No signs of increased overdiagnosis with digital mammo
The switch from film to digital mammography in large, population-based breast cancer screening programs improves the detection of life-threatening cancer without significantly increasing detection of clinically insignificant disease, according to a study published online Oct. 2 in Radiology.

Digital mammography delivers higher sensitivity at detecting breast cancer, which has raised concerns that its introduction into screening programs would increase the diagnosis of clinically unimportant cancers.

Adriana M. J. Bluekens, MD, from the National Expert and Training Centre for Breast Cancer Screening in Nijmegen, the Netherlands, and colleagues conducted a large observational study to compare the clinical relevance of cancers detected by film mammography with those found via digital technology.

The researchers examined radiologic abnormalities and pathologic tumor characteristics detected by more than 1.198 million screening mammograms performed between October 2003 to December 2007. Mammograms were provided as part of the Dutch biennial screening program, which targets women ages 50 to 75 years. A total of 87.3 percent of mammograms during the study period were film-based, and 12.7 percent were digital.

Data analysis showed an increased incidence of ductal carcinoma in situ (DCIS) in the years after the introduction of population-based screening with digital mammography. “Although all grades have potential to progress and become invasive, the development of low-grade DCIS can extend over more than three decades, whereas high-grade DCIS is associated with far more rapid cancer invasion,” wrote Bluekens et al. Thus, low-grade DCIS lesions have been linked with overdiagnosis.

Detection and treatment of high-grade DCIS, in contrast, may help reduce mortality by preventing the development of life-threatening invasive breast cancer.

Recall rates were 2.6 percent for initial film exams and 1.3 percent for subsequent exams. The rates for digital exams were 4.4 percent and 2.1 percent, respectively. Digital mammography detected significantly more cancers than film-based mammography: 6.8 per 1,000 women on initial digital exam compared with 5.6 per 1,000 on initial film exam.

Digital mammography also detected more DCIS and more invasive disease than film-based mammography on both initial and subsequent exams.

“In the mix of low- to high-grade DCIS lesions, there is no shift to the detection of low-grade lesions in digital screening. Instead of this, we noticed a larger amount of high-grade lesions, which are regarded as precursors of high-grade invasive tumors,” Bluekens said in a press release. Detection of high-grade DCIS with digital mammography was 58.5 percent, compared with 50.5 percent for film.

“This gain is largely due to enhanced depiction of microcalcifications with digital mammography resulting in improved detection of DCIS and invasive carcinoma with an intraductal component,” Bluekens said.

“The follow-up period of the different digital screening programs is not sufficiently long enough to analyze mortality effect separately from that of film mammography,” Bluekens added. “However, surrogate parameters, such as stage distribution and tumor characteristics of [digitally] detected cancers, do indicate the continuation of mortality decrease with the transformation of [film to digital] in screening programs.” 

The current study demonstrated that digital is superior to film in the early detection of clinically relevant cancers, the researchers concluded. However, they cautioned that the results were based on analysis of data from the Dutch screening program, with its focus on balancing the rates of detection, recall and false-positives. Numbers from the U.S. screening program, which focuses more on a high detection rate, would likely be different.