There are two types of tissue in the normal breast -lobules where milk gets made and ducts, which transport milk from the lobules to the nipple when breast-feeding (lactation). If a breast cancer develops in the lobule it is called lobular carcinoma. If it begins in a duct it is called ductal carcinoma. It is not better or worse to have a lobular or ductal cancer of the breast. About 85% of breast cancers begin in the duct and about 15% begin in the lobule. There are special types of ductal cancers as well (tubular, mucinous, colloid and medullary carcinoma, for example). If a breast cancer has penetrated through the lining of the duct or lobule it is called infiltrating or invasive carcinoma and has the potential to spread outside the breast to lymph nodes or other places in the body. A cancer that has not yet penetrated the lining is called in situ. These are cancers, but have not yet developed the ability to spread or metastasize to other places in the body. Ductal carcinoma in situ and lobular carcinoma in situ are discussed under the heading of Pathology Diagnoses in this section. The particular characteristics of each tumor are determined by examining tissue under the microscope. This tissue can be obtained by a surgical procedure in the operating room, by a needle biopsy in the office setting, or by using imaging techniques (mammogram or ultrasound) to perform a needle core biopsy of an abnormal area that cannot be felt.

A multitude of breast cancer-specific information is evaluated in determining the best options for treatment. Breast cancer can be thought of as two different diseases. Local-regional disease (the tumor in the breast and status of the lymph nodes under the arm) versus potential for distant disease (possibility of metastatic tumor cells in other locations in the body). Some treatments (i.e. surgery, radiation) are directed locally, while others (chemotherapy, anti-hormonal therapy (TAMOXIFEN) or special drugs (HERCEPTIN) work distantly as well as locally.