The lesions to be considered most often in the differential diagnosis of breast cancer are the following, in descending order of frequency: mammary dysplasia (fibrocystic condition of the breast), fibroadenoma, intraductal papilloma, lipoma, and fat necrosis.

STAGING

Currently, the American Joint Committee on Cancer and the International Union Against Cancer have agreed on a TNM (tumor, regional lymph nodes, distant metastases) staging system for breast cancer. The use of this uniform TNM staging system enhances communication between investigators and clinicians.

Table 17-2 sets forth the TNM classification.

PATHOLOGIC TYPES

Numerous pathologic subtypes of breast cancer can be identified histologically (

Table 17-3). These types are distinguished by the histologic appearance and growth pattern of the tumor. In general, breast cancer arises either from the epithelial lining of the large or intermediate-sized ducts (ductal) or from the epithelium of the terminal ducts of the lobules (lobular). The cancer may be invasive or in situ. Most breast cancers arise from the intermediate ducts and are invasive (invasive ductal, infiltrating ductal), and most histologic types are merely subtypes of invasive ductal cancer with unusual growth patterns (colloid, medullary, scirrhous, etc). Ductal carcinoma that has not invaded the extraductal tissue is intraductal or in situ ductal. Lobular carcinoma may be either invasive or in situ.

Except for the in situ cancers, the histologic subtypes have only a slight bearing on prognosis when outcomes are compared after accurate staging. Various histologic parameters, such as invasion of blood vessels, tumor differentiation, invasion of breast lymphatics, and tumor necrosis have been examined, but they too seem to have little prognostic value.

The noninvasive cancers by definition are confined by the basement membrane of the ducts and lack the ability to spread. However, in patients whose biopsies show noninvasive intraductal cancer, associated invasive ductal cancers metastasize to lymph nodes in about 1-3% of cases.