Because these units are prototypes, costs can range as high as $400,000 to $500,000 per unit. In this age of cost-containment coupled with the extremely small reimbursement for screening mammography, there is little consumer (i.e., mammography providers) interest in purchasing these units. It is hoped that as more data concerning digital screening mammography emerge and the cost of wet processing, film, cassettes, and x-ray film storage are eliminated, consumer demand will increase and price will decline.
The mammogram is a sensitive method for detection of early breast cancer in a screening population. However, it frequently lacks the specificity to separate benign from malignant lesions. These limitations have fueled the interest for magnetic resonance mammography (MRM). Initial examinations were performed without contrast enhancement, and separation of benign from malignant lesions was marginal. However, with the use of contrast agents specifically tailored for MR imaging, improvement in sensitivity and specificity over nonenhanced MRM has been realized.
The basis for contrast-enhanced MR imaging of the breast centers on the vascularity and vessel permeability difference of benign and malignant tumors. Benign lesions frequently are sparsely vascularized, whereas malignant breast tumors require additional blood supply, as they grow much over 1 cm in size. The contrast agents used for MR imaging are paramagnetic agents and tend to concentrate in tissue with more abundant vasculature. Because these agents alter the MR signatures, the conspicuity of areas with greater blood supply than normal is enhanced. This neovascularity associated with malignant tumors is only a partial explanation for their concentration of paramagnetic agents. In addition, increased capillary permeability and local changes in osmolar pressure also contribute to the enhancement characteristics of breast cancer.
General screening for breast cancer with MRM deserves special mention. Screening implies the testing of a large population to identify the small subset that may have preclinical disease. As noted in the section Principles of Cancer Screening, just as the decision to screen an asymptomatic population for disease must meet well-defined benchmarks, so too does the introduction of a new screening technology if it is to replace the existing standard-bearer. MRM as a screening test does not meet these conventional criteria as well as conventional mammography.
We do not have sufficient data regarding sensitivity and specificity, and MRM is also much more costly at present than mammography and is an invasive technique that requires injection of paramagnetic contrast agents. Thus, MR imaging should not be used in a pure screening setting until more data concerning test accuracy and reproducibility are available. Furthermore, MRM would need to undergo the same degree of quality assurance scrutiny and standard setting related to quality control procedures and personnel standards as has screen-film mammography under MQSA. For the interested reader, the article by Weinreb and Newstead is an excellent overview of MR imaging of the breast.
We do not specifically address ultrasound in this chapter, because its use is strictly adjunctive and not meant to be used for screening purposes. However, a recent article by Kolb et al. reports on 11,220 consecutive patients prospectively examined with screening ultrasound, in which all 3,626 women with dense breasts and negative clinical and mammographic exams were included. In this group, 11 cancers (0.30%) were found by ultrasound only, and the cancer detection rate in this group was increased 17%. Although this is an intriguing report, ultrasound requires further evaluation and validation by multiple centers before it can be recommended as a screening tool, even in a subset of extremely dense breasts.
Robert A. Smith and Carl J. D’Orsi
R. A. Smith: Cancer Screening, Department of Cancer Control, American Cancer Society, Atlanta, Georgia
C. J. D’Orsi: Diagnostic Radiology, University of Massachusetts Memorial Medical Center, Worchester, Massachusetts