Breast masses are a common clinical finding, and may be palpable or nonpalpable, benign or malignant. The majority of palpable breast masses are benign, but 10% of women who present with this finding will have a diagnosis of cancer.  Approximately 4% of breast cancers will present with a palpable mass and no mammographic or ultrasonographic evidence of disease. 
A delayed or missed breast cancer diagnosis can severely affect patient outcome (and is a frequent cause of malpractice litigation).
Evaluation of a breast mass is guided by findings on history, physical exam, imaging, and biopsy.  A triple test of clinical breast exam, imaging (e.g., mammography and ultrasonography), and needle biopsy can lead to a definitive diagnosis in nearly all cases.