Application of immunohistochemistry to breast lesions.

CONTEXT Immunohistochemistry has an expanding role in mammary pathology that has been facilitated by a growing list of available antibodies and a better understanding of biology. OBJECTIVE To explore the key role of immunohistochemistry in guiding adjuvant therapy decisions and sentinel node staging in breast cancer, as well as the role of immunohistochemistry as an aid to distinguishing usual ductal hyperplasia from atypical ductal hyperplasia/low-grade carcinoma in situ; subtyping a carcinoma as ductal or lobular, basal or luminal; ruling out microinvasion in extensive intraductal carcinoma; distinguishing invasive carcinoma from mimics; and establishing that a metastatic carcinoma of unknown primary site has originated in the breast. DATA SOURCES Current literature is reviewed, including clinical and pathologic journals. CONCLUSIONS As new, targeted treatments for breast cancer are developed, pathologists can expect additional immunohistochemistry applications in the future.