I saw the "breast within a breast" sign.

The "breast within a breast" sign is used to describe the appearance of a breast hamartoma, also known as a fibroadenolipoma. A breast hamartoma is a benign mass made up of the same tissues that are found in a normal breast, giving the appearance of a breast within a breast on mammography. The "breast within a breast" sign is pathognomonic for a breast hamartoma and can help confidently diagnose these masses as benign without the need for further imaging or biopsy.