Interobserver Agreement and Assay Reproducibility of Folate Receptor α Expression in Lung Adenocarcinoma: A Prognostic Marker and Potential Therapeutic Target

Abstract

Context.—Lung cancer is the leading cause of cancer deaths in the United States and globally. Folate-targeted drugs are among the promising new targeted therapies for lung cancer, provided predictive biomarkers can be identified for optimal patient selection. Objective.—To evaluate the interobserver agreement and reproducibility of an immunohistochemistry assay for folate receptor α as a potential predictive marker for folate-targeted therapies. Design.—Immunohistochemistry using anti–folate receptor α antibody 26B3 was performed on formalin-fixed, paraffin-embedded tissues. The M-score, a semiquantitative measure of staining intensity and proportion of tumor cells staining, was determined for each specimen. Interobserver agreement was assessed using lung adenocarcinoma specimens stained at a single site and evaluated by 3 independent pathologists. Interinstrument reproducibility assessed 20 specimens stained by 3 different automated stainers. Interlaboratory agreement was determined on 5 specimens, repeatedly stained on each of 5 days, at 3 different study sites. Results.—Folate receptor α expression was identified in 39 of 54 cases of lung adenocarcinoma (72%) and 4 of 37 cases of lung squamous cell carcinoma (11%). Agreement among 3 pathologists was found in 24 of 26 cases (92%). Interinstrument reproducibility was observed in 19 of 20 cases (95%). Agreement among 3 laboratories was found for 49 of 50 specimens (98%). Conclusions.—Immunostaining of folate receptor α in lung adenocarcinomas is reproducible across staining platforms and among laboratories. Agreement among pathologists is achieved using a semiquantitative scoring method. An accurate and convenient method for determining folate receptor α expression offers a potentially invaluable tool for selecting patients for folate-targeted therapies.