I found this explanation online:
After
breast surgery or after a breast injury -- from sports, accidents, or a seat belt burn -- you may develop a lump. These types of
breast lumps are called
breast fat necrosis, because they are made of bruised, injured, or dead fatty tissue.
The problem with fat necrosis in the breast is that it often
looks like breast cancer, even though it is benign. Fat necrosis may take many forms and is hard to diagnose. Take a look at the symptoms, causes, and treatments for breast fat necrosis.
What is Fat Necrosis of the Breast?
Fat necrosis is a
benign condition that can occur in your breast. It consists of fatty tissue that has been bruised, injured, or has died. Fat necrosis can result after any type of breast surgery, from biopsy to
reconstruction. Once fatty tissue has been injured or has died, it can gradually change into scar tissue or may collect as liquid within an
oil cyst. Fat necrosis
does not lead to the development of breast cancer, but it may sometimes cause
breast pain.