CASE REPORT CASE REPORT 72 SA JOURNAL OF RADIOLOGY • August 2009 CASE REPORT Introduction Mammography is an established screening tool for breast cancer.1 Early detection of breast cancer has a significant impact on morbid- ity and mortality.1 Correct interpretation of mammograms can avoid false negatives and unnecessary biopsies in respect of benign chang- es. However, interpretation of mammograms can be challenging.2 The medial aspect of the breast is a ‘blind spot’ on the medio-lateral oblique view. As mammographic techniques improve, more atten- tion is being given to imaging the medial breast, and the sternalis muscle will be revealed with increasing frequency.2 A case of the mammographic findings of the sternalis muscle is presented. Features of this normal mammogram variant are described. In addition, the computed tomography findings are described and presented. Case report A 37-year-old woman presented with bilateral ‘lumpy’ breast paren- chyma on palpation. There was no family history of breast cancer and she had not undergone any previous breast surgery. All of her three children had been breastfed. She was not taking any form of hormonal manipulation. A mammogram and ultrasound were performed to further assess the palpable changes. The mammogram revealed bilateral, moderately dense breast parenchyma. Normal skin and nipple con- tours were present. No clustered microcalcifications or architectural distortion were noted. However, in the cranio-caudal view of the right breast, a triangular opacity was noted in the postero-medial aspect of the breast. The medio-lateral oblique view did not contrib- ute to the assessment of this density. Ultrasound did not conclusively identify the density seen on mammogram in this patient. A limited computed tomography (CT) scan of the chest wall was performed to gain clarity of the mam- mographic findings (Figs 1a, 1b and 2). The CT scan demonstrated a flattened band of muscle density lying anterior to the medial margin of the pectoralis muscle. This structure was separated from the underlying pectoralis muscle by a thin band of fat (Fig. 3). The finding on mammography, which was further elucidated by CT scanning, was the sternalis muscle (Fig.4). Mammographic normal variant: the sternalis muscle Debra Meerkotter, MB BCh, FCRad (D) SA Regional Imaging, St Vincents Hospital, Launceston, Tasmania, Australia Fig. 1a. Right cranio-caudal view. Fig. 1b. Left cranio-caudal view. Mammographic.indd 72 8/17/09 3:05:05 PM CASE REPORTCASE REPORT Discussion The sternalis muscle is an uncommon normal variant that may pres- ent a diagnostic dilemma as it requires differentiation from pathol- ogy.2 The sternalis muscle may be unilateral or bilateral. Cadaveric studies demonstrate its incidence in 8% of the male and female population. It is found in all population groups.3 It is twice as often unilateral as bilateral.3 The function of this muscle is uncertain, with various proposed interpretations regarding it. Many surveyed surgeons are unaware of this muscle,1 and many anatomy textbooks do not include it.2 This narrow muscle bundle runs parallel to the sternum, with fibres running at 90 degrees to the pectoralis major muscle. It extends infraclavicularly to the inferior aspect of the sternum. The pectoral nerves provide innervation to the sternalis muscle. It is anatomically distinct from the pectoralis muscle and the abdominal musculature. The origin of this muscle is uncertain, with pectoralis major, rectus abdominus and sternomastoid all being proposed. Mammographic techniques are continually improving, and for this reason we will encounter this muscle with increasing fre- quency. When the cranio-caudal view is properly performed, 30 - 40 % of mammograms will reveal the pectoralis muscle.4 It is seen as a smooth, convex density along the posterior edge of the image; distinct from this, the sternalis muscle may be visualised as a focal density on the medial aspect of the breast. Mammographically, a single focal soft-tissue density will be seen in the medial aspect of the breast only on the cranio-caudal view. The density may be flame-shaped or triangular, it may have an irregular margin, and it is generally surrounded by fat. The maxi- mum dimension of the density is 1 - 2 cm. The density is not seen on the medio-lateral oblique view as the medial breast is a ‘blind spot’ on this view.2 73 SA JOURNAL OF RADIOLOGY • August 2009 Fig. 2. Right medio-lateral oblique view. Fig. 3. Sternalis muscle coronal CT image. Fig. 4. Sternalis muscle sagittal CT image. Mammographic.indd 73 8/17/09 3:05:06 PM CASE REPORT CASE REPORT 74 SA JOURNAL OF RADIOLOGY • August 2009 What are the guidelines in differentiating this feature from pathology? The location on the cranio-caudal view and the normal medio-lateral oblique view are useful guides. In addition, good clinical examination is essential in assessing the breast. The dif- ferential diagnosis is that of a mass (benign or malignant) or vari- able attachment of the pectoralis muscle (this is a slip of pectoralis muscle abutting the sternum and is imaged on both cranio-caudal and medio-lateral oblique views). If doubt still exists, a CT scan or magnetic resonance imaging (MRI) can confirm the presence of the sternalis muscle. Unnecessary biopsy of this normal variant can be avoided if one is aware of the entity. On CT scanning, the longitudinal course of the muscle can be defined. It lies parallel to the sternum and superficial to, and on the medial aspect of, the pectoralis muscle. Often, the sternalis is separated from the pectoralis muscle by fat. CT scanning in the decubitus position encourages the muscle to bulge forward, provid- ing improved visualisation of the muscle.2 MRI scanning will provide superior soft-tissue resolution and will define the muscle clearly. MRI scanning can be optimised by placing the patient prone, which utilises gravity to pull the muscle away from the chest wall by dependent breast tissue.1 Conclusion The sternalis muscle is an unusual normal variant which is seen on occasion on the cranio-caudal view of a mammogram. The main differential is that of significant pathology. There are certain features that will guide the radiologist in the interpretation of the mam- mogram, so avoiding unnecessary biopsy. If there is any doubt in interpreting the mammogram, further correlation with CT or MRI will enable a conclusive diagnosis of this normal variant. 1. Berg WA, Birdwell RL. Diagnostic Imaging: Breast, 1st ed. Manitoba, Canada: Amirysis, 2006. 2. Bradley FM, Hoover HC, Hulka CA, et al. The sternalis muscle: an unusual normal finding seen on mam- mogram. Am J Roentgenol 1996; 166: 33-36. 3. Jeng H, Shuen-Jing S. The sternalis muscle: an uncommon anatomical variant among Taiwanese. J Anat 1998; 193: 287-288. 4. Ramirez-Escobarr MA, Salmeron IR. What is this breast mass? Br J Radiol 1998; 71: 573-574. Mammographic.indd 74 8/17/09 3:05:06 PM