Analysis of a two-marker immunohistochemistry panel (TTF-1 and p40) for distinguishing lung adenocarcinoma from squamous cell carcinoma on destained direct cytologic smears

Cytologic sampling is widely practiced for diagnosis of advanced lung cancer. In the present era of targeted therapy, distinguishing adenocarcinoma (ADC) from squamous cell carcinoma (SCC) is pivotal for tailoring the tissue for molecular testing. Studies on application of immunocytochemistry (ICC) on direct smears remain limited in literature. This study was aimed to assess the utility of TTF -1 and p40 antibodies on direct cytologic smears to distinguish ADC from SCC. Lung cytologic samples from 2016 to 2022, that were morphologically non-small cell carcinoma (NSCLC) were subjected to ICC using TTF-1 and p40 antibodies which were applied directly on destained 95% ethanol-fixed Papanicolaou- stained smears. Based on the expression, the results were compared with the cytomorphological and histopathological diagnosis. Of the 44 cases of adenocarcinoma that were positive for TTF-1 by immunocytochemistry, 43 were positive by immunohistochemistry (IHC) in paraffin-embedded biopsy, with a sensitivity of 97.2% and positive predictive value (PPV) of 100%. All cases of SCC were positive for p40 both by ICC and IHC; sensitivity and PPV were 100% each. The agreement between ICC and IHC for both the markers were significant with a Kappa = 1.0. A two-marker panel consisting of TTF-1 and p40 antibodies on direct smears serves as a valuable tool in distinguishing ADC from SCC, thus overcoming the morphologic pitfalls of cytomorphology. Cytologic smears can be conveniently used for ICC, thus negating the need for immunohistochemistry and serving as a measure to conserve tissue for molecular testing.