Breast cancer remains the third leading cause of death among women in the United States and the leading cause of death in women ages 40-55. In the year 2003, 211,300 new cases of invasive and 55,700 new cases of in situ breast cancer will be diagnosed. In this time frame it is also anticipated that 40,200 deaths from breast cancer will occur. In an effort to address this significant public health risk, mammographic screening was introduced. Screening mammography reduces mortality from breast cancer through early tumor detection; this benefit has been demonstrated in multiple randomized controlled trials. Indeed, the introduction of screening mammography into the general population has shown the ability to decrease the mortality rate from breast cancer by as much as 50%. In the United States alone, the death rate from breast cancer has dropped almost 20% in the last decade, after having been stable for the prior 50 years. Although this dramatic reduction in mortality may be partially attributable to advances in therapy, the temporal course of the decrease suggests that the effect is largely due to the widespread implementation of screening mammography. However, while it is clear that mammographic screening for breast cancer has proven invaluable in the early detection and diagnosis of breast cancer, it remains an imperfect tool.
Mammography’s Shortcomings
Conventional two-view film screen mammography does not detect all cancers. In fact, as many as 20% of cancers that become clinically evident over the course of 1 year will not have been visible by screening mammography performed within that year. A major factor contributing to this limitation of mammography is the “structured noise” that is created by the overlap of normal structures within the breast, which are superimposed on each other in a standard two-dimensional mammogram. These overlapping structures can obscure a lesion, making it impossible for the radiologist to perceive it on a mammogram, and this effect becomes more pronounced as the density of the breast increases. It is this recognition that a significant percentage of breast cancers are not detected by screening mammography that has prompted recent interest in looking to other modalities as possible adjunctive screening tools. In particular, magnetic resonance imaging and whole breast screening ultrasound have shown promise in detecting mammographically occult cancers, and the incremental benefit over screening mammography has been most pronounced in the radiographically dense breast. But while interest in both MRI and screening breast ultrasound continues to grow, it must be borne in mind that, unlike mammography, neither of these techniques has been shown to decrease the mortality rate from breast cancer. Tomosynthesis, on the other hand, capitalizes on the strengths and proven abilities of mammography; indeed, tomosynthesis is mammographyonly better.
Tomosynthesis is a three-dimensional x-ray technique for imaging the breast. The breast is compressed in a manner analogous to conventional mammography, and during the 7-second compression, the tomosynthesis unit acquires a series of images of the breast from multiple angles. Because tomosynthesis imaging is a three-dimensional technique, each breast is compressed only one time for a complete examination. The projection images are then combined to “synthesize” thin slices through the breast, rendering individual planes in sharp detail. The total radiation dose for the tomosynthesis study is less than the dose of a standard two-view mammogram.
Proven Clinical Benefits
In conjunction with a vendor of digital mammography technology, Massachusetts General Hospital has developed the only full-field digital breast tomosynthesis system currently in clinical use, and the vendor has licensed the technology for commercial development. Multiple pilot research studies conducted at Massachusetts General Hospital have demonstrated the clinical benefits of tomosynthesis over conventional mammography. Display of the three-dimensional breast volume with each plane in isolated sharp focus enhances lesion visibility and facilitates analysis of lesion margins by removing the confusing overlap of adjacent structures. This may allow the radiologist to not only find more cancers, but also make more accurate diagnoses of breast lesions. In our initial pilot study, 16% more cancers were detected using tomosynthesis imaging compared with conventional mammography. Furthermore, the acquisition of the information as a three-dimensional volume also permits precise lesion localization within the breast, overcoming another challenge in the interpretation of standard mammograms.
Furthermore, the ability of tomosynthesis imaging to eliminate “pseudo-lesions” produced by the superimposed projection of normal structures on the two-dimensional image also suggests that the technique will result in fewer women being recalled unnecessarily for additional imaging from screening mammography. Currently, approximately 25% of women called back from screening mammography for additional diagnostic imaging are shown to have no demonstrable abnormality, their possible lesion having been caused by the superimposition of normal breast tissues. These represent false-positive screening studies. Our initial work suggests that approximately 85% of these callbacks can be avoided using tomosynthesis. Such a reduction would have a dramatic impact on screening for breast cancer in the United States and could translate to approximately a million women every year who would not be unnecessarily recalled for additional imaging, thus avoiding additional radiation exposure, emotional anxiety, and increased health care costs.
Although screening mammography has had an undeniable positive impact, breast cancer remains a significant cause of mortality in women, underscoring the need for other options in the screening armamentarium. Digital tomosynthesis will fill this role. One of the distinct advantages offered by tomosynthesis is that it extends the capabilities of conventional mammography, an imaging modality that is affordable, available, well studied, and proven to be effective in decreasing the mortality rate from breast cancer. The simultaneous increase in the cancer detection rate, coupled with a decrease in the callback rate, at a radiation exposure comparable to that of conventional mammography represents a clear advance in breast cancer screening. Tomosynthesis represents the future of breast imaging, and it is my belief that the continued development and implementation of tomosynthesis will result in further reductions in mortality from breast cancer.
Elizabeth Rafferty, MD, is associate director of breast imaging, Avon Comprehensive Breast Center, Massachusetts General Hospital, Boston.
