For all their workflow advantages, digital mammograms do not improve health outcomes compared to older film detection technology, according to a new study.

Researchers at the University of Sydney in Australia found that a small increase in cancer detection following the transition from film to digital mammography did not correlate with a reduction in cancers diagnosed in between scheduled screenings. They concluded that while providing easier storage and management of images, digital mammography does not provide greater health benefits to women over film.

"A change to a screening program that increases cancer detection rates, as found in the systematic review, is only beneficial if the additional cancers detected would have otherwise presented at a later stage and caused morbidity and premature mortality," the study's lead author, Rachel Farber, told HCB News. "Interval cancers are cancers that are diagnosed after a woman has a negative screening result and before her subsequent scheduled screening. If the increase in detection rate in technology has increased the clinical utility of screening, then we would hope to have a subsequent decrease in the interval cancer rate. This would suggest that some of the additional screen detected cancers were clinically important and would otherwise have rapidly progressed if left undetected. However, in the systematic review we found there was no change in the interval cancer rate."

The transition to digital mammography began in the U.S. starting in 2000 when the FDA approved use of the technology, based on studies that suggested it would potentially be more specific in its findings. Proponents asserted that it would reduce the number of callbacks for positive findings, find more diseases, and result in fewer cancers diagnosed in between screenings (interval cancers).

The team sifted through seven databases and systematically reviewed 24 studies that compared film to digital mammography within the same population of asymptomatic women. Their meta-analysis included 16,583,743 screening examinations, with 10,968,843 on film and 5,614,900 digital.

They recorded an increase of 0.51 per 1,000 screens for cancer detection and a recall rate increase of 6.95 per 1,000 screens following the transition from film to digital mammography. The increase in cancer detection was largely attributable to more detection of DCIS (ductal carcinoma in situ), with little difference in invasive cancer detection.

These findings, according to Farber, indicate that additional cancers detected may be mostly overdiagnosed or slowly progressing cancers. The practice shift from film to digital mammography also increased the false positive rate, which can cause increased short-term anxiety for affected women, she says, raising questions about proposals for the adoption of new mammography and other imaging technologies for population screening.

"This study highlights the importance of carefully considering the effect a new technology has on intermediate and long term benefits and harms," she said. "Currently, tomosynthesis (3D mammography) and other imaging technologies are proposed for adoption in population screening in many countries worldwide. The existing evidence indicates that tomosynthesis improves initial screen detection measures (cancer detection and/or recall rates); however, new research in tomosynthesis screening should focus on evaluating both the short term and the intermediate- to long-term outcomes, including effect on interval cancer rates."

A number of studies have since been conducted to assess the value of transitioning from film to digital mammography, specifically whether it results in improved health outcomes for women. One conducted in 2018 in the U.K. found digital mammography to be “substantially higher” in detecting grade 1 and 2 early-stage, invasive cancers than film, including for ductal and lobular cancers. They showed no difference in recall rates or the detection of grade 3 invasive cancers.

“Looking back at film screen mammography at the beginning of the study, we see a similar difficulty with detecting grade 3 cancers,” Dr. Rosalind Given-Wilson, a radiologist at St. Georges University Hospitals NHS Foundation Trust in London, and a co-author, told HCB News at the time. “It does not appear to have worsened with digital, but also has not improved.”

The findings for the study out of the University of Sydney were published in JNCI: Journal of the National Cancer Institute.