JULY 1, 2022 BY CANCER ROSE
By Søren R Christiansen, Philippe Autier, Henrik Støvring
The effect of breast cancer screening is declining
A new study raises the debate about the progressive decrease in the benefits of breast cancer screening which would be at a too low level compared to their consequences in terms of overdiagnosis and overtreatment.
Researchers from the University of Aarhus, Department of Public Health, Denmark, and the International Prevention Research Institute (IPRI), Lyon, France, are the authors of the study.
They state that breast cancer mortality has decreased over the past three decades due to improvements in patient management and better therapies, while the number of women needed to be invited to mammography screening in Denmark to prevent one cancer death in 10 years has doubled.
"As the beneficial effects of mammography screening declines ever more, we should consider abandoning the current mammography screening program with biennial mammograms for everyone aged 50-70. Perhaps a more targeted, high-risk screening strategy could be an alternative, if studies showed the strategy's beneficial effects," Støvring, associate professor in the department of public health at Aarhus University declared in an interview.
"I think we are approaching a point where just continuing might become untenable from an ethical point of view, as fewer and fewer women will experience gains due to screening (they would not die from breast cancer anyway due to improved treatment), but the number of women harmed due to overdiagnosis and overtreatment remains constant," he noted.
H.Støvring believes that for breast cancer the evidence for mammography screening is not convincing. He declared: "I think it is critical that we reassess screening programs as new evidence becomes available”.
In conclusion, improvements in cancer therapy over the past 30 years have reduced mortality, which may erode the benefit-harm balance of mammography screening.
In addition, future improvements in the management of patients with breast cancer will increasingly reduce the benefit-risk ratio of screening.
The benefit of mammography in terms of reduced mortality declines while the harms such as overdiagnosis are unaffected. Screening leads to both overdiagnosis and overtreatment, which has a cost both on a human level and in terms of the economy.
Interview with the main author, June 24, 2022 by Helle Horskjær Hansen
Screening for breast cancer has a cost. This is shown by a Danish/Norwegian study that analysed 10,580 breast cancer deaths among Norwegian women aged 50 to 75 years.
"The beneficial effect of screening is currently declining because the treatment of cancer is improving. Over the last 25 years, the mortality rate for breast cancer has been virtually halved," says Henrik Støvring, who is behind the study.
According to the researcher, the problem is that screenings lead to both overdiagnosis and overtreatment, which has a cost both on a human level and in terms of the economy.
Overdiagnosis and overtreatment
When the screening was introduced, the assessment was that around twenty per cent of the deaths from breast cancer among those screened could be averted. While this corresponded to approximately 220 deaths a year in Denmark 25 years ago, today the number has been halved.
The study shows that in 1996 it was necessary to invite 731 women to avoid a single breast cancer death in Norway, you would have to invite at least 1364 and probably closer to 3500 to achieve the same result in 2016.
On the other hand, the adverse effects of screening are unchanged.
"One in five women aged 50-70, who is told they have breast cancer, has received a 'superfluous' diagnosis because of screening – without screening, they would never have noticed or felt that they had breast cancer during their lifetime," says the researcher.
One in five corresponds to 900 women annually in Denmark. In addition, every year more than 5000 women are told that the screening has given rise to suspicion of breast cancer – a suspicion that later turns out to be incorrect.
Peaceful, small nodes – but in who?
Henrik Støvring notes that the result is not beneficial for the screening programmes.
According to the researcher, the challenge is that we are not currently able to tell the difference between the small cancer tumours that will kill you and those that will not.
Some of these small nodes are so peaceful or slow-growing that the woman would die a natural death with undetected breast cancer, if she had not been screened. But once a cancer node has been discovered, it must of course be treated, even though this was not necessary for some of the women – we just do not know who.
"The women who are invited to screening live longer because all breast cancer patients live longer, and because we have got better drugs, more effective chemotherapy, and because we now have cancer care pathways, which mean the healthcare system reacts faster than it did a decade ago,” says Henrik Støvring.
Abstract of the study
Søren R Christiansen, Philippe Autier, Henrik Støvring, Change in effectiveness of mammography screening with decreasing breast cancer mortality: a population-based study, European Journal of Public Health, 2022;, ckac047, https://doi.org/10.1093/eurpub/ckac047
Reductions in breast cancer mortality observed over the last three decades are partly due to improved patient management, which may erode the benefit-harm balance of mammography screening.
We estimated the numbers of women needed to invite (NNI) to prevent one breast cancer death within 10 years. Four scenarios of screening effectiveness (5–20% mortality reduction) were applied on 10,580 breast cancer deaths among Norwegian women aged 50–75 years from 1986 to 2016. We used three scenarios of overdiagnosis (10–40% excess breast cancers during screening period) for estimating ratios of numbers of overdiagnosed breast cancers for each breast cancer death prevented.
Under the base case scenario of 20% breast cancer mortality reduction and 20% overdiagnosis, the NNI rose from 731 (95% CI: 644–830) women in 1996 to 1364 (95% CI: 1181–1577) women in 2016, while the number of women with overdiagnosed cancer for each breast cancer death prevented rose from 3.2 in 1996 to 5.4 in 2016. For a mortality reduction of 8.7%, the ratio of overdiagnosed breast cancers per breast cancer death prevented rose from 7.4 in 1996 to 14.0 in 2016. For a mortality reduction of 5%, the ratio rose from 12.8 in 1996 to 25.2 in 2016.
Due to increasingly potent therapeutic modalities, the benefit in terms of reduced breast cancer mortality declines while the harms, including overdiagnosis, are unaffected. Future improvements in breast cancer patient management will further deteriorate the benefit–harm ratio of screening.
Assuming a relative effect of mammography screening at 20% on breast cancer mortality, the number of women who needs to be invited to save one life has increased by 87% from 1996 to 2016. (Editor's note: this means that it is currently necessary to screen an ever increasing number of women in order to have a breast cancer death that would be prevented by screening, so it is more difficult to find a woman who has benefited from screening, while the adverse effects do not decrease (overdiagnosis)).
The number of women overdiagnosed with breast cancer per woman saved from dying of breast cancer has increased substantially from 1996 to 2016.
The deterioration in benefit-to-harm ratio of breast screening will continue due to steady improvement in therapies.
This study supports the need for re-evaluation of national screening programmes in high-income countries.
“A cost both on a human level and in terms of the economy... “
...According to the lead author.
Another recent study raises the issue of additional costs associated with declining screening effectiveness: https://www.sciencedirect.com/science/article/pii/S0277953622003793
In this paper, the authors exploit a natural experiment resulting from the phased geographic rollout of a national mammography screening programme in Ireland to examine the impact of screening on breast cancer outcomes from both a patient cohort and a population perspective.
Ireland is one of the few countries where, for operational reasons, the rollout of screening has resulted in a cohort of unscreened women that has existed long enough to serve as an appropriate comparison group.
Using data from 33,722 breast cancer cases diagnosed between 1994 and 2011, the authors employ a difference-in-differences research design using ten-year follow-up data for cases diagnosed before and after the introduction of the programme in screened and unscreened regions.
They conclude that, although the programme produced the intended intermediate effects on breast cancer presentation and incidence, these failed to translate into significant decreases in overall population-level mortality, though screening may have helped to reduce socioeconomic disparities in late stage breast cancer incidence.
Highlights of the study
- Screening increased detection of asymptomatic and early stage cancers.
- There was no significant effect on population breast cancer or all-cause mortality.
- Screening may have reduced socioeconomic disparities in late stage incidence.
- Results call in to question the overall effectiveness of this common intervention.