By: Denise D. Farleigh, M.D.
Mammography, along with clinical breast examination, has been proven to reduce breast cancer mortality in randomized clinical trials. The use of screening mammography is associated with the detection of breast cancer at an earlier stage and smaller size, resulting in a reduction in mortality. While the successful use of film-screen mammography in breast cancer screening is one of the major achievements in medical imaging, there are numerous limitations inherent in the technology. The identification of fine microcalcifications and subtle soft tissue masses is key to the detection of early breast cancer. Film-screen mammography is neither perfectly sensitive nor highly specific. Not all breast cancers are detected with conventional mammography and there are a significant number of false positive examinations, which require additional imaging and biopsy.
 Film-screen mammography has important limitations in detecting subtle soft tissue lesions, particularly in the presence of dense glandular tissue. Perhaps the major limitation of traditional mammography is that the film serves simultaneously as the image receptor, display medium and long term storage medium for the image. This limitation can lead to loss of image contrast, critical in identifying subtle abnormalities. Recognition of these limitations has led to a search for alternative imaging technologies that might address current problems. Digital mammography is one of the most promising newer imaging techniques.
One of the principal advantages of any digital imaging system is the separation of image acquisition, process and display, allowing optimization of each of these steps. Additionally, advanced applications such as computer aided detection (CAD) can be applied easily to the digital mammogram, assisting in image interpretation.
Four types of digital mammography systems are currently being evaluated. Each uses significantly different technology to produce digital images of the breast. Three of these systems have been approved by the FDA for use in the United States, the fourth has undergone testing in Europe and Japan.
There have been several clinical trials evaluating digital mammography. The first evaluated the GE Senograph 2000D, with 641 women enrolled from several institutions. Each woman received both a digital mammogram and a film-screen study. All studies were read from printed images. Additional trials have utilized similar formats for other currently available systems. Larger clinical trials have also been performed, demonstrating a statistically significant reduction in false positive results with digital mammography, compared with film-screen examinations. In each of the early trials, the detection of breast cancer was similar between film-screen studies and digital mammograms. In the Oslo II Study there was a higher cancer detection rate with FFDM in women ages 50-69, although this did not reach statistical significance. The later trials have all used soft-copy reading stations, rather than printed images for digital mammography. This approach allows the interpreting physician to manipulate the mammographic image, allowing real time assessment of areas that ordinarily might have required additional work-up.
The American College of Radiology Imaging Network (ACRIN), funded by the National Cancer Institute, completed enrollment of more than 49,000 women from 35 sites in the United States and Canada into a trial evaluating five digital mammography systems. Like the previous trials, all participants received both a digital study and a film-screen mammogram. Follow-up will continue until 2008. The large number of participants will allow detection of small differences in sensitivity and specificity between digital and film-screen examinations. The relative cost-effectiveness of digital mammography will also be studied in the trial. Preliminary results from this trial are expected this fall.
Benefits offered by digital mammography include operational advantages as well as advantages in diagnostic ability and confidence. These improvements are difficult to measure, but positively impact the patient and her clinicians.
Digital mammography, like other digital modalities, allows digital storage and transmission of images, eliminating lost studies and eventually eliminating the need for a film library. Images can be sent electronically to several treating physicians or given to the patient without a loss of quality. This is an important improvement in patient management, particularly for the patient with a new diagnosis of breast cancer. In addition, there is virtually complete elimination of film artifacts. The variability of the film processor is also eliminated as a source of error.
From the perspective of the patient, the major advantage of digital mammography is speed. This is especially true of diagnostic studies, allowing quick review of the images by the radiologist and rapid determination regarding additional images or studies needed. In addition, both screening and diagnostic patients benefit since there is no wait while films are processed and reviewed. Images are available to the technologist and radiologist within seconds. There are positive benefits for the patient, technologist, physician and radiology administrator as patients can be scheduled with maximum efficiency.
Further advances using digital mammography are expected. These include increased utilization of computer aided detection, tomosynthesis, and contrast-enhanced digital mammography. Some of these modalities are currently under investigation, others (CAD) are already in use in many centers.
Providence Imaging Center recently installed three full field digital mammography (FFDM) units, setting a new standard for excellence in breast imaging in Alaska. PIC has completed the transition from film-screen mammography to become the first completely full field digital mammography facility in the state. The digital equipment selected, Hologic's Selenia, has the highest image quality available in today's market. The digitally formatted images of the breast are displayed on a physician's workstation within a short period of time for review by the radiologist. The radiologist can manipulate the image, optimizing the diagnostic information available. Messages can also be sent to the technologist, allowing additional images to be obtained promptly and the study to be individualized for the patient.
There have been major advances in digital technology applied to mammography over the past several years. Digital technology has clear advantages over tradition film-screen mammography, particularly in terms of efficient operation of the breast imaging center. While improved cancer detection has not been demonstrated in most of the clinical trials reported to date it seems likely that this will change in the coming years with additional developments in this rapidly changing field.
Published November 2005, Physician Practice.
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