Abstract
Breast lesions with a prominent myoepithelial cell component constitute a heterogeneous group of benign and malignant neoplastic proliferations. These lesions are often dual epithelial-myoepithelial, but may be purely myoepithelial cell in nature. Benign epithelial-myoepithelial lesions typically maintain the morphology and immunophenotype of the normal bilayer epithelial myoepithelial structures. However, the distinction between the two cell components is not always clear-cut in malignant lesions in which the histogenesis of myoepithelial cells remains uncertain. Neoplastic biphasic epithelial-myoepithelial lesions of the breast include adenomyoepithelioma (AME), pleomorphic adenoma and adenoid cystic carcinoma. Four histological patterns of classical AME have been described: tubular, lobulated, spindle-cell and adenosis variants. Overlapping patterns occur and some AMEs display an intraductal papillary pattern that may represent a fifth variant. AME can be benign or malignant. Classical AME may show atypical features, which are not sufficient for the diagnosis of malignancy (atypical AME). Atypical AME is recognised as a lesion of uncertain malignant potential with limited metastatic capability. Based on the histological features, we propose a classification of malignant AME (M-AME) into three variants: M-AME insitu, M-AME invasive and AME with invasive carcinoma. In this review, we provide an overview of myoepithelial lesions of the breast focusing on the classification of AME to improve not only the consistency of reporting but also help to guide further management decision-making.
Highlights
The human breast incorporates a branching ductal network lined by an inner layer of polarised luminal epithelial cells and an outer layer of myoepithelial cells (MECs), separated from the stroma by a lamininrich basement membrane
5 CD10 CD10 is similar to Smooth muscle myosin heavy chain (SMMHC) staining in the majority of cases but in our experience, it is less robust with expression absent in some cases for unknown reasons
Focal background staining of stromal myofibroblasts can be seen with both CD10 and SMMHC, but CD10 shows a higher rate of non-specific staining of epithelial cells and it does not stain blood vessels
Summary
The human breast incorporates a branching ductal network lined by an inner layer of polarised luminal epithelial cells and an outer layer of myoepithelial cells (MECs), separated from the stroma by a lamininrich basement membrane. SMA is a robust marker that is usually retained in poorly fixed or infarcted tissue It is typically strongly positive in myoepithelial cells (MEC). If the myoepithelial layer is attenuated, interpretation of p63 can be difficult, as there are large gaps between positive nuclei It is a squamous cell marker and can be expressed by lesional cells in some tumours. CD10 is similar to SMMHC staining in the majority of cases but in our experience, it is less robust with expression absent in some cases for unknown reasons. It can be useful when it is retained and positive. Focal background staining of stromal myofibroblasts can be seen with both CD10 and SMMHC, but CD10 shows a higher rate of non-specific staining of epithelial cells and it does not stain blood vessels
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