The longest study yet to examine women who undergo mammography shows that it reduces deaths from breast cancer by at least 30%, a finding that many doctors say may help ease the recent controversy surrounding the procedure.
The three-decade study in Sweden showed that one breast cancer death can be prevented for every 414 to 519 women who are screened, a much lower number than the 1,000 to 1,500 that had been projected in previous studies.
"What this tells us is that, in the long term, screening for breast cancer is a very good investment," said epidemiologist Robert A. Smith, director of cancer screening at the American Cancer Society and a coauthor of the paper appearing in the journal Radiology.
Critics had argued that overuse of the procedure produced too many false positives, requiring many women to undergo unnecessary invasive procedures — which have risks of their own — to rule out the presence of a tumor.
"There is little question that there is some overdiagnosis," Smith said, "but the number we have identified is really quite small. We estimated it is less than half the number of lives saved, so it is really pretty low."
The findings are "a really big deal," said Dr. Loretta Lawrence, chief of breast imaging at North Shore University Hospital in Manhasset, N.Y. "A 30% reduction in mortality translates to 15,000 to 20,000 lives saved by mammography screening each year."
The new study does not address the controversy surrounding the value of screening women in their 40s because the results did not stratify women by age. Nor does it address the controversial issue of how frequently a woman should have a mammogram: In the U.S., annual mammograms are generally recommended, and some scientists have argued that the period between screens should be lengthened. The women in the study were screened less frequently: every 24 months for women aged 40 to 49, and every 33 months for older women.
The findings are unlikely to change clinical practice in this country because professional groups already recommend routine screening. But because the screening seems so effective, they might reduce the pressure to extend the period between mammograms and to limit the tests for younger women.
But Dr. H. Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice in Lebanon, N.H., who has argued frequently that women are being overscreened, said the study didn't take into account the tremendous advances in breast cancer therapy over the last two decades. As treatments have improved, the vast majority of patients have done well, regardless of how early their tumors were diagnosed.
"That is probably the explanation for why more recent trials of mammography have found less effect or no effect," he said.
The Swedish Two-County Trial, run by a team headed by cancer screening specialist Stephen W. Duffy of Queen Mary University of London, examined women with no history of breast cancer in two counties in Sweden. The women were divided into 48 geographic groups, which were then randomly assigned either to receive an invitation to enter a screening program or to receive normal care. About 77,080 women were in the invitation group and 55,985 in the normal care group. The screening phase of the study lasted about seven years.
The team concluded that regular screening reduced deaths from breast cancer by 31% during the 29 years of follow-up. Because only about 70% of the women invited to enter the screening program actually did so, said Dr. Lawrence Bassett, a professor of breast imaging at UCLA's Geffen School of Medicine, the reduction in mortality among those who were actually screened was 40%.
Duffy noted that most of the deaths prevented by screening would have occurred more than 10 years after the screening started. The long-term benefit of the screening is thus actually more than double the benefit reported in shorter studies, he said.
Moreover, the screening technique used in the Swedish study "is nothing near what it is now … and probably underestimates the benefit," Lawrence said. The physicians in Sweden used only one view per breast, while radiologists now use two views. And radiologists use digital imaging, which is much more sensitive than the film used in the Swedish study.
"The study probably underestimates what the benefit would be today because the technology is so much better now," Bassett added.
Much of the current controversy about mammography was triggered by a November 2009 report by the U.S. Preventive Services Task Force, which recommended that most women under 50 did not need routine screening. Although the chance that a woman in her 40s will be diagnosed with invasive breast cancer is small — about 1.44% — such tumors account for about 17% of breast cancer deaths, according to the American Cancer Society.
Most organizations that set guidelines for screening have chosen to ignore the task force's recommendations and continue to urge that women be screened beginning at age 40. The American Cancer Society, for example, recommends a mammogram every year starting at 40, the National Cancer Institute recommends the test every one to two years starting at 40, and the American College of Obstetricians and Gynecologists recommends a test every one to two years for women in their 40s and every year for those over 50.
Medicare and most insurance companies have continued to pay for the tests.
"Mammography has taken quite a hit in recent years" from short-term studies, said Dr. Stamatia Destounis, a radiologist at Elizabeth Wende Breast Care in Rochester, N.Y. She said radiologists know it is valuable, but family doctors, ob-gyns and others that might recommend mammography have been confused. "I'm hoping that this study will convince them about the long-term value of screening," she said.
Welch agreed that screening helped some women, "but it comes at a real human cost" of anxiety, false alarms and even unnecessary breast cancer treatments for others.
"There is no simple right answer for this stuff," he said. "Women who feel good about screening should continue to have it, and those who don't feel comfortable shouldn't be coerced into undergoing it."
Nobody's arguing that mammography doesn't save lives, said Dr. Susan Love, head of the Dr. Susan Love Research Foundation in Santa Monica. "Really, what has been the source of more controversy is: What is the right schedule? This doesn't really add to that discussion."