Frozen section examination demonstrates fibrosis and cystic spaces. Also seen are areas of compressed glands with a lobular orientation. The glands are lined by a single layer of epithelial cells with oval nuclei and regular arrangement. No true invasion of glands into the adjacent stroma is seen. This patient probably has which of the following?

The lesion described is sclerosing adenosis. The tip-offs in the description are the references to compressed glands and lobular orientation. Sclerosing adenosis is a common component of fibrocystic disease, and occurs when fibrosis distos the normal lobular architecture. The result can be some fairly bizarre, but completely benign, compressed glands that may mimic carcinoma on both frozen and permanent sections. Low power examination is often helpful, as this emphasizes the lobular character of the lesion. Note: Lesions actually involving the epithelium of the duct system typically have multiple layers of cells and range from usual ductal hyperplasia (with low risk of invasive carcinoma and characterized by the presence of both myoepithelial cells and epithelial cells within the duct) through atypical ductal hyperplasia (with medium risk of invasive carcinoma and characterized by ductal carcinoma in situ-like features only involving poions of a duct) to ductal carcinoma in situ (with relatively high risk of invasive carcinoma and characterized by clearly abnormal features such as loss of myoepithelial cells and formation of cribriform patterns involving complete cross-sections of ducts). Lobular carcinoma in situ typically produces lobular units whose lumina are completely filled with epithelial cells. Ref: Chandrasoma P., Taylor C.R. (1998). Chapter 56. The Breast. In P. Chandrasoma, C.R. Taylor (Eds), Concise Pathology, 3e.