Three experts who were approached by Medscape Medical News say this is further evidence that regular screening mammography significantly reduces the risk of dying from breast cancer, but one expert questioned the methodology used in the study.
The primary goal of cancer screening is to detect tumors at an early stage, when they are most treatable. The hope is that this will reduce the number of advanced cancers associated with poor prognosis and hence the risk of dying from that cancer.
So far, for mammography, the data have been somewhat conflicting. For example, some evidence suggests that widespread breast cancer screening may catch more small, slow-growing tumors that are unlikely to be fatal but will not curb the number of cancers that are diagnosed at a late stage.
The new study, published online in Cancer, refutes this view.
It followed a Swedish cohort of 549,091 women (covering approximately 30% of the Swedish screening-eligible population) who underwent regular mammography.
For the women in this cohort, there was a statistically significant 41% reduction in the risk of dying of breast cancer within 10 years and a 25% reduction in the incidence of advanced disease, compared to women who did not undergo screening. “Even in this age of effective treatments, early detection confers a substantial and significant additional reduction in risk of dying from breast cancer,” said lead author Stephen W. Duffy, MSc, from the Center for Cancer Prevention at Queen Mary University, London, United Kingdom.
The current study confirms the findings of a smaller earlier study (Cancer. 2019;125:515-23) from the same investigators. “It finds the same result with an extremely large evidence base, with more than half a million women, and it also adds further to the evidence that screening achieves this reduction in the context of routine healthcare, not only in the research context,” Duffy commented. “The results are generalizable to other populations, particularly in North America, Western Europe, and Australasia, where the epidemiology and demographics of breast cancer are similar,” said Duffy. “Clearly, more intensive screening is likely to achieve a greater benefit, but a trade-off between costs, both financial and human, and benefits always has to be made specific to each societal and healthcare environment.”
In Sweden, the policy regarding breast cancer screening is to screen women aged 40 to 54 years every 18 months. For those aged 55 to 69 years, screening is recommended every 24 months.
“The use of the incidence-based endpoints means that there is accurate classification of both the breast cancer cases and the whole study population in terms of exposure to screening and avoids a number of biases seen in other studies of service screening,” Duffy told Medscape Medical News.
“I have never seen persuasive evidence for the assertion that breast cancer screening does not reduce deaths from metastatic disease – indeed, the randomized trials seem to show the opposite,” said Duffy. “This may have arisen from a misunderstanding about the mechanism whereby screening works. It primarily works by diagnosing cancer early so that treatment is successful and recurrence with distant metastases, followed by death, does not occur some years later. I suspect some colleagues have confused this with distant metastases at initial diagnosis,” he added.