Progress in both detection and management of breast cancer may have saved the lives of hundreds of thousands of women over the past three decades, according to a new report.
The cumulative number of breast cancer deaths that have been averted since 1989 ranges between 384,000 and 614,500, depending on different background mortality assumptions.
Looking at 2018 alone, an estimated 27,083 to 45,726 breast cancer deaths were avoided, with the expected mortality rate of breast cancer reduced by 45.3%–58.3%
These estimates come from a study published online today in Cancer
"The take-home message from the paper could not be more clear," said coauthor Jay Baker, MD, professor of radiology and division chief of breast imaging at Duke University, Durham, North Carolina. "The combination of early detection through screening mammography plus improved treatments has saved the lives of somewhere around half a million women living in the United States alone."
Putting that into context, he added, "That is the equivalent of saving the life of every screening-aged woman living in Chicago today. Or saving every man, woman and child living in Wyoming."
However, what remains unclear is how much of the improvement is because of screening and how much as a result of improved treatment, as the study was unable to differentiate between the two, he noted.
We cannot separate the lives saved due to early detection at screening versus improved treatments. Dr Jay Baker
The study analyzed data from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) database.
"The is one of the most important breast cancer databases in the United States, but, unfortunately, it does not include information on whether or not a patient's cancer was discovered at screening," he said.
"Therefore, we cannot separate the lives saved due to early detection at screening versus improved treatments," he commented.
Decrease in Breast Cancer Rates
Mortality rates related to breast cancer increased from 1975 to 1990 by 0.4% per year in the United States, but since 1990 rates have been dropping.
Mortality rates from breast cancer decreased by a statistically significant 1.8% per year from 1990 to 1995, 3.4% per year from 1995 to 1998, and 1.8% per year from 1998 to 2015, according to National Cancer Institute statistics.
While this latest study cannot separate out the contributions made by screening and treatment to this decline in breast cancer deaths, a previous study did.
A study published last year in JAMA and reported at the time by Medscape Medical News looked at the reduction in mortality rates from breast cancer from 2000 to 2012, and reported that 63% of the overall reduction in breast cancer mortality was as a result of treatment, and 37% with screening.
Screening mammography has been a hot-button issue, with some conflicting results regarding the absolute mortality benefit and the risk of harms. Some evidence suggests that widespread breast cancer screening may catch more small, slow-growing tumors that are unlikely to be fatal, without curbing the number of cancers that are diagnosed at a late stageAs an example, a 2016 analysis in the New England Journal of Medicine reported that overdiagnosis of breast cancer via mammography screening is "larger than is generally recognized." An overdiagnosis was defined as a tumor detected on screening that never would have led to clinical symptoms.
Other studies have found a mortality benefit with mammography screening, such as a large observational study in the United Kingdom that reported a "substantial," statistically significant reduction in breast cancer mortality between 1991 and 2005 that was associated with the introduction of a national breast screening program.
Conversely, a Dutch study found that screening mammography over a period of 24 years among women ages 50 to 74 years has had little effect on reducing rates of advanced breast cancer or mortality from breast cancer. The authors noted that screening dramatically increased the detection of early-stage breast cancer, both ductal carcinoma in situ (DCIS) and stage I, and in so doing, contributed to increased rates of overdiagnosis.
However, a study from 2014 found somewhat opposite results, and concluded that a 37% decrease in the incidence of late-stage breast cancer during the past three decades can be attributed to mammography. The detection of early-stage breast cancer also increased 48% during that period.
In an interview with Medscape Medical News, Baker commented that mammography screening has been controversial "but it shouldn't be."
"Every few years, a different concern is raised about why mammography screening doesn't work or is a bad idea, and each one of those concerns has been proven false," he said. "The rate of overdiagnosis has been wildly inflated in several papers because those studies did not take into account the fact that the incidence of breast cancer has been steadily increasing at a rate of about 1% per year since at least the 1940s."
The whole discussion of overdiagnosis needs to be considered in light of the fact that since the start of screening mammography, the death rate for breast cancer in the US has declined by about 40%, he explained.
"It's a trick of statistics without taking into account the context, and in this case, the context is that the increasing incidence of breast cancer had been steady for decades before screening started," he said. "It's not reasonable to assume that the continued increase — at the same rate as before — is suddenly due to screening. That's what you would have to accept to believe in rampant overdiagnosis."
While screening guidelines can't be based on this study alone, Baker noted that some of the literature "confirms that starting screening at 40 and continuing as long as a woman remains in good health saves the most lives."
Study Details
Baker and colleagues analyzed breast cancer mortality and population data for US women ages 40 to 84 years during a 30-year period.
Four different assumptions regarding background mortality rates (in the absence of screening mammography and improved treatment) were used by looking at differences between SEER-reported mortality rates and background mortality rates for each 5-year age group, which was multiplied by the population for each 5-year age group. SEER data were then used to estimate annual and cumulative breast cancer deaths that had been averted in 2012 and 2015 and extrapolated SEER data were used to estimate deaths averted in 2018.
The analysis showed that, when looking at single years, the number deaths averted ranged from 20,860 to 33,842 in 2012, from 23,703 to 39,415 in 2015, and from 27,083 to 45,726 in 2018.
Reductions in breast cancer mortality rates ranged from 38.6% to 50.5% in 2012, from 41.5% to 54.2% in 2015, and from 45.3% to 58.3% in 2018.
Cumulative breast cancer deaths averted since 1989 ranged from 237,234 to 370,402 in 2012, from 305,934 to 483,435 in 2015, and from 384,046 to 614,484 in 2018
Experts Weigh In
Asked comment on the study, Anthony Miller, MD, professor emeritus, Dalla Lana School of Public Health, University of Toronto, Canada, pointed out that "an important neglected issue in this analysis is the almost complete cessation of hormone replacement therapy (HRT) and the subsequent reduction in breast cancer incidence and almost certainly mortality in the early 2000s."
Miller believes that the effect of breast screening by mammography has been overestimated. The main randomized screening trial that is believed to have demonstrated that fact, he said, was the long running Swedish Two-County trial, which showed a substantial absolute reduction in mortality from breast cancer.
"That study had many flaws, not [the] least that the reduction in breast cancer mortality claimed in the screened group was balanced by an increase in all-cause mortality in that group, in comparison to the control group," he said. "Thus, the reduction in breast cancer mortality we have seen in the general population must largely be attributed to improved treatment."
Also asked to comment on the study, Nancy Keating, MD, a professor of healthcare policy and medicine at Harvard Medical School and a physician at Brigham and Women's Hospital in Boston, Massachusetts, explained that the current paper updates prior work that showed both screening and treatment have decreased breast cancer deaths.
"But unfortunately, it doesn't get us any closer to the answer of why the mortality has decreased," she said. "I believe it's a combination of screening and treatment, but we don't know how much each one contributes, and this study doesn't tell us that.
"This is methodological paper and helpful for population estimates, and even for other countries thinking how to calculate cancer deaths averted," she explained. "Not only for breast cancer but other cancer types."
There has also been considerable debate as to the optimal age to begin screening and the interval between screens. There are variations in guidelines, Keating acknowledged. "It would be nice to have one simple answer."
"What we do know from other papers is that women will benefit from making individualized decisions about screening," she continued. "We see more of a benefit if we screen higher risk women than lower risk women, and we also know that there are tradeoffs to screening. We do want to balance the benefits and harms, but then we need to think through what are the benefits and what are the harms."
Overall, she said, "incorporating women's values and preferences into screening decisions makes a lot of sense."
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