MAMMOGRAPHY REDUCES RISK OF ADVANCED CANCER

New data from a large Swedish study show that mammography screening for breast cancer reduces the rate of both advanced and fatal breast cancer.

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 screeningmay 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;125:515-523) 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.

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"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.

One Expert Questions Methodology

One of the experts who was approached by Medscape Medical News to comment on the new study, Philippe Autier, MD, MPH, PhD, University of Strathclyde Institute of Global Public Health at the International Prevention Research Institute, Dardilly, France, questioned the methodology of the study." This method is incorrect simply because women attending screening are different from women not attending screening," he said. "The former are more health aware and have healthier behaviors than the latter, and this is a well-known fact and supported by the literature."

Autier emphasized that it is practically impossible to control for that bias, which is known as confounding by indication.

"The statistical methods used for attenuating the so-called self-selection are very approximate and based on unverified assumptions," he said. "For this reason, the Handbook on Breast Cancer Screening produced by the International Agency for Research on Cancer [IARC] clearly stated that 'observational studies based on individual screening history, no matter how well designed and conducted, should not be regarded as providing evidence for an effect of screening,' and the methodology in this paper has never been recommended by the IARC."

A better way of conducting this type of study would have been to show the incidence trends of advanced-stage breast cancer in Sweden for the entire female population aged 40 years and older, he asserts. Autier used that methodology in his own study in the Netherlands, as previously reported by Medscape Medical News. That study found that in the Netherlands, screening mammography over a period of 24 years among women aged 50 to 74 years had little effect on reducing rates of advanced breast cancer or mortality from the disease.

Experts Applaud the New Findings

Three of the experts who were approached by Medscape Medical News to comment on the new findings applauded the efforts of Duffy and colleagues in providing evidence that mammography can reduce breast cancer–related mortality.

Marie Quinn, MD, director of diagnostic radiology at Roswell Park Comprehensive Cancer Center, Buffalo, New York, said this study adds to the growing body of scientific evidence that confirms that women who undergo regular screening mammography significantly reduce their risk of dying from breast cancer.

"Women who underwent regular screening also had a 25% reduction in the incidence of advanced-stage breast cancer," she said. "This is important, because breast cancers are less fatal and often require less treatment when picked up at an earlier stage. We know the risk reduction benefit detected in this well-designed study can be attributed to screening mammography and not advances in cancer treatment, due to the long-term follow-up and outcome of cancer death within 10 years."

The findings from this study support the guidelines recommending routine screening mammography in the United States, Quinn continued, but she pointed out that some aspects of screening (eg, the age at which to begin screening and how often to screen) can vary. "This can be confusing for patients and providers," she said. "Overall, research has shown us that women who undergo regular screening mammograms reduce their risk of dying from breast cancer. For women of average risk, the benefit of mammography is maximized with annual screening beginning at age 40," she said.

Jay A. Baker, MD, FACR, FSBI, chief of the Division of Breast Imaging at Duke University Medical Center, Durham, North Carolina, emphasized that this is yet another study that confirms that the improvement in breast cancer mortality is not the result of improved treatments alone, as some have speculated. "Others have tried to model the benefit of screening vs treatment, but this study is a more direct measurement," he said. "This conclusion is important for both patients and physicians to hear."

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Although the study strongly supports regular screening for all women, it does not specifically address which set of screening guidelines is optimal, Baker commented. "Fortunately, even though some organizations in the US curiously suggest a delayed start to screening, all organizations and professional societies agree that the most lives and the most years of life are saved by yearly screening beginning at age 40," he added. "This new study tells us that new treatments alone aren't enough and confirms that screening saves at least one third more lives."

Another expert, Bonnie N. Joe, MD, PhD, professor in residence and chief of breast imaging in the Department of Radiology and Biomedical Imaging at the University of California, San Francisco, agreed that the study shows the mortality benefits of regular screening mammography. "Notably, these benefits were related to participation in mammography screening and independent of any advances in treatment," she said, "And these findings in this study support regular screening mammography to reduce advanced-stage breast cancers and to reduce a woman's risk of dying from breast cancer."

Joe noted that overall, this was a "well-done, large-scale screening study with long-term outcomes and should be applicable to other populations. In the US, we know that peak cancer incidence is in the 40s for minority women, and the results of this study support regular screening starting at 40."

The study was supported by the American Cancer Society through a gift from the Longaberger Company's Horizon of Hope Campaign. Additional financial support was provided by Brostcancerförbundet, Sweden. Duffy, Autier, Quinn, Joe, and Baker have disclosed no relevant financial relationships. One coauthor of the study has disclosed relationships with industry, as noted in the original article.

Cancer. Published online May 11, 2020. Full text

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