A screening mammogram actually consists of two X-ray films, taken from the side (referred to as the “mediolateral oblique view”) and from above (referred to as the “craniocaudal view”), for each breast. A diagnostic mammogram, which may include additional views or magnifications, is usually performed following a suspicious finding on a screening mammogram or when a woman has a new symptom such as a breast lump. To create a uniform system of assessing mammography results, the American College of Radiology developed the Breast Imaging Reporting and Data System (BIRADS) (Table 1-3). This system includes five categories of assessment with increasing suspicion of malignancy, along with standard follow-up recommendations for each category.
Ultimately, mammograms do not detect cancer per se. Rather, they provide a means for the identification of tissue abnormalities, which are subject to interpretation by human observers who introduce variability into the process on the basis of their prior experience, training, perceptual capabilities, vigilance, and so on. Radiologists look for microcalcifications (tiny calcium deposits), architectural distortions, asymmetrical densities, masses, and densities that have developed since the previous mammogram. In some cases, breast ultrasound may be ordered as a follow-up to mammography to rule out cysts (fluid-filled lesions) or to better characterize the lesion and its solid components. For women with a BIRADS score of 4 or 5, biopsy and histological examination are generally necessary to determine whether the abnormality identified by imaging methods is benign and harmless or malignant and life threatening.
Traditionally, a diagnostic biopsy entails an open surgical incision to remove a lump or tissue sample. In the case of nonpalpable lesions, the surgeon may be guided to the position of the abnormal tissue by “wire localization,” in which a fine wire is inserted into the suspicious area and placement of the tip is confirmed by mammography. More recently, two minimally invasive biopsy methods have been developed for breast cancer diagnosis: fine-needle aspiration (FNA) for cytology and core-needle biopsy (CNB) for pathological assessment of breast tissue (Fajardo and DeAngelis, 1997; Morrow, 1995; Scott and Morrow, 1999; Sneige, 1991). A very thin needle and syringe can be used to remove either fluid from a cyst (standard FNA) or clusters of cells from a palpable mass (fine-needle aspiration biopsy [FNAB]). It is most frequently used to confirm a cyst by removing the cyst fluid. If the mass proves to be solid or only partially filled with fluid, the needle is quickly pushed back and forth through the solid tissue while suction is applied to free some cells, which are aspirated and analyzed by a cytopathologist. The sensitivity of FNAB cytology depends on the quality of the sample obtained and the experience of the cytologist, but it generally falls in the range of 65 to 98 percent (Scott and Morrow, 1999). Unfortunately, the ability to obtain samples of sufficient size from nonpalpable lesions is limited, and the retrieval of such samples can be done only with imaging guidance (ultrasound or mammography).
Thus, FNA is not as useful in such instances (Pisano et al., 1998b). Other limitations of this procedure are that it cannot distinguish between invasive cancer and in situ lesions, and it may produce inconclusive findings that require an additional core or open biopsy procedure.
CNB, although more traumatic for the patient, affords a higher rate of sensitivity. CNB uses a larger needle with a special cutting edge. The needle is inserted, under local anesthesia, through a tiny incision in the skin, and 5 to 10 small cores of tissue are removed. Tissue cores obtained by CNB are usually sufficiently large to allow pathologists to distinguish between invasive and noninvasive types of breast cancer. For nonpalpable lesions, CNB must be combined with an imaging modality (X ray, ultrasound, or less frequently, MRI) to target the suspicious tissue. The choice of guidance method depends on the experience of the radiologist or surgeon, the equipment available, and the type of lesion.