Case 36 Answer 3
BI-RADS 2, Benign. This is classic fat necrosis. Fat Necrosis looks ugly on ultrasound! It can be hypoechoic or mixed echogenicity, well- or poorly defined, and can have a lot of shadowing. The ultrasound pictures just confuse things, and may lead to an unneccessary biopsy, as happened in this case.
Just to give you another example, here is another classic example of fat necrosis:
See related case below.