There are many types of breast cancers, and correctly identifying each one is important to determine the proper treatment.

Breast cancers can be divided into two main overarching groups: the carcinomas and the sarcomas. Carcinomas are cancers that arise from the epithelial component of the breast. The epithelial component consists of the cells that line the lobules and terminal ducts; under normal conditions, these epithelial cells are responsible for making milk. Carcinomas comprise the vast majority of all breast cancers, and will be further discussed below. Sarcomas are rare cancers that arise from the stromal (connective tissue) components of the breast. These stromal component cells include myofibroblasts and blood vessel cells, and cancers arising from these "supportive" cells include phyllodes tumors and angiosarcoma. Sarcomas account for less than 1% of primary breast cancers.

Under the Microscope: Selected Types of Breast Cancer

  • Ductal Carcinoma

    Ductal carcinoma in situ (DCIS)

    DCIS is a pre-invasive lesion with atypical cells that grow within ducts and lobules and do not invade the breast tissue. DCIS has the potential to become invasive; thus, the goal of treating DCIS is to prevent the development of invasive carcinoma.

  • Invasive Ductal Carcinoma

    Invasive ductal carcinoma

    Invasive ductal carcinoma is the most common type of breast cancer. The malignant cells attempt to form small “ducts” or tubules to various degrees. This example is an intermediate grade (histologic grade 2) invasive ductal carcinoma.

  • Invasive Lobular Carcinoma

    Invasive lobular carcinoma

    Invasive lobular carcinoma is the second most common type of breast cancer. The malignant cells form single file lines or single cells and often show vacuoles within the cytoplasm. This example is a low-grade (histologic grade 1) invasive lobular carcinoma.

Types of Breast Carcinomas

Within the large group of carcinomas, there are many different types of breast cancer. The first major division is between in situ and invasive carcinoma. In situ carcinoma is "pre-invasive" carcinoma that has not yet invaded the breast tissue. These in situ cancer cells grow inside of the pre-exisiting normal lobules or ducts. In situ carcinoma has significant potential to become invasive cancer, and that is why it must be adequately treated to prevent the patient from developing invasive cancer. Invasive cancers have cancer cells that infiltrate outside of the normal breast lobules and ducts to grow into the breast connective tissue. Invasive carcinomas have the potential to spread to other sites of the body, such as lymph nodes or other organs, in the form of metastases.

Approximately 80% of breast carcinomas are invasive ductal carcinoma, followed by invasive lobular carcinomas which account for approximately 10-15% of cases. Invasive ductal carcinomas and invasive lobular carcinomas have distinct pathologic features. Specifically, lobular carcinomas grow as single cells arranged individually, in single file, or in sheets, and they have different molecular and genetic aberrations that distinguish them from ductal carcinomas. Ductal and lobular carcinomas may have different prognoses and treatment options, depending upon all of the other features of the particular cancer.

The remaining cases of invasive carcinoma are comprised of other special types of breast cancer that are characterized by unique pathologic findings. These special types include colloid (mucinous), medullary, micropapillary, papillary, and tubular. It is important to distinguish between these various subtypes, because they can have different prognoses and treatment implications.