Lack of supportive evidence for the use of immunohistochemical staining to identify occult regional lymph node metastases in primary lung cancer.

Immunohistochemical (IHC) staining for the identification of nodal occult metastases (OM), not detected by routine histological examination, has been proposed for improved staging, prognostication and decision of adjuvant treatment in surgically treated primary lung cancer. In a prospective study, we analysed 178 cases of primary lung cancer stage I-III (N0-N1) for OM by immunostaining lymph node tissue using a broad-spectrum anti-cytokeratin antibody. OM were found in 7 (4 %) of the 178 cases. Using Kaplan-Meier analysis, overall survival was not significantly different between cases with stage I and cases upstaged to stage II because of OM (n = 3), or between cases with stage II and cases upstaged to stage III (n = 4). Likewise, the presence of OM was not significantly correlated with overall survival in univariable or multivariable Cox proportional hazards regression models, also when disregarding OM <0.2 mm in size. Given the low frequency of OM and lack of significant impact on survival in our study, the justification for including IHC staining of lymph nodes in lung cancer in clinical practise does not appear convincing. Moreover, we report several potential pitfalls in the use of broad-spectrum cytokeratin IHC staining for OM detection, for example staining of intra-nodal mesothelial cells.