Medullary carcinoma is a “well circumscribed carcinoma composed of poorly differentiated cells with scant stroma and prominent lymphoid infiltration” (1). It accounts for less than 5% of breast carcinomas (2,3,4). Patients with medullary carcinoma tend to be relatively young, and it constitutes up to 10% of carcinomas diagnosed in women ages 35 or younger. The mean age in several series ranged from 45 to 54 years (5,6,7,8). Bilateral carcinoma is uncommon in patients with medullary carcinoma in one breast, and synchronous or metachronous medullary carcinoma involving both breasts is a rare event (3,6,9). The size distribution of medullary carcinomas is not appreciably different from that of infiltrating duct carcinomas (5,6).

Because they have circumscribed margins and a firm consistency, medullary carcinomas can be mistaken clinically and radiologically for fibroadenomas (10). No ultrasound or mammographic criteria are specific for medullary carcinoma or for distinguishing medullary from circumscribed nonmedullary carcinoma (10,11). However, a tumor with an irregular or “jagged” margin on ultrasonography is unlikely to be a true medullary carcinoma (12).

Ipsilateral axillary lymph nodes tend to be enlarged in medullary carcinoma patients, even when no nodal metastases are present, a phenomenon that may complicate clinical staging (13). The average number of lymph nodes recovered from the axillary dissection specimen of a patient with medullary carcinoma is greater than for other types of carcinoma. This difference probably results from the greater ease of finding enlarged, hyperplastic lymph nodes that exhibit reactive changes when examined microscopically.

The typical intact medullary carcinoma is a moderately firm, discrete tumor. A distinct margin usually outlines the tumor when bisected and distinguishes it from the surrounding breast tissue, but some small medullary carcinomas have poorly circumscribed borders resulting from an intense lymphoplasmacytic reaction extending beyond the immediate perimeter of the tumor (5). The tumor has a lobulated or nodular structure that might be apparent on the cut surface. Necrosis is not unusual. As the amount of necrosis increases, there is a greater likelihood that the tumor will have cystic foci.

Medullary carcinoma is defined by a constellation of histopathologic features: prominent lymphoplasmacytic reaction, microscopic circumscription, growth of tumor cells in sheets (syncytial pattern), high nuclear grade, and high mitotic rate. When most, but not all, of these components are present, the tumor may be described as an “invasive duct carcinoma with medullary features.” A carcinoma is not medullary if it has one or more of the following features: invasive growth at the periphery of the tumor, a diminished lymphoplasmacytic reaction, nuclear cytology that is not high grade, or conspicuous glandular or papillary growth.

The diagnosis of medullary carcinoma cannot be made conclusively by needle core biopsy because of the limited sample provided by such materials. It is appropriate to report that the findings raise the possibility of medullary carcinoma and that the final classification depends on complete evaluation of the excised tumor.