The search for alternative methods of screening for breast cancer is stimulated by a desire to increase accuracy and to overcome some of the technical barriers associated with film screen mammography. We do not address adjunctive modalities but discuss only those with a possible use as a breast cancer screening tool. Many screening methods have and are being suggested, including electrical impedance, light scanning, and infrared imaging of the breast. There has been no substantiation of a benefit accruing from these technologies for detection of early breast cancer in a screening situation. However, digital full-field mammography and magnetic resonance (MR) mammography seem to hold promise in this regard.
All current routine screening of the breast is performed with dedicated equipment, due to the requirement of high spatial resolution and contrast for mammography. Digital techniques promise to fulfill these requirements. There are two general techniques of performing digital mammography. The first and less desirable method is digitizing a film screen mammogram. Although this allows for some of the desirable features of digital mammography, it also preserves some undesirable attributes of the film screen technique. The second method replaces the film screen cassette combination with a specialized detector capable of transforming the latent x-ray image into an electronic digital image.
There are major benefits and significant barriers to digital mammography use for routine screening. Because there is no film processing involved, image acquisition is much faster, producing a mammogram in approximately 6 seconds as compared to the 90 to120 seconds required for film development.
In a situation in which 40 or more screening examinations are done per day, this higher performance could increase throughput. Once a digital image is acquired, it can be manipulated to adjust contrast and brightness to the individual patient. Regions of interest may also be magnified. This can all be accomplished with the single x-ray exposure used to produce the original image and may be particularly useful in the dense breast, in which a masking effect for early cancer detection can exist.
Issues of concern associated with screening mammography are the variability of interpretation and the false-positive rate involved with screening. With the evolution of digital screening mammography, computer video diagnosis and computer-aided detection (CAD) is becoming a reality. Using detection algorithms for specific mammographic features associated with malignancy, this software is able to indicate these findings on the digital image. However, this benefit is balanced by the indication of densities that may be fictitious or due to overlapping structures within the breast. Thus, if there were too many false-positives, CAD would lose its use.
However, if these false-positives could be kept to a minimum, perhaps one to two per image, and detect a high degree of true findings, perhaps 85% to 90%, then real benefit could be realized. Recent presentations indicate that these factors are real. CAD could then function as a second reader and possibly have a positive impact on variability and false-positive diagnoses while not diminishing the high sensitivity of screening mammography. Underserved areas, especially rural settings and remote facilities that provide mammographic services, could benefit from the ability to transport digital images almost anywhere. This can be done without loss of image fidelity and rapidly enough to be of clinical use. Preliminary results from a screening trial comparing routine film screen images with digital images on the same patient suggest at least equivalence in detection of breast cancer and a significantly lower recall and false-positive rate for digital screening mammography.
Significant barriers to use of a digital technique for screening mammograms exist and must be addressed before its full potential can be realized. In order to attain the benefit of image processing, the digital images must be viewed on specialized monitors. These must have the ability to present the entire image at full resolution with a small pixel size.
At present, these are prohibitively expensive, and one would require at least two monitors in routine practice. Additionally, the brightness of these monitors does not approach that of a mammographic view box. This could affect detection, especially microcalcifications, which can be an important finding of DCIS. Additionally, although the ability to portray many more levels of grey increases soft-tissue display, the monitor only allows for small segments of this enhanced grey-scale display per monitor image. Thus, image manipulation is required, which could increase the time required for interpretation. These problems are currently being addressed by novel new methods of displaying digital images and improving workstation software. Finally, at present, full-field digital screening mammography units are very costly.