“Women who undergo breast cancer screening cut their risk of dying from the disease by 40%, according to a global panel of experts,” The Guardian reports.
Critics argue that this benefit is outweighed by the problem of overdiagnosis, where women are diagnosed as having cancer and treated, when the cancer would never have caused any harm. This treatment carries the usual impacts and side effect for these overdiagnosed individuals, but does not offer them any benefit.
The balance of benefits and risks from breast cancer screening is a hotly debated topic. The latest attempt to settle the debate is a new review published by the International Agency for Research on Cancer (IARC): a working group of cancer experts from across the world.
The review has been published in the peer-reviewed medical journal The New England Journal of Medicine.
The IARC concluded, based on an evaluation of the available evidence, that the benefit of inviting women aged 50 to 69 years of age for mammography screening outweighs the potential harms. In the UK, women in this age group are invited for this screening every three years.
How was the report developed?
IARC brought together a working group of 29 international experts from 16 countries to assess the benefits and harms associated with breast cancer screening. These experts were selected based on their areas of expertise and for not having any known conflicts of interest.
IARC staff searched for available studies on breast cancer screening, and the experts added any other relevant studies they were aware of in their areas. The experts reviewed and debated this evidence in their specialist areas, and came to an initial conclusion. This conclusion was then reviewed by the working group as a whole and a consensus position reached.
Why was the report needed?
This report was part of the IARC’s ongoing work to review and evaluate the effects of preventing different cancers. They had last reviewed the evidence on breast cancer screening in 2002. As new research continues to be carried out, it is important to consider this new evidence, and whether it affects their conclusions. Particular areas they highlighted as needing consideration were:
- improvements in treatments for late-stage breast cancer
- concerns around overdiagnosis (diagnoses of breast cancer that would never have been diagnosed otherwise and would never have caused the women any harm)
- what age groups of women should be offered screening and how frequently
- effects of screening through self or health professional breast examination, or approaches other than mammography
- screening in women at high risk of breast cancer
What evidence did the expert group consider on mammography?
In their last report in 2002, the IARC concluded that the evidence for the efficacy of mammography screening in women aged 50 to 69 years old was sufficient, based on the available randomised controlled trials (RCTs). Reassessment of all available RCTs up to the time of the current assessment by the expert group confirmed that this was still the case.
The expert group also considered evidence from recent, high-quality observational studies, as the RCTs were carried out more than two years ago and there have been improvements in screening and treatment since then. They focused on cohort studies with a long duration and which used the best methods for avoiding confounding and other potential limitations.
Case-control studies were also considered, particularly in areas where there were no cohort studies. 20 cohort studies and the same number of case-control studies from the developed world countries were considered for assessing the effectiveness of mammography.
What did the group conclude about mammography?
Overall, the group concluded that the benefits of mammography screening outweigh the adverse effects for women who are 50 to 69 years of age.
The results of 40 case-control and cohort studies from high-income countries suggested that women in this age group who went for screening had around a 40% reduction in risk of death from breast cancer. If all women who were invited for screening were considered, the average reduction in risk of death from breast cancer was 23%. The evidence did not clearly show how frequently women needed to be screened to gain maximum benefit.
There was judged to be sufficient evidence that women aged 70 to 74 years who went for screening also had a reduced risk of death from breast cancer. Evidence in women aged under 50 was limited, meaning that conclusions could not be drawn.
There was sufficient evidence that mammography screening does lead to overdiagnosis. Once women have been identified as having breast cancer, it is impossible to tell which of them have been “overdiagnosed” but there are ways to estimate the proportion of women it affects. The studies assessed by the expert groups estimated that 1- 11% of women identified as having breast cancer through screening are overdiagnosed.
There was also sufficient evidence that women experience short-term adverse psychological effects if they are given a false positive result on mammography (that is, a positive result that turns out not to be breast cancer on further investigation). Studies from organised screening programmes suggested that about 1 in 5 women who are screened 10 times between the ages of 50 and 70 years would be expected to have a false positive. Less than 5% of false positives lead to an invasive procedure, such as a needle biopsy.
What were the expert group’s other conclusions?
The group also drew conclusions on the other issues they were covering in their report. For a lot of the issues they were interested in, they concluded that the evidence was as yet limited or inadequate to be able to draw firm conclusions. For example, the evidence about whether breast self-examination could reduce death from breast cancer if taught and practised competently and regularly was judged as being inadequate. The full report, including the conclusions, is available from the IARC website.
Does this mean that all scientists now agree and the debate is over?
Probably not. Evaluating the evidence relating to breast cancer screening is complex, and different scientists have analysed and interpreted it in different ways. For example, a Cochrane review from 2013 estimated that the overdiagnosis rate could be as high as 30% based on RCT evidence.
The current report is the considered opinion of the IARC, based on their evaluation of the evidence available to date. However, this doesn’t mean that all other scientists will agree, as they may interpret the studies and weigh up the benefits and harms differently. The IARC will continue to review their conclusions as new evidence becomes available.
What is important is that women who are invited for screening are provided with clear information, so they know the potential benefits and risks, and about the best estimation of their chances of experiencing these. This enables them to make decisions about whether they want to attend screening.
Sarah Williams from Cancer Research UK sums this up in a quote on the BBC website: “There isn’t one definitive answer to the question of how the benefits and harms of breast screening stack up – individual women will have different views on the factors that matter most to them, and also there are a number of different ways to bring together and interpret the evidence.”