Any mass seen by mammography, or ultrasound, can be benign or malignant.
Masses seen in the posterior aspect of the mammogram (usually behind the normal shadow of the true mammary tissue) are more suspicious for an abnormal growth, than those seen in the normal plane of mammary tissue on the mammogram. All these posterior growths merit special attention, and usually biopsy. Unfortunately far posterior lesions are often difficult to biopsy by ultrasound or stereotactic mammogram, but a center with a dedicated breast MRI device can usually approach these for biopsy.