Radiology Resident Case of the Week
Etiology/Pathophysiology
This is a normal variant.
Pathology:
There is no associated pathology, however, this muscle may be misinterpreted as a breast mass and undergo attempted surgical resection, only to discover during surgery that there is no breast mass, only a muscle.
Miscellaneous
Imaging
The muscle is usually identified first on the cranio-caudal projection mammogram in the medial/deep breast, typically unilateral, although it may be bilateral. The mediolateral oblique projection usually fails to demonstrate the sternalis muscle.
On the CC view, the muscle typically appears as a triangular or "flame" shaped structure, sometimes with ill-defined margins, and frequently with fat surrounding it.
CT or MRI clearly demonstrate the sternalis muscle as a longitudinal structure with a parasternal course. When supine, the this muscle is flattened and bandlike. When decubitus or prone, the sternalis muscle is mobile and may have a bulging appearance. The classic description on CT or MRI is that of a deep, vertically oriented parasternal tubular structure surrounded by fatty tissue.
DDX
None.
Key references
Bradley F, Hoover H Jr., Hulka C, et. al., The Sternalis Muscle: An Unusual Normal Finding Seen on Mammography.
ACR Code
Keywords: