Public discussion about the risks of over-diagnosis of breast cancer have left some women wondering whether they should take part in the government’s breast screening program.
Let’s take a look at what the evidence says and how women might be enabled to make their own decisions about whether to participate.
Last year Sir Michael Marmot, Professor of Epidemiology and Public Health at University College London, was asked to chair an independent panel to review the benefits and harms of screening mammography in the United Kingdom. The panel focused on the findings from randomised trials, even though most of these trials were carried out a long time ago, in the 1970s and 1980s.
A report on the findings of the review and an accompanying editorial were published in the Lancet late last month. In summary, the report said that screening resulted in an estimated ratio of cases over-diagnosed, to deaths from breast cancer prevented, of 3:1. An over-diagnosed case is one where screening has identified a breast cancer that will never cause harm.
As it is not currently possible to distinguish breast cancer identified through screening which will never cause harm from cancer that will, all breast cancer identified by screening is treated – with surgery and various combinations of radiotherapy, chemotherapy, endocrine and biologic therapies.
A woman treated needlessly will have no way of knowing that her cancer was harmless and that her well-being has been sacrificed, without her knowledge or consent, for the sake of others who may have benefited from screening.
Assessing the benefits of screening
The benefits of screening appear to have been over-estimated.
Advances in treatment have changed the outlook for women with breast cancer so profoundly over the past two decades that the potential for screening to further reduce deaths from breast cancer has been considerably reduced since the original randomised trials were performed.
Furthermore, limiting the estimated benefit to reduced deaths from breast cancer is misleading because radiotherapy after surgery for breast cancer materially increases the risk of dying from heart disease.
The magnitude of overdiagnosis has also been underestimated. An estimate of overdiagnosis from Australia – adjusted for confounders and lead time and assuming 60% of invited women attend – found an over-diagnosis rate of 30% to 40% depending on the expected incidence without screening, rather than the 19% in the Lancet report.
The Nordic Cochrane Center review of screening mammography has estimated the ratio of cases over-diagnosed to deaths from breast cancer prevented at 10:1. Our estimate of the ratio in Australia, using a lower estimate of deaths prevented due to the impact of improved treatment and the higher rate of over-diagnosis, is closer to 15:1.
The clear message is that over-diagnosis exists, it happens regularly as part of the screening process and women need to understand this.
An informed decision
When an asymptomatic woman requests screening mammography through her doctor or when a doctor recommends that an asymptomatic woman be screened, the doctor has a responsibility to inform her about the potential harms and benefits so she can decide whether she will have the mammogram in the context of her situation and preferences.
But when a woman receives an invitation for screening from a government body that makes little or no reference to screening-related harm, she may reasonably expect that that there is no real likelihood of harm. It could be argued that the weight of responsibility for full and frank disclosure of risks and benefits is even greater when the invitation comes from a trusted government agency that has been historically responsible for promoting screening.
It’s time to review the routine invitation for screening to all women aged 50 to 69 years.
The proportion of women in the invited age group who currently attend for screening in Australia is between 50% and 60% so not all women currently respond to the invitation by attending.
Stopping the routine invitation for screening would not prevent women being screened. Women with a high risk of cancer – those with a family or personal history of breast cancer – should still be screened. There may be others who should consider screening because they have a combination of less powerful risk factors which might put them at above average risk.
Other women, not at above-average risk, could request screening if this is what they want.
If women elect not to be screened and only enter the medical system when a clinical problem presents itself, they can be reassured that they will have the benefit of the recent improvements in breast cancer treatment.
Government should undertake a review of the invitation for screening. But this will take time.
For the time being, it is imperative that all women be provided with comprehensive information about the disadvantages as well as the advantages of mammographic screening. The pamphlet produced by the Nordic Cochrane Center, which presents the information in terms of the impact of screening on absolute risks and benefits, would make an excellent model.
But a pamphlet alone is insufficient. Given the risks, each woman must be given the opportunity to discuss her options and decide what is best for her, with those who are in a position to provide accurate and helpful advice.