Dedicated breast computed tomography: the optimal cross-sectional imaging solution?

Mammography has widespread acceptance as a screening modality for breast cancer, with mortality reduction of 30–40% in screened populations (1). However, mammography has limitations. Its sensitivity for cancer detection when used for screening lies in the 80% range, ranging from 70.8% in the 40–44 age group to 84.5% for 75–89 year old patients (2). In addition, there is an accepted recall rate of 10% (with 90% of those recalled findings proving to be non actionable) and an expected negative biopsy rate in range of 60–75% for mammographically-identified suspicious lesions, both of which result in added cost, anxiety, and potential morbidity. One inherent limitation of planar mammography is that a three-dimensional, volumetric structure (the breast) is imaged, but the resultant data are displayed in a two- dimensional manner. This leads to superimposition of tissue and limits effectiveness, especially in women with dense breasts. Carney et al showed a decrease in mammographic sensitivity as a function of increasing breast density (87.0% sensitivity in women with fatty breasts versus 62.9% in women with extremely dense breasts) (3). Berg and colleagues demonstrated an even lower sensitivity rate of 45% among women with extremely dense breasts (4). Yankaskas and colleagues showed that as breast density increases, so does recall rate from 2.4% in almost entirely fat breasts to 6.9% in extremely dense breasts (5). This makes intuitive sense, as most non-calcified cancers and normal fibroglandular tissue share similar X-ray attenuation values; as a result, the presence of a significant amount of this “background noise” (heterogeneously or extremely dense parenchymal pattern) can lead to misrepresentation of normal tissue as a potential abnormality requiring recall (false positive) or, more ominously, non-detection of a cancer obscured by adjacent or overlapping tissue (false negative). Obtaining two near-orthogonal mammographic views (rather than a single view) serves as an attempt at three-dimensionality, helping to sort out real from spurious findings, and aiding in localization of potential abnormalities. However, even when additional views (e.g. 90° lateral, step-obliques, rolled view s) are obtained, it remains the mammographer’s burden to construct a mental 3-D virtual image of the breast, and superimposition of tissue in women with any significant degree of density persists as a limiting constraint. Full-field digital mammography (FFDM), with its improved dynamic range, tissue contrast, and ability to post-process digital images, was expected to improve cancer detection, particularly in dense-breasted women. This was confirmed in the Digital Mammographic Imaging Screening Trial (DMIST), which showed statistically significant improved sensitivity for cancer detection with FFDM compared to SFM, but only in women under 50, pre- or peri-menopausal, and with dense breasts, the latter factor appearing to be the key (6,7). However, the issue of tissue superimposition lingers, for while the digital images can be manipulated in many ways, they remain planar. Imaging the breast with ultrasound (US) more closely approximates a 3-D examination, with 2 planes (depth and the chosen transducer orientation plane) visualized at any single moment with the third dimension added by the operator’s real time sweep of the transducer. In most cases, all tissue from skin to chest wall can be interrogated in a single frame, such that superimposition of tissue is not a factor. The effectiveness of US as an adjunctive breast cancer screening tool has been demonstrated recently, with American College of Radiology Imaging Network (ACRIN) protocol 6666 showing a supplemental detection yield (over and above cancers found by mammography alone) of 4.2 cancers per 1000 high risk women undergoing screening (8). However, the real time nature of breast US introduces operator-dependence to a far greater degree than is found in mammography and US is most effectively used when the imaging physician performs or is present during scanning. This can be time consuming, and therefore physician labor-intensive and cost-ineffective. Another limitation of breast US, as demonstrated by Berg and colleagues, is a relatively low positive predictive value for biopsy at 8.9% versus 22.6% for mammography, in the screening setting (8). Evolving technological enhancements, including elastography and 3D/4D breast US, may improve its performance in the near future. An additional exciting bonus to the advent of FFDM is its ability to allow new, novel computer-based technologies. Digital breast tomosynthesis (DBT) is one example that partially addresses the 2-D limitations of planar mammography. DBT is performed using a mammographic X-ray tube but differs significantly from standard digital mammography in that it obtains multiple low-dose images at angled increments along an arc centered around the compressed breast. This information is computer-compiled and subsequently displayed on a workstation that allows the breast to be examined on a slice-by-slice basis, at a variety of slice thicknesses, chosen by the interpreter. Each individual slice has the appearance of a planar mammographic view (either craniocaudal or mediolateral oblique, depending on how the breast was positioned at acquisition), albeit displaying information from only one tomographic plane at a time. The third dimension is added as the examiner scrolls through the breast from skin to skin. Using this imaging technique reduces the element of tissue superimposition, theoretically improving lesion detection capability and mass margin characterization. Smith et al showed that DBT combined with FFDM resulted in improved clinical performance (as measured by area under the ROC and recall rate reduction) compared with the use of FFDM alone, independent of radiologist experience (9). However, despite its theoretical promise, DBT has not yet found its niche in routine clinical practice, in part perhaps due to lack of a validated optimal viewing protocol, well-defined indications, dose considerations and the large number of reconstructed images that must be reviewed. Stereoscopic digital mammography (SDM) is another spin-off of FFDM whose purpose is to mitigate the limitations of superimposed breast tissue. The technique is patterned after the concept of binocular vision; having two eyes separated by several centimeters allows each eye to receive slightly discrepant views of the world that the brain is able to compile into a 3-D construct. In SDM, two “discrepant” images are obtained in both the CC and MLO projections, each angled +/− 5° relative to standard tube position. These slightly angled images are sent to a specially-designed workstation where each image is projected separately, then filtered through a polarizing screen. The observer, who is wearing cross-polarized viewing glass, can see only one of the two angled images with each eye. From this data set, the observer’s brain constructs a virtual 3-D image of the breast. While tissue is still superimposed (the way trees viewed in a forest would be), the viewer can perceive the relative depth and location of structures, helping to differentiate significant findings from spurious ones related to simple superimposition. Getty et al, demonstrated a PPV of true lesion detection of 32% for SDM and 26% for standard 2D digital mammography (10). However, these promising results came with a “cost”. Since each of the two stereo views was obtained at a standard dose, each stereo acquisition in the MLO and CC views was twice that of the standard dose. This is untenable in a population based screening situation and work is underway to duplicate the study with a dose similar to standard digital mammography. While strides have been made toward advancing breast imaging beyond planar mammography, each modality described above has significant limitations. As a result, the search for other promising technologies continues. Dedicated breast CT (DBCT) represents a new technology which may allow true three dimensional imaging of the breast using the same fundamental x-ray contrast mechanism with which breast imagers are already familiar.