This week’s New England Journal of Medicine includes an article*on the potential benefit of breast cancer (BC) screening by mammography. The paper, authored by a group of epidemiologists and biostatisticians from the Cancer Registry of Norway, Harvard University, the Dana Farber Cancer Institute and Stockholm’s Karolinska Institutet, suggests that mammography has a relatively small influence on survival.
The work, along with an accompanying editorial* by H. Gilbert Welch, M.D., M.P.H., got front-page attention in the Times and elsewhere. My friends want to know why this particular research study was featured and, really, what it showed.
So here’s my take –
The researchers studied chronological trends in BC diagnosis and mortality in Norway. To see if mammography had an effect, they divided the country into two groups, “screening” and “non-screening,” based on when a state-sponsored mammography screening program was implemented in each of 19 counties there. The national plan, which required that each region establish a centralized, multidisciplinary BC care team before participation, began in 4 counties in 1996 and gradually expanded to include all of Norway by 2005. According to the authors, all Norwegian women between the ages of 50 and 69 years have been asked to participate in screening mammography since 2005; 77% have done so; Norway’s nationwide cancer registry is nearly 100% complete.
They evaluated a total of 40,075 women (“subjects”) who received a diagnosis of BC between 1986 and 2005.
For women between the ages of 50 and 69, BC-associated mortality** fell from 25.3 to 18.1 in counties where a government-sponsored mammography program was implemented early on, and from 26.0 to 21.2 in counties where mammography was not covered, over a similar time frame. Because BC-associated mortality declined in all regions of Norway, regardless of whether mammography was offered, the authors conclude that screening can’t account for all the reduction in mortality.
By their calculations, mammography accounts for roughly 10 percent of the enhanced survival. (This finding was not statistically significant.)
The authors suggest that recent progress in BC survival – which in their study improved significantly in all regions of Norway – comes, for the most part, from better care and treatments.
What’s wrong with the paper? I see several key flaws:
1. The average follow-up is only 2.2 years after diagnosis, with a maximum follow-up of 8.9 years (“Results,” p. 1206). This is far too short a follow-up interval to measure the benefit of mammography or any sort of intervention in women with breast cancer. When BC recurs it’s often after several years and, occasionally, decades later.
2. Among women under the age of 50 there was a slight increase in BC-mortality noted: A non-significant relative increase in mortality, of 4%, after the introduction of the screening program for older women (p. 1207, Table 1). This worrisome finding is not adequately addressed by the authors; one might wonder – did fewer women in their forties go for mammograms after 1996, since they were only recommended and covered for older women? (My concern is that reduced screening, now, in younger women might lead to an increase in BC mortality.)
3. Digital mammography was not evaluated in this study.
4. The authors detected the greatest benefit of screening among women with Stage II BC; there was a “marked” 29% reduction in mortality relative to the historical counterparts for that group who were screened, as compared to only a 7% reduction in mortality for women with Stage II tumors in counties where screening was not available over the same historical interval (p. 1207). This observation suggests that mammography screening is most life-saving for women with Stage II tumors. As an oncologist, I find this highly-plausible; the purpose of mammography is to identify tumors in early stage and spare women morbidity and mortality associated from advanced disease.
5. There’s no mention of the absolute number of lives saved by the procedure according to the authors’ calculations, but I think this is an important number to keep in mind when we assess the procedure’s value. If the paper’s conclusion is true – that mammography reduces BC-associated deaths by just 10 percent – then in Norway, with a total population of 4.8 million and where some 4,791 women in the study died (p. 1206), these results support that mammography spared approximately 480 lives in those 20 years.
Mortality in the U.S. from breast cancer has declined by roughly a third since the implementation of wide-spread mammography screening. Here, where some 45,000 women die each year of BC, we’d save 4500 lives per year if the added value of mammography is just 10 percent, as suggested by the new study. If the benefit of screening mammography is higher – in the range of 45 percent, as was supported by a 2007 paper, also published in the NEJM – then the value would exceed 20,000 women’s lives per year. If the benefit is only 25 percent in terms of reduced mortality, that would result in over 11,000 lives saved, per year in the U.S.
As for the editorial, first I’ll say that the opening statement – that “no screening test has ever been more carefully studied than screening mammography,” is misleading. While this was, indeed a well-organized and careful study, among other issues it was far too short in patients’ follow-up to measure the impact of mammography on BC survival. The Annals papers, which caused so much controversy last year, relied heavily on old data and did not at attempt to examine the efficacy of digital mammograms.
What’s needed, still, for public health policy in the U.S. is evidence regarding the long-term outcomes after digital mammography performed in FDA-regulated, modern facilities by skilled, board-certified radiologists applied every other year in women who are over the age of 40 in the context of modern, adjuvant treatments and current pathology methods.
As for the analysis by G. Kolata in the Times, where she wrote:
“…it indicates that improved treatments with hormonal therapy and other targeted drugs may have, in a way, washed out most of mammography’s benefits by making it less important to find cancers when they are too small to feel.”
I’d say the opposite is true:
It’s precisely because there are effective treatments for early-stage disease that it’s worth finding breast cancer by mammography. Otherwise, what would be the point?
Metastatic breast cancer is quite costly to treat and, even with some available targeted therapies, remains incurable. Despite so many advances in treating early-stage BC, the survival rate at 5 years is under 25 percent for women with Stage IV disease.
*abstract available, otherwise by subscription
**mortality rates: per 100,000 person-years