- Kenny Lin, MD, MPH
Although digital breast tomosynthesis (DBT; 3D mammography) was approved by the U.S. Food and Drug Administration a decade ago and has since been rapidly adopted by breast imaging centers, no studies have shown that it is more beneficial or less harmful for breast cancer screening than traditional digital mammography. In a Diagnostic Tests review in the April 1 issue of AFP, Drs. Kathleen Barry and Chelsea Evans noted that DBT offers a "modestly increased cancer detection rate" and lower recall rate, but also costs about 40% more per test and exposes patients to a higher dose of radiation. It is unclear if additional cancers detected by DBT would have eventually become symptomatic, and "no studies have evaluated mortality as an outcome in women screened with DBT compared with digital mammography."
To address these important questions, in 2017 the U.S. National Cancer Institute (NCI) and the Canadian Cancer Trials Group launched a $100 million randomized trial, TMIST (Tomosynthesis Mammographic Imaging Screening Trial). With a planned enrollment of 165,000 women at 100 North American clinical sites, TMIST was designed to compare the incidence of advanced breast cancer after 4.5 years of follow up in women receiving either digital mammography or DBT. However, by early 2020, investigators had managed to enroll fewer than 23,000 women and were forced to expand to include overseas sites in Asia and Europe. The reason, according to an article in Medscape: already convinced that DBT was a superior technology, large numbers of U.S. and Canadian radiologists were declining to participate. Then the COVID-19 pandemic hit, further hindering recruitment efforts.
Meanwhile, a national study of Breast Cancer Surveillance Consortium sites found substantial racial and ethnic disparities in DBT access, with Black, Asian, and Hispanic women significantly less likely than White women to be screened at a facility with DBT or to receive DBT if the facility offered both DBT and digital mammography. If DBT is truly superior to digital mammography, this apparent disadvantage may worsen existing disparities in breast cancer outcomes. If it isn't, then widespread DBT use is unnecessary and wasteful.
After the NCI director suggested in the fall of 2020 that the "feasibility and relevance" of TMIST was in jeopardy, a working group was formed to reevaluate the trial. In its report released last month, the group recommended that the trial continue, but with protocol revisions that included reducing the sample size to 102,000 and developing specific targets for enrolling women from racial and ethnic minority groups. Unlike European trials that are also evaluating DBT, the group noted, TMIST is the only study that is representative of the U.S. population, includes women younger than age 50, and includes multiple rounds of screening.
In a 2019 commentary, Drs. Joy Melnikow and Joshua Fenton observed of DBT:
Diffusion of medical technology ahead of definitive evidence is common in the United States. ... Societal attitudes that place high value on innovation and technology create a fertile environment for the rapid adoption of novel but unproven interventions. ... When the evidence from randomized clinical trials catches up, interventions shown to add little value to previous approaches are often already embedded in practice, widely covered by health insurance (sometimes by mandate), and difficult to withdraw.
Initial findings from TMIST will be available in 2027 at the earliest if investigators can reach its lower enrollment goal. As radiology facilities across the country continue to "upgrade" from digital mammography to DBT and more states mandate insurance coverage of the newer but unproven technology, it remains to be seen whether this study's findings will have any effects on clinical practice.