A fat necrosis results from an injury to breast tissue, as occurs in auto accidents when the seatbelt squeezes the breast tissue. The injury causes a lump under the skin that is sometimes painful, according to WebMD. The skin around a fat necrosis can become red and have a bruised appearance.
The fat necrosis is an area of scar tissue that becomes hardened. According to the American Cancer Society, the individual is able to feel the lump through the skin. It is difficult for a woman to distinguish between a fat necrosis and a cancerous tumor. Even mammograms do not always provide enough evidence to determine if the lump is cancerous, making a biopsy necessary to determine what is causing the lump. Women with large breasts are more likely to form a fat necrosis than other women are, but the formation of this type of scar tissue does not increase the woman's chance of developing breast cancer.
Some damaged fat tissue responds to injury by dying and releasing the fat from inside the cell. This fatty fluid collects in a sac-like structure forming an oil cyst. The cyst is also a lump under the skin. With oil cysts, the doctor uses a fine needle to aspirate the fluid from the sac. Aspirating the fluid reduces the size of the cyst if it is bothersome, according to the American Cancer Society.