women aged 50 to 74 every two to three years with mammography. The recommendations suggest not routinely screening women aged 40 to 49.
There appear to be four broad areas of disagreement about the benefits and harms of mammography:
- how to present the results of the randomized controlled trials of mammography to women, clinicians and policy makers
- how much importance to attach to the results of non-randomized studies
- whether the early detection of a breast cancer in a woman means that mammography saved her life
- how to balance the benefits and risks of mammography when making a decision about whether or not to undergo mammography.
We will be posting a series of brief articles about each of these topics between now and the winter holidays. This article focuses on the evidence for screening women between the ages of 40 to 49.
Differences between the Canadian Task Force on Preventive Health Care and the Canadian Breast Cancer Foundation
The Canadian Task Force on Preventive Health Care (Task Force) notes that slightly more than 2000 women “would need to be screened … about once every two years over a median of about 11 years to prevent a single death from breast cancer.
A press release from the Canadian Breast Cancer Foundation (CBCF) responding to these recommendations quotes their CEO, Sandra Palmaro, as saying “scientific evidence demonstrates that earlier detection and diagnosis can save lives among women 40-49 by at least 25%.”
Are the statements from these two organizations compatible?
There appears to be two areas of disagreement. The first is about the magnitude of the benefit of screening – 2018 women needing to be screened to prevent one breast cancer death according to the Task Force, versus a 25% increase in lives saved according to the CBCF. The second is about how the benefit of screening is measured – a single death from breast cancer prevented from the Task Force, versus lives saved from the CBCF.
The difference between the relative and absolute benefits of screening
The estimates of the magnitude of the benefit from screening used by these two organizations may not be as far apart as it first appears. The Task Force based its estimate on the benefit of screening from 8 studies involving almost 350,000 women. Study participants were randomized to either undergo screening mammography or not. These studies found that the risk of death from breast cancer was decreased by 15%, which isn’t that much different from the 25% risk reduction quoted by the CBCF. It is important to clarify where the estimates of benefit come from.
The difference is that the Task Force presented its figures as absolute risk reductions, while the CBCF presented its figures as relative risk reductions.
The infographic below uses the Task Force data to explain the difference between describing the benefits of mammography in relative and absolute terms.
What is an absolute risk reduction?
After 11 years of screening, 0.32% (or 1 in 313) of women in the randomized trials who were not screened, had died of breast cancer. If the risk of dying from breast cancer is decreased by 15% with screening (which is what the Task Force suggests), this means that 0.27% (or 1 in 370) of women who are screened for 11 years would die from breast cancer. The absolute difference between these two numbers is just 0.05%. Another way of saying this is that about 2000 women will need to be screened to prevent one death from breast cancer, 11 years after beginning regular screening.
The CBCF states that the relative benefit from screening is actually 25% rather than the15% that the Task Force suggested. This would mean that 0.24% (or 1 in 417) of women who are screened would die from breast cancer, which would require that 1250 women need to be screened to prevent one death from breast cancer (rather than the 2100 estimated by the Task Force). In either scenario, well over 1000 women need to be screened to prevent one death from breast cancer.
When talking about the benefits of any medical test or treatment in health care, it is important that people are informed about the absolute benefits. Some may argue that a focus on absolute benefits is biased against preventive interventions because the absolute benefits of prevention are usually small. For example, the absolute benefits of wearing a seat belt while driving are likely smaller than the absolute benefits of mammographic screening, yet it is illegal not to wear a seatbelt. However wearing a seatbelt causes virtually no harm (we will address the potential harms of mammography in a subsequent article) and the cost of installing seatbelts in cars and wearing a seatbelt is small.
Deaths from breast cancer versus death from any cause
One wouldn’t expect a screening programme for breast cancer to decrease death from any cause other than breast cancer. Therefore, it makes sense to present the benefits of a mammography screening program in terms of the number of deaths from breast cancer prevented.
At the same time, women contemplating mammography screening for breast cancer may be interested in knowing whether having a mammogram can decrease their risk of dying from all causes.
The Task Force described the benefits of mammography in terms of deaths from breast cancer. However, this doesn’t provide any information about the impact of screening on women’s risk of death from any cause. Breast cancer is a common cancer in women, and most women aged 40 to 49 are not yet at a high risk of heart disease and other “competing” causes of death. Therefore, one might think that their risk of death from any cause would be decreased by mammography. However, that is not the case – the Task Force review notes that there was no difference in the likelihood of death in women who were screened and those who were not screened in the 350,000 women in the randomized trails of mammographic screening in women 40-49 years of age.
The CBCF press release mentions deaths from breast cancer and deaths from any cause interchangeably, even though they are different. When presenting the benefits of screening programs to women, it is important to be very clear about whether one is talking about deaths from breast cancer or deaths from any cause.