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Ductal carcinoma in situ is a very early form of breast cancer that does not spread. It arises in the cells that line the ducts, the tube-like structures that carry milk to the nipple.

This form of cancer is not invasive, meaning it is not found beyond the duct walls or in adjacent breast tissue. It is not considered life threatening. However, if left untreated, ductal carcinoma in situ can sometimes turn into an invasive cancer.

Sometimes abnormal cells can grow in the milk-producing glands, or lobules, a condition called lobular carcinoma in situ. This is not considered breast cancer, but it can raise a woman’s risk of developing breast cancer.

Invasive ductal carcinoma is breast cancer that starts in the milk ducts and spreads to the surrounding tissue. It is the most common form of breast cancer. Another common type, invasive lobular carcinoma, starts in the milk-producing glands and spreads to nearby breast tissue.

Both types of invasive cancer can spread to other parts of the body through the lymphatic system, a network of organs, vessels, and nodes that drains excess fluid from the body’s tissues and traps foreign substances, such as viruses and bacteria. Invasive breast cancers can also spread to distant organs through the blood.

There are many types of rare breast tumors. Inflammatory breast cancer accounts for a small percentage of all breast cancers. It is a fast-growing cancer that can sometimes be mistaken for a breast infection, because symptoms include swelling and skin redness.

Paget’s disease refers to breast cancers that develop under the nipple and areola. The condition causes a change in the appearance of the nipple and, sometimes, the areola.

Rare forms of invasive ductal carcinoma include medullary breast cancer, in which the tumors are soft and fleshy. Other types include mucinous breast cancer, in which tumors contain mucus, and tubular breast cancer, the cells of which appear to be tube-shaped when examined under a microscope.

Metaplastic breast cancer also occurs rarely. It’s believed that this form of cancer develops in one type of breast cell, which then changes into another type. Papillary breast tumors are also rare; these cancers contain small finger-like projections in their cells.