Sternal muscle simulating asymmetry on mammography

The sternal muscle is an unusual accessory muscle of the chest wall, located adjacent to the sternum, medial to the pectoralis major muscle. Its function is uncertain. It is important to know it so as not to be confused with an asymmetry in the medial quadrants of the craniocaudal (CC) view of mammography. It is not usually identified in the mediolateral oblique view (MLO).

Computed tomography of the same case, showing the sternal muscle.