Teaching Case

A 45-year-old female patient with a previous history of invasive ductal carcinoma in the right breast, who had undergone mastectomy, performed the annual follow-up mammogram of the left breast.

The study revealed a rounded density on the craniocaudal mammogram along the sternal edge of the film. In the mammograms of the previous years this density was absent.
The patient also had a previous thoracic CT, which showed a thin vertical muscle band in the anterior chest wall, superficially to the left major pectoralis muscle.

The sternalis muscle is an uncommon anatomic variant of the chest wall musculature with uncertain teleology and function [1].
The sternalis muscle is usually a long, thin, flat muscle, arising from the infraclavicular part of the anterior chest wall, running longitudinally superficial to the medial part of pectoralis major in a paramedian position. It frequently extends up to the caudal end of the sternum to insert on the costal cartilages, sternum, pectoral fascia or the aponeurosis of the external oblique within the anterior layer of the rectus sheath [1, 2].

The incidence of this anatomic variant varies widely between nationalities, with an overall incidence of 1-18%. It can be unilateral or bilateral [1].

The mammographic appearance of sternalis muscle is variable [2, 3]. It may be visible as a rounded, irregular, triangular or flame-shaped density on the craniocaudal mammogram along the sternal edge of the film. Its contours may be well-defined, irregular or spiculated. Furthermore, distinct pulling of the breast may lead to variations in the appearance of the muscle on yearly mammograms. The muscle is usually not seen on the standard MLO or ML views [2, 3].
With improved mammographic positioning, it will be seen more frequently [3]. In this particular case it was not visible in the previous studies, raising concern about a malignant lesion. Knowledge of this entity allowed the radiologist to make the right diagnosis.

CT and MR imaging are diagnostic when they show the longitudinal extent of the muscle, which lies anterior to the medial margin of the pectoralis major muscle [2].

Take home messages:
The sternalis muscle may have a variety of mammographic appearances that should be familiar to the radiologist to avoid confusion with a malignant lesion.
In the majority of cases, knowledge of this entity allows the correct diagnosis. However, in the rare cases in which the aetiology is uncertain, CT or MR imaging can be useful, allowing to confirm the diagnosis.