Digital breast tomosynthesis (DBT) is more commonly known as “3D mammogram” because it creates a 3-dimensional image of breast tissue by taking x-rays through multiple tissue planes. DBT has become an increasingly popular technology in recent years. From 2015 to 2017, use of 3D mammograms more than tripled, from 12.9% of screening examinations in early 2015 to 43.2% in late 2017.
3D mammograms have been marketed intensely to doctors and patients, promising more accurate results. According an analysis of Open Payments data in Kaiser Health News, manufacturers of 3D mammogram equipment paid doctors and teaching hospitals more than $9.2 million for research, speaking fees, consulting, and meals related to 3D mammograms from 2013-2019. 3D mammogram companies also spent $14 million on patient advertising from 2015-2019.
One of the big selling points of 3D mammograms is their potential to reduce false positive results. False positives can seem like no big deal, but they result in anxiety and often unnecessary follow-up treatments, which can cascade into even more tests and procedures.
False positive results for mammograms are quite common. For every 10,000 women screened for ten years starting age 40, more than 6,000 will have a false positive. Reducing this rate would be enormously beneficial. However, a new study shows that 3D mammograms don’t reduce false positives as much as one would think.
“Unfortunately, the growing availability of DBT does not substantially change the likelihood that women will experience a false-positive result over years of regular mammograms.”
Dr. Lydia Pace, Associate Physician in the Divisions of Women’s Health and General Internal Medicine at Brigham and Women’s Hospital
In JAMA Network Open, University of California-Davis researcher Dr. Thao-Quyen Ho and colleagues examined about 3 million screening results for more than 900,000 patients in the Breast Cancer Surveillance Consortium. They compared the probability of patients receiving a false positive over ten years of screening using different types of mammogram technology. They found that 50% of patients who received 3D mammograms once a year had a false positive, compared to 56% of those who received traditional mammograms. Patients who had 3D mammograms were slightly less likely to have an unnecessary biopsy than those who had traditional mammograms (11.2% vs 11.7%).
Comparing patients that received screenings every other year (which is recommended by the US Preventive Services Task Force), the difference in false positive rate between the two screening technologies was smaller (36% for 3D mammograms vs 38% for traditional). There was no significant difference between rates of unnecessary biopsy for patients that had biennial screenings. For women with extremely dense breasts (for which mammogram results are more difficult to interpret), there was no significant difference in false positives between 3D and traditional mammograms.
Screening every two years with traditional mammograms results in far fewer false positives than screening every year with 3D mammograms.
Any reduction in false positives from mammograms is a step in the right direction. However, this study shows that 3D mammograms are not as effective as some had hoped in reducing false positives over years of screening. Even with 3D mammograms, about half of women will have a false positive over ten years.
For patients trying to balance the benefits and harms of screening, it’s important to know that the age at which you start screening and how often you get screened affects the rate of false positives much more than the type of screening technology used.
For example, screening every two years with traditional mammograms results in far fewer false positives than screening every year with 3D mammograms. For patients not at high risk of breast cancer, starting screening at age 50 rather than 40 (as recommended by the USPSTF) with traditional mammogram also results in fewer false positives than starting screening at age 40 with 3D mammograms (58% vs 61%).
“Unfortunately, the growing availability of DBT does not substantially change the likelihood that women will experience a false-positive result over years of regular mammograms,” writes Dr. Lydia Pace, Associate Physician in the Divisions of Women’s Health and General Internal Medicine at Brigham and Women’s Hospital, in an accompanying editorial. “Even if we practice in a center where DBT is routinely offered, clinicians should continue to counsel patients that false-positive results are an expected outcome from mammography screening,” Pace writes.
There are other downsides of 3D mammograms that should be made clear to patients. There have been no studies of whether or not using 3D mammograms actually improve morbidity, mortality, or quality of life. These scans may detect more cancers, but there isn’t evidence that the cancers being detected would have harmed patients, so 3D mammograms may also lead to more overdiagnosis and overtreatment. Older 3D mammogram machines also expose women to more radiation than conventional mammography — and they cost more to patients and to the health system.
Reducing false positives for mammography is a worthy goal, however, 3D mammograms clearly are not the solution.