A blow to the breast can split open the fat cells. Free fat in the tissues is a slightly irritant and can cause scaring and the feeling of a lump. Sometimes the cause is obvious such as from a seat belt injury or fall, but, especially in women with large breasts, fat necrosis can happen without an apparent injury.
It can look very much like cancer but the appearances under the microscope will clearly distinguish it from cancer. It is important that a biopsy is performed before a lump is called fat necrosis if there is no clear history of a significant blow to the breast in that particular area. Even then it is best to follow the woman until the lump disappears, which it will over the next few weeks if it is only fat necrosis.
If it is shown to be fat necrosis, then no treatment is needed. There is a belief that a blow to the breast will increase the risk of breast cancer in the future, but this is NOT true.