Women deserve to know the facts about breast cancer and screening. As last week’s NEJM paper on overdiagnosis got so much press, I thought it worth reviewing some basics. The number one problem with that analysis is the assumption that the rate of “clinically meaningful” cancer has been constant.
The unfortunate truth is that the rate of invasive* breast cancers in U.S. women has been going up. Between 1975 and 2012, the incidence of invasive breast cancer climbed from 103 to nearly 130 cases per 100,000 women, in the United States. This 25% increase in the disease rate is based on SEER data for women of all races. (It does not include DCIS or other in situ abnormalities.) Here’s a graph:
“…We started with the assumption that the underlying probability that clinically meaningful breast cancer would develop was stable, an assumption we believe was warranted given the stable incidence of metastatic breast cancer for more than three decades…
As the graph above demonstrates, the rate of “clinically meaningful breast cancer” is not steady. Rather, the incidence of invasive breast cancer has been rising, clearly, among American women.
The authors' assumption would be true only if you believe that invasive breast cancer is insignificant and nothing to worry about unless it’s bigger than 2 centimeters (most of an inch) in diameter (Stage 2), or large enough that you might feel it, but that would be a mistaken belief. The other problem, in that same key sentence, is the notion that a stable incidence of metastatic disease implies that the incidence of “clinically meaningful breast cancer” would be stable. Apart from the issue of circular reasoning, it is a non-fact.
In the context of a 25% increase in invasive breast cancers, a stable incidence of metastatic disease strongly suggests that screening has made an impact. If U.S. women weren’t getting screened, the metastatic disease rate would be even higher.
The graph shows a near-flat rate of mortality from breast cancer since 1975.** (Due to lack of a research “team” at my desktop, I was unable to access the stage 4 rate from the SEER data, as Dr. H. Gilbert Welch and his colleagues show in their paper.) The rate of breast cancer mortality in the population, above, essentially parallels what's presented in the NEJM (Figure 1, lower curve) for women who are first diagnosed with stage 4 (metastatic) disease. This correspondence makes sense, and is not surprising, as metastatic breast cancer is terminal.
Despite a significant increase in the rate of breast cancer since 1975, the incidence of women diagnosed with late-stage (metastatic) disease, and mortality, have both been stable or slightly improved. Both trends are a testament to screening’s value.
As things stand, only around two-thirds of women for whom the USPSTF panel recommends screening actually go for it. So whatever benefits of screening we can see in population-based SEER data, are diluted. Yet even in the data presented in Figure 2 of the paper by Welch and co-authors, the incidence of breast cancers bigger than 2 cm. at diagnosis, between 3 and 5 cm., and greater than 5 cm. in size, are down, a likely and favorable consequence of screening.
I have written many times that mammography is imperfect, and yet it has helped individuals and the population, as evident by the graph above. I agree with Dr. Welch and his colleagues that better treatment is a factor in improved – extended – survival after a breast cancer diagnosis, but it has not significantly changed the mortality rate for those with stage 4 disease.
Imagine if we had better screening, today, than we did 20 years ago! The fact is, we do. Rather than dismissing screening’s value, doctors might provide women with current information and encourage them to take advantage of the best options available.
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*Invasive cancers are usually defined as those that have invaded the nearby body tissue, such as when malignant cells lining breast milk ducts penetrate the supportive breast tissue and fat. Pathologists distinguish invasive breast cancers from “stage 0” noninvasive tumors, like DCIS or LCIS, that have not yet entered the nearby normal tissue. Invasive cancers can arise from any breast cancer cell type, whether it’s called by an old or new name: like ductal, lobular and inflammatory breast cancer; or hormone-receptor positive, triple negative, etc.
**The slight decrease in the mortality rate, shown here at the population level based in SEER data, should be distinguished from the dramatic drop in mortality from breast cancer after diagnosis, of approximately 36%, in recent decades.
