When DCIS is diagnosed, the pathologist (a doctor specializing in diagnosing disease from tissue samples) will look for an area of dead or degenerating cancer cells, called tumor necrosis, within the tissue sample. If necrosis is present, the tumor is considered more aggressive. The term comedocarcinoma is often used to describe DCIS with necrosis.

Lobular carcinoma in situ (LCIS): Although not a true cancer, LCIS (also called lobular neoplasia) is sometimes classified as a type of noninvasive breast cancer, and this is why it is included here. It begins in the milk-producing glands but does not grow through the wall of the lobules.

Most breast cancer specialists think that LCIS itself does not become an invasive cancer very often, but women with this condition do have a higher risk of developing an invasive breast cancer in the same breast or in the opposite breast. For this reason, women with LCIS, in particular, should pay close attention to having regular mammograms (see below for guidelines).

Invasive (or infiltrating) ductal carcinoma (IDC): This is the most common breast cancer. It starts in a milk passage, or duct, of the breast, has broken through the wall of the duct, and invaded the fatty tissue of the breast. At this point, it can metastasize, or spread to other parts of the body through the lymphatic system and bloodstream. About 80% of invasive breast cancers are infiltrating ductal carcinomas.

Invasive (or infiltrating) lobular carcinoma (ILC): Invasive lobular carcinoma starts in the milk-producing glands, or lobules. Similar to IDC, it also can spread (metastasize) to other parts of the body. About 10% of invasive breast cancers are ILCs. Invasive lobular carcinoma may be harder to detect by a mammogram than invasive ductal carcinoma.

Inflammatory breast cancer: This uncommon type of invasive breast cancer accounts for about 1% to 3% of all breast cancers. Usually there is no single lump or tumor. Instead, inflammatory breast cancer (IBC) makes the skin of the breast look red and feel warm and gives the skin a thick, pitted appearance that looks a lot like an orange peel. Doctors now know that these changes are not caused by inflammation or infection, but by cancer cells blocking lymph vessels or channels in the skin. The affected breast may become larger or firmer, tender, or itchy. Inflammatory breast cancer is often mistaken for infection in its early stages.

Inflammatory breast cancer has a higher chance of spreading and a worse outlook than typical invasive ductal or lobular cancer. Inflammatory breast cancer is always staged as stage IIIB unless it has already spread to other organs at the time of diagnosis which would then make it a stage IV

Mixed tumors: Mixed tumors describe those that contain a variety of cell types, such as invasive ductal combined with invasive lobular breast cancer. In this situation, the tumor is treated as if it were an invasive ductal cancer.