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A 55-year-old woman with no significant medical history presented with a self-palpated mass in her breast. Physical examination revealed a 2-cm firm, mobile mass in her left breast. Imaging studies showed an irregular 1.7-cm hypoechoic solid mass with angular margins and no associated calcifications. Prior annual screening mammograms were negative. An ultrasound-guided core needle biopsy revealed a malignant spindle cell neoplasm with extensive chronromyxoid differentiation and a minor invasive ductal component, consistent with a metaplastic breast carcinoma (Figure 1). A proliferation of small, round glands with open lumina and eosinophilic secretions was present adjacent to the metaplastic carcinoma. These glands were lined by a single layer of uniform epithelial cells with clear to vacuolated cytoplasm (Figure 2). An immunohistochemical stain for p63 confirmed the absence of myoepithelial cells around the small glands while demonstrating scattered expression in the spindle cell component of the metaplastic carcinoma (Figure 3A and B). S100 showed strong and diffuse nuclear and cytoplasmic staining in the glandular epithelium, and moderate expression throughout the metaplastic carcinoma, specifically in the chondroid elements (Figure 3C and D). Taken together, a diagnosis of metaplastic carcinoma with associated microglandular adenosis (MGA) was rendered. All components of the tumor, including the MGA, were negative for estrogen receptor (ER), progesterone receptor (PR), and HER-2/neu. MGA, a rare finding in the breast, is histologically characterized by an infiltrative proliferation of uniform small round glands with open lumina, eosinophilic secretions, and lack of a myoepithelial cell layer within normal breast stroma. MGA has a distinct immunophenotype of diffuse cytokeratin and S100 expression, lack of myoepithelial cells, and lack of ER, PR, and HER-2/neu expression. MGA may mimic well-differentiated invasive breast carcinoma; however, clues to MGA include the lack of 697822 IJSXXX10.1177/1066896917697822International Journal of Surgical PathologyMuller and Jorns research-article2017