Fat necrosis is a noncancerous lump that forms in an area of fatty breast tissue. The lump forms in response to damage to the tissue—for example, following an injury, breast surgery, a breast biopsy (taking a sample of tissue for analysis), or radiation therapy.
The lump is usually firm and painless, and the surrounding area may look red or sometimes dimpled; occasionally, the nipple may be inverted (retracted inward).
Fat necrosis often clears up by itself, although surgery may be advised if it persists or gets larger. A biopsy may also be advised to exclude the possibility of breast cancer.