The march of mammography
National Cancer Insitute/Bill Branson
Mammography machine 1991 is shown.
A series of nine randomized clinical trials (RCTs) between the 1960s and 1990s formed the foundation of the clinical use of mammography. These studies enrolled more than 600,000 women in the United States, Canada, the United Kingdom, and Sweden. The nine main RCTs of breast cancer screening were the Health Insurance Plan of Greater New York (HIP) trial, the Edinburgh trial, the Canadian National Breast Screening Study, the Canadian National Breast Screening Study 2, the United Kingdom Age trial, the Stockholm trial, the Malmö Mammographic Screening Trial, the Gothenburg trial, and the Swedish Two-County Study.3
These trials incorporated improvements in the technology as it developed, as seen in the fact that the earliest, the HIP trial, used direct-exposure film mammography and the other trials used screen-film mammography.3
Meta-analyses of the major nine screening trials indicated that reduced breast cancer mortality with screening was dependent on age. In particular, the results for women aged 40-49 years and 50-59 years showed only borderline statistical significance, and they varied depending on how cases were accrued in individual trials. “Assuming that differences actually exist, the absolute breast cancer mortality reduction per 10,000 women screened for 10 years ranged from 3 for age 39-49 years; 5-8 for age 50-59 years; and 12-21 for age 60-69 years.”3 In addition the estimates for women aged 70-74 years were limited by low numbers of events in trials that had smaller numbers of women in this age group.
However, at the time, the studies had a profound influence on increasing the popularity and spread of mammography.
As mammographies became more common, standardization became an important issue and a Mammography Accreditation Program began in 1987. Originally a voluntary program, it became mandatory with the Mammography Quality Standards Act of 1992, which required all U.S. mammography facilities to become accredited and certified.
In 1986, the American College of Radiology proposed its Breast Imaging Reporting and Data System (BI-RADS) initiative to enable standardized reporting of mammography; the first report was released in 1993.
BI-RADS is now on its fifth edition and has addressed the use of mammography, breast ultrasonography, and breast magnetic resonance imaging, developing standardized auditing approaches for all three techniques of breast cancer imaging.6
The digital era and beyond
With the dawn of the 21st century, the era of digital breast cancer screening began.
The screen-film mammography (SFM) technique employed throughout the 1980s and 1990s had significant advantages over earlier x-ray films for producing more vivid images of dense breast tissues. The next technology, digital mammography, was introduced in the late 20th century, and the first system was approved by the U.S. FDA in 2000.
One of the key benefits touted for digital mammograms is the fact that the radiologist can manipulate the contrast of the images, which allows for masses to be identified that might otherwise not be visible on standard film.
However, the recent meta-analysis discussed in the introduction calls such benefits into question, and a new controversy is likely to ensue on the question of the effectiveness of digital mammography on overall clinical outcomes.
But the technology continues to evolve.
“There has been a continuous and substantial technical development from SFM to full-field digital mammography and very recently also the introduction of digital breast tomosynthesis (DBT). This technical evolution calls for new evidence regarding the performance of screening using new mammography technologies, and the evidence needed to translate new technologies into screening practice,” according to an updated assessment by the U.S. Preventive Services Task Force.12
DBT was approved by the Food and Drug Administration in 2011. The technology involves the creation of a series of images, which are assembled into a 3-D–like image of breast slices. Traditional digital mammography creates a 2-D image of a flattened breast, and the radiologist must peer through the layers to find abnormalities. DBT uses a computer algorithm to reconstruct multiple low-dose digital images of the breast that can be displayed individually or in cinematic mode.13
Early trials showed a significant benefit of DBT in detecting new and smaller breast cancers, compared with standard digital mammography.
In women in their 40s, DBT found 1.7 more cancers than digital mammography for every 1,000 exams of women with normal breast tissue. In addition, 16.3% of women in this age group who were screened using digital mammography received callbacks, versus 11.7% of those screened using DBT. For younger women with dense breasts, the advantage of DBT was even greater, with 2.27 more cancers found for every 1,000 women screened. Whether such results will lead to clinically improved outcomes remains a question. “It can still miss cancers. Also, like traditional mammography, DBT might not reduce deaths from tumors that are very aggressive and fast-growing. And some women will still be called back unnecessarily for false-positive results.”14
But such technological advances further the hopes of researchers and patients alike.