Diagnosis: Sternalis Muscle - a normal variant. BI-RADS Category 2 - Benign.
Why It's Important: The sternalis muscle is a normal anatomic variant, present in 6 - 8% of people. It can be misinterpreted as a potentially worrisome mass on a screening mammogram, leading to unnecessary extra views, diagnostic workups and even biopsies.
If present, the sternalis courses longitudinally along the medial border of the pectoralis muscle, extending from the inferior clavicle to the inferior aspect of the sternum. There is great variation in the appearance of the sternalis muscle, ranging from just a few muscle fibers, to a thick, well-developed muscle. It is most commonly unilateral, but rarely can be present bilaterally. Occasionally, the sternalis can be used as a muscular flap in reconstructive surgery of the head & neck or chest wall.
Teaching Points: Imaging Findings - The pectoralis muscle can be seen posteriorly on the CC view approximately 30% of the time. A much smaller percentage of the time, the sternalis muscle is visible medially at posterior margin of the CC view mammogram.
It has a variable appearance on the mammogram. These are some of the common ways in which it can appear. - Round mass with circumscribed margins - Irregular mass with ill-defined margins - Flag shape triangular mass - Flame shape Vary in size from 3 – 15mm or more Aunt Minnie appearance Very rarely can be seen on MLO or lateral mammographic views Usually unilateral, but may be bilateral Isodense mass, surrounded by fat, intermittently visualized depending on positioning, partially visualized Can be seen on US – but blends in with normal musculature, isoechoic to fat, ellipse shape with hyperechoic fascia surrounding Cross-Sectional Imaging Best seen on MRI or CT scan with patient in prone or decubitus (rarely done) position as pulled away from chest wall
How differentiate from pathology? - characteristic location in posteromedial breast - May have characteristic shape (flag, etc) - Not visible on MLO view - can try to pick out on US - if need to can see on cross-sectional CT or MRI exam as a muscle band DDX = mass (benign or malignant) or variable attachment of pectoralis muscle
CT appears as a flat parasternal muscle, longitudinal in orinetation
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Resources For More Information: Bradley FM, Hoover HC, et al. The Sternalis Muscle: An Unusual Normal Finding Seen on Mammography. AJR 1996; 166:33-36. Snosek M, Tubbs RS, Loukas M. Sternalis muscle: what every anatomist and clinician should know. Clin Anat 2014;27(6):866–884. Jelev L, Georgiev G, Surchev L. The sternalis muscle in the Bulgarian population: classification of sternales. J Anat 2001;199(Pt 3):359–363. Trastour C, Machiavello JC, Chapellier C, et al. Sternalis muscle in breast surgery. Ann Chir. 2006;131(10):623-5. Raikos et al. Sternalis muscle: an underestimated anterior chest wall anatomical variant. Journal of Cardiothoracic Surgery 2011, 6:73. Demirpolat G, et al. Mammographic features of the sternalis muscle. Diagn Interv Radiol 2010; 16: 276-278. Ozbalci EA, et al. THE MUSCULUS STERNALIS: ULTRASONOGRAPHIC VERIFICATION OF A RARE BUT BENIGN MAMMOGRAPHIC FINDING. J Breast Health 2013; 9: 169-71. MEERKOTTER, D. Mammographic normal variant : the sternalis muscle. South African Journal of Radiology, [S.l.], v. 13, n. 3, p. 72, aug. 2009. Young Lee B, et al. The sternalis muscles: incidence and imaging findings on MDCT. J Thorac Imaging. 2006 Aug;21(3):179-83. Snosek M, et al. Sternalis muscle, what every anatomist and clinician should know. Clin Anat. 2014 Sep;27(6):866-84. 2014 Jan 16.