Metaplastic breast cancer is a rare form of breast cancer, accounting for fewer than 1% of all breast cancers. It differs from the more common kinds of breast cancer in both its makeup and in the way it behaves.
Like invasive ductal cancer, metaplastic breast cancer begins in the milk duct of the breast before spreading to the tissue around the duct. What makes a metaplastic tumor different is the kinds of cells that make up the tumor.
When the cells of an invasive ductal tumor are examined under a microscope, they appear abnormal, but still look like ductal cells. Metaplastic tumors may contain some of these breast cells, too, but they also contain cells that look like the soft tissue and connective tissue in the breast. It is thought that the ductal cells have undergone a change in form (metaplasia) to become completely different cells, though it is not known exactly how or why this occurs.
Metaplastic breast cancers can also behave more aggressively than other kinds of breast cancers.