Low grade intraductal breast carcinoma with apocrine features as a precursor of infiltrating apocrine carcinoma

A 50-year old patient was referred to the Department of Radiology for an annual screening mammogram. Radiologically, a non-palpable right breast lesion with microcalcifications was located in the upper external quadrant. On light microscopy, two adjacent but different lesions were identified that were both associated with apocrine adenosis, radial scar and proliferative fibro-cystic disease. The two areas of 8 respectively 10 mm diameter consisted of apocrine ductal carcinoma in situ (ADCIS) of low grade that intimately merged with distorted and compressed small clusters or tubules of apocrine cells with low grade nuclei and large amount of eosinophilic granular cytoplasm. However, these clusters lacked myoepithelial cells on both microscopic and immunohistochemical examinations and consecutively were interpreted as infiltrating areas of well differentiated apocrine carcinoma (AC). In this case, where small sized multifocal low grade infiltrating AC developed on a complex background of apocrine adenosis and sclerosing lesion, immunohistochemistry was instrumental in order to establish the absence of myoepithelial cells. Similar to other types of infiltrating carcinomas, low grade infiltrating AC may develop from low grade ADCIS, since both lesions have similar morphological and immunohistochemical profiles.