Answer: Clear cell hidradenoma

Histology: This is a highly cystic lesion, with polypoid infoldings of stroma giving a papillary appearance. The lesion has a bland lining which resembles normal urothelium. Mitosis are present but not numerous, and there is essentially no cytologic atypia. The lining cells often demonstrate nuclear grooves, and there appears to be a superficial layer of ductal cells at the top. By immunohistochemistry, the lesion does not demonstrate myoepithelial cells by smooth muscle myosin heavy chain or actin stains. Instead, the lesion is diffusely immunoreactive for p63, while the more luminal cells demonstrate accentuated labeling for EMA.

Discussion: Papillary carcinomas of the breast typically demonstrate diffuse immunoreactivity for estrogen receptor. They should demonstrate greater cytologic atypia than the current lesion, which resembles benign urothelium. Papillary carcinomas of the breast may have peripheral myoepithelial cells which label for p63, but would not show diffuse immunoreactivity for p63. This lesion does not demonstrate invasive growth as would be seen with an invasive carcinoma arising from a papillary carcinoma of the breast.

Clear cell hidradenomas have been reported within the breast, and in this case the polypoid/papillary architecture simulated that of a papillary carcinoma. The absence of immunoreactivity for actin and smooth muscle myosin heavy chain accentuates the difficulty.