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Evolution of Mammography Screening: From Film Screen to Digital Breast Tomosynthesis
Plain Language Summary
Mammography screening has been adopted in many countries and population health programs because randomized trials showed that it reduces the risk of dying from breast cancer. These randomized trials used screen-film mammography (SFM) which means that the X-ray beams are captured on a film cassette. The films are then developed and reviewed on a light-box by the physician. However, technical developments have witnessed an evolution from screen-film to digital mammography (DM) which means that a specially designed digital so-called “detector” captures the X-rays (replacing the film) and converts the information to a digital image, which is displayed on a high-resolution computer monitor, and transmitted and stored just like computer files. The evidence prompting a change to DM screening came from studies comparing detection for SFM and DM (one of these was a randomized trial) that showed roughly similar or higher cancer detection using DM compared to SFM. There were also some technical efficiencies underlying a general shift to digital imaging in medical practice. In very recent years, a new technology known as digital breast tomosynthesis (DBT, or tomosynthesis), essentially a quasi-three-dimensional mammogram, has become available. So far, tomosynthesis seems capable of detecting more breast cancers then standard DM and it could potentially reduce false-alarms (known as false recalls or false-positive screen) but different studies show mixed findings on this issue. Although some countries have started to use tomosynthesis for screening, and several trials have been or are being done to address evidence gaps around this new technology, there are no recommendations to change to tomosynthesis screening because it is not yet known whether using tomosynthesis adds health benefit above what might be achieved with standard mammography screening.
Introduction
Since the 1980s mammography screening has been implemented in many countries, including population-based programs, following the results of the randomized controlled trials (RCTs) of mammography screening, as described in chapter “Estimates of Screening Benefit: The Randomized Trials of Breast Cancer Screening.” Alongside widespread adoption of mammography screening, there has been a continuous and substantial technical development from SFM to full-field digital mammography (FFDM) 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.
This chapter has three main sections, beginning with a brief overview of the technical background and development of the different mammography-based modalities. The second part focuses on the trials and studies that form the basis of the evidence for screening with FFDM versus SFM, and the rapidly emerging evidence on the introduction of DBT in breast screening. The third part pinpoints the evidence needed if and when breast tomosynthesis is adopted routinely into service screening for breast cancer.
Technical Background
Mammography
Screen-film mammography
Mammography is an X-ray examination of the breast used for decades in diagnosis of breast disease and as a screening modality. In analog mammography, also called SFM, the X-ray beams are captured on a film cassette. The films are then developed and reviewed on a light-box by the physician. Breast cancer is not one single disease entity; it rather comprises a wide range of types, growth rates, and growth patterns which is also mirrored in the X-ray image of the breast. Hence, the radiographic appearance of breast cancer ranges from barely detectable, minimal signs to apparent cancer growth and hence a clearly visible abnormality. Some radiographic patterns of breast cancer are readily detected when relatively small (at an early stage), such as spiculated lesions and tumors presenting with calcifications, whereas other lesions are challenging to detect such as tumors causing only a nonspecific density or areas with subtle architectural distortion ( Fig. 13.1 ).
Therefore, mammography is one of the most technically challenging areas in radiography since it requires a high spatial resolution, for fine details like microcalcifications, and outstanding soft-tissue contrast to enable visualization of soft-tissue lesions such as tumors. Moreover, a low radiation dose is crucial since the breast is a radiosensitive organ. SFM was hence refined to get high-resolution images of both soft-tissue lesions and calcifications. The randomized trials in mammography screening were all performed with SFM systems and hard copy (film) reading. Still, far from all screening centers have transitioned to DM, due to costs inherent in changing workflow from analog to digital and due to image storage issues.
Full-field digital mammography
Along the development of digital imaging, flat panel detectors were developed enabling so-called FFDM, which has been broadly used in diagnostic and screening practice since 2006. In FFDM a specially designed digital detector captures the X-rays (replacing the film) and converts the information to a digital image, which is displayed on a high-resolution computer monitor, and transmitted and stored just like computer files. Before the introduction of DM into practice, it was questioned whether the image quality and especially the spatial resolution of DM was sufficient for detection of minimal calcifications. It was concluded, however, that even if the spatial resolution of DM commonly was inferior to SFM due to technical reasons, the resolution is good enough for breast cancer diagnostics. The digital technique provided a number of other advantages compared to SFM, such as elimination of film-related inefficiencies, contrast resolution, quick transfer/teleradiology, and simplified storage of images in picture archiving and communication systems and image processing. It also reduces the need for technical repeats, because the radiographer can immediately see whether the image fulfills the quality standards or not and the woman does not need to come back for a new visit due to technical failure. Furthermore, digital images allows for potential application of computer-aided detection and other advanced applications.
Digital Breast Tomosynthesis
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