Fat necrosis is a condition in which painless, round, firm lumps caused by damaged and disintegrating fatty tissues form in the breast tissue. This is usually the result of injury to the breast. Necrosis can sometimes occur after surgery or radiation therapy, as a result of the body trying to repair the damage to the breast by forming scar tissue.

It can be difficult to distinguish between breast cancer and areas of fat necrosis by conducting a breast physical exam since they may feel the same when touched. A needle biopsy, or in some cases a surgical excision, may be necessary to determine if cancer is present.

Some areas of fat necrosis will respond differently to injury. Instead of forming scar tissue, the fat cells die and release their contents. This can form a collection of fluid called an oil cyst. Oil cysts can be diagnosed by fine needle aspiration.