Pathologic examination of the sentinel lymph node: what is the best method?

Sentinel lymph node biopsy (SLNB) has become an acceptable alternative to complete axillary dissection to determine whether breast cancer has spread to axillary lymph nodes. Yet the best method for pathologic examination of the sentinel lymph node (SLN) remains controversial. For years there has been speculation that micrometastases in axillary lymph nodes were clinically insignificant and thus lymph nodes did not require sectioning at close intervals. Yet essentially all studies, including a recent large prospective study, have found a significantly poorer prognosis associated even with metastases less than 2 mm in size-the most common definition of micrometastasis-suggesting that such small metastases cannot be safely overlooked. The use of immunohistochemistry (IHC) to detect keratin proteins will reveal metastatic breast carcinoma in about 18% of axillary lymph nodes that appear negative on routine stains. The preponderance of evidence to date suggests a significantly poorer prognosis in patients with such occult metastases, although data from large prospective studies are lacking. Molecular techniques such as polymerase chain reaction (PCR) offer even more sensitive methods for detecting occult metastasis in SLNs, although false positives are a particular problem in techniques that do not permit morphologic correlation, and for now they remain a research tool. Intraoperative examination of the SLN permits a completion axillary dissection to be performed during the same procedure if metastatic tumor is found; however, intraoperative techniques such as cytologic examination and frozen section lack sensitivity, and can result in loss of up to 50% of the SLN tissue. A proposal for optimal pathologic examination of the SLN is offered based on the above data.