When it comes to cancer, we’ve heard that screening saves lives. But a new analysis conducted by Dartmouth University researchers and others is questioning the value of mammograms to screen for breast cancer.
The study, published online this month in the New England Journal of Medicine, points to overdiagnosis as a major problem linked to breast cancer screenings. The study analyzed more than 35 years of data between 1975 and 2012 and concluded that “women were more likely to have breast cancer that was overdiagnosed than to have earlier detection of a tumor that was destined to become large.”
The researchers found that with mammography, the incidence of large tumors decreased by 30 cases per 100,000 women (from 145 to 115) and the incidence of small tumors increased by 162 cases of breast cancer per 100,000 women (from 82 to 244), but only 30 of those small tumors were likely to become dangerous.
The findings explained
Lead author H. Gilbert Welch, M.D., M.P.H., of the Dartmouth Institute for Health Policy and Clinical Research explains the findings in a YouTube video (see below). He says that for every woman who benefited from mammography and early detection, about four women were overdiagnosed. Overdiagnosis is a concern because it can result in unnecessarily treating cancers that wouldn’t have otherwise grown or spread.
The study also notes that while mammography increased the detection of small tumors (those smaller than 2 centimeters), it didn’t produce the expected hallmark of effective screening: a significant decrease in the detection of larger, presumably more dangerous tumors. It also didn’t appear to be responsible for breast cancer’s declining mortality rate, a trend the researchers chalked up instead to improved cancer treatments.
“The better we are able to treat a disease, the less important it becomes to find it early,” Welch notes in the video. He adds, “Women who feel good about screening can feel good about continuing to do so. Those who do not can feel equally good about not pursuing it.”
At the heart of study is a sometimes difficult concept for patients to grasp: the idea of nonprogressive cancers, or those that are technically considered cancer by pathology standards but will never cause symptoms or problems. However, it’s a concept now recognized with other cancers—for example, prostate cancer—and has heralded in treatment options such as active surveillance.
What the critics say
The study had some limitations, as critics are quick to point out. The research assumes that the incidence of breast cancer has not increased; rather, mammography has increased the number of cancers detected. This not known for certain—an editorial that accompanied the study notes that researchers’ estimates were “imprecise” while acknowledging overdiagnosis is a problem.
The American College of Radiology (ACR) issued a statement disputing the study’s findings. It says: “The numerical data … clearly show that mammography screening catches more cancers early and reduces the number of women with cancers of advanced size. Smaller cancers result in better outcomes for women. The data do not support the author’s conclusion that improved therapy is more key to breast cancer survival than mammography screening. Nor does the data support that mammography leads to (marked) overdiagnosis.”
The professional society cites a 2015 study to support its case and recommends women stick with mammography screenings starting at age 40 and continue as long as they are in good health.
What the medical groups say
Most medical societies—including the American Cancer Society (ACS), the American College of Obstetricians and Gynecologists (ACOG), and the ACR decline to suggest an upper age limit to annual screening mammograms, instead leaving the decision to women and their doctors. The groups suggest weighing the potential benefits and risks as well as the individual’s overall health and expected longevity.
For younger women, the American Cancer Society now recommends that annual mammograms be optional for those ages 40 to 44. Women 45 and older should start getting annual mammograms and continue with yearly screening till 54, according to the ACS guidelines. Women 55 and older can transition to having mammograms every other year, though they may choose to continue annual screenings, so long as the woman is in good health with a life expectancy of at least 10 years.
The other medical societies continue to recommend that women at average risk begin getting annual mammograms at age 40. However, the U.S. Preventative Services Task Force (USPSTF) recommends that women start mammography screening later, at age 50, every two years.
The uncertainty surrounding which cancers will ultimately prove to be dangerous and which ones won’t has led our experts to err on the side of caution when it comes to mammography.
“I concur with the ACS, ACOG, and ACR recommendations,” says Edward E. Wallach, M.D., professor emeritus of gynecology and obstetrics at the Johns Hopkins University School of Medicine in Baltimore and a member of the HealthAfter50 board of advisors. “The biological growth rate of cancer can differ from individual to individual, and a physician just can’t be perfect in predicting the course of cancer growth.”