Overview

Metaplastic breast cancer, also called metaplastic carcinoma of the breast, is a rare type of breast cancer that is very different from the typical ductal or lobular breast cancer. It describes a cancer that begins in one type of cell (such as those from the glands of the breast) and changes into another type of cell. Metaplastic carcinoma of the breast describes a range of cancers of mixed epithelial cells (cells that line the breast) and mesenchymal cells (the connective tissue of the breast). Most cases of metaplastic breast cancer start in the epithelial cells, and then change into squamous (nonglandular) cells. Because the cells that give rise to metaplastic breast cancer are not part of the normal breast gland, this cancer does not have estrogen receptors (ERs), progesterone receptors (PRs), or HER2 (a protein found in 25% of breast cancers). See Diagnosis for more information.

Metaplastic breast cancer is considered an invasive cancer, meaning that it has already spread beyond the duct or lobe at the time of diagnosis.

The breast is mainly composed of fatty tissue. Within this tissue is a network of lobes, which are made up of tiny, tube-like structures called lobules that contain milk glands. Tiny ducts connect the glands, lobules, and lobes, carrying the milk from the lobes to the nipple, located in the middle of the areola (darker area that surrounds the nipple of the breast). Blood and lymph vessels run throughout the breast; blood nourishes the cells, and the lymph system drains bodily waste products. The lymph vessels connect to lymph nodes, the tiny, bean-shaped organs that normally help fight infection.

Cancer may begin as a single, genetically abnormal cell. As this one cell divides, it eventually becomes a tumor (a mass of cells) and develops a blood supply to nourish its continued growth. At some point, cells may break off from the primary mass and move to other parts of the body in a process called metastasis. Metaplastic breast cancer can metastasize to the lymph nodes and other areas of the body, especially the lungs.

Statistics

Metaplastic carcinoma of the breast is rare.

Find out more about basic cancer terms used in this section.