Fat necrosis and oil cysts are two different conditions that result from the same process: damage to fatty tissue inside the breast. This can happen as a result of an injury to the breast or treatments such as surgery and radiation therapy.
Fat necrosis develops when the body replaces damaged cells with firm scar tissue. Both during clinical breast exams and on a mammogram, it’s often impossible to tell the difference between fat necrosis and breast cancer. So biopsy is needed for diagnosis.
With oil cysts, the fat cells don’t turn into scar tissue but instead die and release their contents. This causes fluid-filled sacs to form within the breast. Oil cysts are usually diagnosed with fine needle aspiration.