Fig. 43.
(A) A normal terminal duct lobular unit (TDLU) composed of numerous acini lined by bland epithelium. (B) A group of acini are cystically dilated with tall cuboidal epithelium: ie, apocrine metaplasia. When imaged with ultrasonography, the TDLU in B is seen as clustered microcysts. (Reprinted from Warner JK, Kumar D, Berg WA. Apocrine metaplasia: mammographic and sonographic appearances. AJR Am J Roentgenol 1998;170:1375–9; with permission.)