The rate of breast cancer that doctors diagnose only after it has spread has remained steady through the last three decades. But rates have dropped for prostate cancer, report researchers in the New England Journal of Medicine, who suggest the reason for the difference underscores the limits of breast cancer science and screening.
Three researchers, who include an oncologist and a surgical urologist, propose that the steady rate of metastatic breast cancer points to a failure by mammography “to identify at an earlier stage, before symptoms appear, cancers that are destined to become metastatic.” The study comes during a nationwide debate over the efficacy and necessity of early screening.
Dr. H. Gilbert Welch, a professor at Dartmouth College's Institute for Health Policy and Clinical Practice, and his co-authors argue that current breast cancer screening may be ineffective because not all cancers metastasize identically, according to theories of how tumors spread.
One theory says that “cancer arises at a single location, grows there and eventually migrates” to nearby lymph nodes before traveling farther. Another describes cancer as a “systemic disease by the time it is detectable.” Mammograms would not detect such an aggressive cancer before it spreads. Under a third theory, cancers may do both.
Screening tests that overlook that heterogeneity put patients at risk, Welch said. “The risk largely comes from therapy that can't help you, but can hurt you and by the way change your whole self-perception of health,” he said. “Not to mention you may be made bankrupt in the process. Screening tests may be free but subsequent treatment can be really expensive.”
Cancer experts agreed the paper highlights the limits of oncology research.
“All cancers look the same under the microscope,” and that limits medicine's ability to effectively screen and treat the disease, said Dr. George Sawaya, an obstetrics and epidemiology professor at the University of California at San Francisco.
But they are not the same, and scientists are working to identify critical differences between the tumor that is harmless and one that will go “from zero to 100 and does not allow any time for early detection.”
More research is also needed to measure the harms of overtreatment and to create guidelines to prevent it, he said.
Dr. Constance Lehman, co-director of the Avon Comprehensive Breast Evaluation Center at Massachusetts General Hospital in Boston, agreed on the need to better understand tumor biology, but cautioned that women should be assured that mammograms can save lives. "The challenge and the debate is what is the best age to start and what age should you be screened,” she said.
She also questioned the analysis by Welch and colleagues because it relied on data that did not identify whether or not women received mammograms.
The authors used data from the National Cancer Institute's surveillance, epidemiology and end results program and found no change in rates of metastatic disease even as mammography became more prevalent in the 1980s.
Welch acknowledged that the data do not identify which women received mammograms, but said nationally, 66% of women routinely receive breast cancer screening, citing data from the Centers for Disease Control and Prevention's National Health Interview Survey.
He challenged the possibility that women who get mammograms happen to be women who do not need them, which would explain how rates of metastatic breast cancer remained the same before and after mammograms became more widespread. “It is possible we are screening exactly the wrong population,” he said. “I find that an extremely far-fetched explanation.”
The study comes after a national debate over when and how often women should be screened intensified after the American Cancer Society last week released new guidelines that call for later and less frequent routine mammograms. Women between ages 40 and 44 can now skip the screening unless they feel strongly otherwise, the group said. The U.S. Preventive Services Task Force recommends women wait until age 50 for routine mammograms, but two medical societies say women should start annual screening a decade before that.
The use of mammograms has grown contentious as more women are found to have tumors that do not put their health at risk. The harm instead is from overdiagnosis and anxiety, say authors of studies on the subject of healthcare overuse. Guidelines that call for later screening reflect that concern, said Dr. Nancy Lynn Keating, a professor of healthcare policy and medicine at Harvard University. "The absolute risk is so low and the benefit is so small" and the harms are clear, she said.
But some say drawbacks have been overhyped. “Let's stop overemphasizing the “harms” related to mammogram callbacks and biopsies,” three doctors wrote on Wednesday in the New York Times.