This is a biopsy confirmed lobular carcinoma of the breast.

This case has several teaching points:

1. Not all hypoechoic lesions are cysts.

2. A simple cyst should be simple. Look at the ultrasound carefully; this is not a simple cyst.

3. Beware of any developing lesions in the breast of a postmenopausal woman. It's a red flag.

The article in the reference is a very good summary of why lobular carcinomas are what they are and why we miss them. 

Teaching point: not all hypoechoic structures in the breast are cysts. Compared with case 21370.