Double immunostaining with p63 and high‐molecular‐weight cytokeratins distinguishes borderline papillary lesions of the breast

Papillary breast lesions remain a source of diagnostic confusion because the full range of epithelial proliferations may arise within, or secondarily involve, papilloma. The expression of p63 and high-molecular-weight cytokeratins (HMWCK) was studied simultaneously in 33 papillary lesions including intraductal papilloma (IP, n = 10), atypical papilloma (AP, n = 8) and intraductal papillary carcinoma (IPC, n = 15) by double immunostaining. The myoepithelial cell nuclei were stained dark brown whereas the cytoplasms of usual ductal hyperplasia (UDH) and myoepithelium were stained purple. The myoepithelial layer was recognized as a dark brown dotted line at the epithelial stromal junction in all IP (10/10), most AP (7/8) and some IPC (7/15), suggesting that the retained myoepithelial layer in the papillary processes does not necessarily guarantee benignity. However, the malignant epithelial cells in AP and IPC were typically recognized as monotonous populations unstained with either chromogen. These monotonous cells contrasted with the proliferating cells of UDH in papilloma, which had intense purple cytoplasm in a mosaic-like fashion. The present data suggest that the double immunostaining with the two popular antibodies p63 and HMWCK is a useful tool for reproducible classification of papillary breast lesions.