The ultrasound signs of breast masses are explained by the histopathological data. Ultrasound masses are classified according to their shape and margin. Round or oval masses are benign when their margins are circumscribed (fibroadenoma, intramammary lymph node); on the other hand, with non-circumscribed margins (microlobulated or irregular), masses that are round or oval may be cancers. Seven histological types of round cancers have been identified: grade III invasive ductal carcinoma, colloid or mucinous carcinoma, medullary carcinoma, intramammary metastases, intracystic papillary carcinoma, lymphoma and high-grade phyllodes tumors. Irregularly shaped ultrasound masses with non-circumscribed margins are predominantly cancers but may in some cases be benign lesions such as sclerosing adenosis, a radial scar, fibroadenoma or phyllodes tumor.