Tuesday October 30 2012
Breast cancer screening has both benefits and risks
The media reports that breast cancer screening is "harming thousands", with The Guardian claiming "breast cancer screening causes more damage than previously thought".
These headlines have been prompted by the findings of an independent panel that reviewed the benefits and harms of breast cancer screening in the UK. The panel was set up to examine the issue, as there has been ongoing debate about the balance of benefits and harms from breast screening, both in the UK and internationally.
The panel was commissioned by the national cancer director for England and Cancer Research UK, and included experts in medical epidemiology (the study of disease patterns and their causes), statistics, breast cancer diagnosis and treatments, as well as a patient advocate. The panel reviewed the published evidence and submissions from experts who have been involved in the debate.
Breast cancer screening programmes are unable to predict the individual outcome (prognosis) if a person is found to have breast cancer. In some cases, the cancerous cells can spread rapidly, posing a significant risk to health. In others, the cancerous cells are much less aggressive, so the cancer has no impact on life expectancy. This uncertainty leads to what the researchers term ‘overdiagnosis’ – where women are given treatment and are exposed to all of its harms, but receive no benefit.
Based on the evidence available, the panel estimated that for every 10,000 women invited to screening from the age of 50 for 20 years:
- 681 breast cancers will be diagnosed
- 129 of these diagnoses will be overdiagnosed
- 43 deaths from breast cancer will be prevented
Therefore, for every death prevented, there are estimated to be three cases of overdiagnosis. This means that, in around 307,000 women aged 50-52 who are invited to screening each year in the UK, about 1,320 deaths from breast cancer will be prevented and about 3,960 women will be overdiagnosed. The panel noted that there is uncertainty around these estimates, and that they should be seen as approximate.
Although the headlines focus on the harms, the panel concluded that, overall the UK breast cancer screening programme provides significant benefit and should continue to be offered.
The decision of whether to attend for breast screening when invited ultimately rests with each individual woman.
The panel emphasised the need for clear communication so that women can make informed decisions. Hopefully, the review’s findings will provide clear and understandable estimates of the potential benefits and harms associated with breast cancer screening that can be used for this purpose.
How did the media report the review?
While the overall reporting on the findings of the review was accurate, most of the headlines focused on the negative findings – the problem of overdiagnosis.
These headlines do not provide balanced coverage of the findings of the panel and seem to undermine the efforts of those involved in breast cancer screening.
What is important is to communicate clearly both the potential benefits and risks to allow women to make informed decisions.
Why was the independent review needed and who carried it out?
There has been ongoing heated debate about the balance of benefits and harms from breast screening, both in the UK and internationally.
In light of this, the Independent UK Panel on Breast Cancer Screening was commissioned to review the benefits and harms of breast screening. The panel included experts in epidemiology, statistics, breast cancer diagnosis and treatment, as well as a patient advocate. Cancer Research UK provided support for the panel and additional funding was provided by the Department of Health.
An article in the peer-reviewed medical journal The Lancet summarises their conclusions, and is available free online.
A summary of the research is also available on the Cancer Research UK website.
What did the review involve?
The overall aim of the review was to carry out an up-to-date assessment of the benefits and harms associated with population breast screening programmes. The panel assessed published research, as well as spoken and written evidence from experts who have been contributing to the debate. The process aimed to be a rigorous review of the evidence by an independent panel, but not a formal systematic review.
The panel’s assessment of the quantitative benefits of breast cancer screening on the relative risk of death from breast cancer was based on evidence from systematic reviews of randomised controlled trials (RCTs).
They also reviewed evidence from observational studies.
Using the estimates of relative benefit they obtained and information about the breast screening programme in the UK, they estimated the absolute benefit of breast cancer screening. That is, they worked out how many deaths are prevented for every 10,000 women invited for screening.
They assumed that women would not receive any benefit for the first five years of screening, but that the reduction in mortality risk would continue for 10 years after screening ended.
They also considered the harms of breast cancer screening, mainly overdiagnosis. This is where some women who are diagnosed as having breast cancer by screening and are treated are subsequently found to have cancers that would never have caused symptoms in their lifetime. This means that their treatment turns out to have been unnecessary and they would not have died from breast cancer.
The panel said that the ideal study to identify the rate of overdiagnosis does not exist. This, they say, would be a study comparing the number of breast cancer cases in women screened for 20 years and followed up until death, with the number of cases in a comparable unscreened population.
Therefore, instead they estimated overdiagnosis based on the available evidence from RCTs and observational studies. There are different ways to calculate overdiagnosis, and the panel felt that there was no single best way to do this.
The panel selected what they believed to be the two most useful approaches for their calculations.
What is the effect of breast cancer screening on relative risk of death from breast cancer?
The panel’s main conclusions on the effect of breast cancer screening on risk of death from breast cancer were based on the data from a Cochrane Collaboration’s systematic review and meta-analysis of randomised controlled trials (RCTs).
This review found that after 13 years of follow-up, breast cancer screening reduced the risk of death from breast cancer in women invited to screening by 20% compared with the controls (relative risk [RR] 0.80, 95% confidence interval [CI] 0.73 to 0.89). The panel noted that other systematic reviews have had different approaches (for example, including different trials) but that they have generally given a similar estimate of the relative risk – ranging from around 0.77 to 0.85.
The panel also looked at estimates of the benefits of breast cancer screening from observational studies, as the RCTs were carried out 20 to 30 years ago, and there have been improvements in breast cancer treatment since then. The panel noted that, although because of the nature of these studies they could overestimate the benefits of screening, their results also suggested a reduction in deaths from breast cancer with screening.
The panel concluded that the best evidence on the benefits of screening comes from the RCTs, and that their estimates of benefit probably still apply, although they do have limitations, and there is statistical uncertainty around the estimates of effect.
What is the absolute benefit of breast cancer screening in terms of deaths from breast cancer?
In the UK, where women are first invited to breast screening at the age of 50, and continue to be invited for 20 years, the panel estimated that 43 breast cancer deaths could be prevented for every 10,000 women invited for screening. This is equivalent to one breast cancer death being prevented for every 235 women invited for screening. Among women who actually attend screening, one breast cancer death is prevented for every 180 women.
What about the harms of breast screening?
The major harm considered by the panel was overdiagnosis – as described above. These women receive treatment, such as the surgical removal of a portion of breast tissue, chemotherapy or radiotherapy and are exposed to the potential side effects, but they do not experience any potential benefits from reduced risk of death from breast cancer.
The panel considered that the best available estimates of overdiagnosis came from three trials in Sweden and Canada where women in the control groups were not invited for screening after the study. Pooling these trials found that in the time period where active screening was taking place, 19% of cancers diagnosed among women invited for cancer screening (95% confidence interval 15 to 23%) were estimated to be overdiagnoses.
The panel noted that these figures were not tailored to the UK screening programme, or a 20-year screening period. However, the evidence from the observational studies supported that overdiagnosis occurs.
On the basis of the 19% overdiagnosis figure, they estimated that for every 10,000 UK women aged 50 years invited to screening for the next 20 years, 129 cases of breast cancer (invasive and non-invasive) would be overdiagnosed.
What were the panel’s overall conclusions?
The panel concluded that breast cancer screening reduces deaths from breast cancer, but that overdiagnosis does occur. Based on their estimates of the absolute benefits and risks of breast cancer screening, they estimated that:
- for each woman with breast cancer whose death is prevented by screening, about three women would be overdiagnosed and treated
- of the 307,000 women aged 50–52 years who are invited to begin screening every year, just over 1% would be overdiagnosed with cancer in the next 20 years
However, the panel said that there is substantial uncertainty in the estimates due to the limitations of the studies available, and that the figures should be seen as an approximate guide.
What were the panel’s recommendations?
The panel recommended that:
- the NHS Breast Screening Programme should continue, as it provides significant benefit – if the estimates of the researchers are correct, the programme saves around 1,320 lives each year
- transparent and objective information should be made available to women invited to screening so that they can make informed decisions about breast cancer screening
What does it all mean?
This review provides an independent assessment of the benefits and harms of breast cancer screening in the UK. It provides estimates of how many women may benefit from the programme, and how many may be harmed by overdiagnosis.
The independent panel acknowledged that there are limitations to these estimates, and as part of their report they made recommendations for additional research that could help to reduce this uncertainty.
The panel emphasised the importance of clear communication of risks and benefits to women, and hopefully the findings and recommendations of the review will lead to improvements in this area, so that women can make better informed decisions about breast screening.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.