Ductal carcinomas are breast cancers that start in the breast ducts. Ductal carcinomas also can be divided into numerous subtypes. The two major subtypes are ductal carcinoma in situ and invasive ductal carcinoma.

Ductal carcinoma in situ (also known as DCIS or intraductal carcinoma) refers to cancer cells confined to the cells lining a breast duct. DCIS may be further subdivided by how the cancer cells grow in and along the ducts. Some tumors may be solid while others have holes or projections. DCIS may be multifocal, which means it can be found along the breast duct in more than one area of the breast. Depending on the number of ducts that contain DCIS, it can be referred to as either localized or diffuse.

Some DCIS cells may have small calcium deposits called microcalcifications. DCIS is frequently discovered by the appearance of these microcalcifications on screening mammograms. This type of cancer is typically noninvasive and is considered the earliest stage of breast cancer. Some experts view this type of cancer as a precancerous condition.

The other major subtype of ductal carcinoma is invasive ductal carcinoma (IDC),  also called infiltrating ductal carcinoma. IDC occurs when carcinoma in a duct breaks through the duct wall and invades other breast tissue. This form of ductal carcinoma is by far the most common of all invasive types of cancer, accounting for about 80 percent of all cases, according to the American Cancer Society.