Routine screening mammography.
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Mass-like, asymmetric opacity at the medial posterior edge of the breast on the left CC projection.
The sternalis muscle is a normal anatomic variant of the chest wall. Mostly it appears on medial posterior edge of the breast on CC views, but may also be detected on MLO projections. Can be unilateral in 70%. Has varying shapes, contours and sizes and may mimic a mass lesion on mammogram. Knowledge of anatomy and mammographic appearance is important for avoiding confusion with a malignant lesion.
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