Myoepithelial cell staining patterns of papillary breast lesions: from intraductal papillomas to invasive papillary carcinomas

We evaluated 25 intraductal papillomas and 18 papillary carcinomas (invasive, 4; micropapillary ductal carcinoma in situ [DCIS], 5; cases originally classified as intracystic/intraductal papillary carcinoma, 9) by calponin, smooth muscle myosin heavy chain (SMM-HC), and p63 immunostains. Calponin, SMM-HC, and p63 labeled myoepithelial cells (MECs) in all intraductal papillomas and all micropapillary DCIS cases. The invasive papillary carcinoma cases were uniformly negative for all stains. The 9 cases originally diagnosed as intracystic/intraductal papillary carcinoma showed more variable results, with identification of an MEC layer in only 4 cases. Comparison of staining of MECs by these 3 stains showed that calponin was more sensitive and intense than SMM-HC; however, there was cross-reactivity with myofibroblastic cells. Staining with p63 was discontinuous, making interpretation of an intact myoepithelial layer difficult. Of 9 cases originally classified as intraductal papillary carcinoma, 5 showed absence of a basal MEC layer by immunohistochemical analysis. The lack of a basal MEC layer in these cases suggests a spectrum of progression from in situ to invasive disease and might help explain distant metastases from previously reported "intraductal papillary carcinoma."