Objectives: To determine the diagnostic accuracy of cytomorphology for subclassifying non-small cell lung cancer into adenocarcinoma (AC) and squamous cell carcinoma (SqC), and the utility of immunocytochemistry (ICC) for poorly differentiated cases. Study Design: Preoperative cytologic diagnoses of SqC, AC, or non-small cell carcinoma (NSCC) were compared with surgical resection diagnoses. NSCC cases with adequate cell block material were stained with CK7, CK5/6, TTF-1 and p63 antibodies and subclassified as SqC, AC, or equivocal. Results: 123 of 140 (88%) preoperative cytologic specimens had a malignant diagnosis, including 36 SqC, 72 AC, 6 adenosquamous carcinomas (ASC), and 9 large cell carcinomas (LCC). Accurate cytologic diagnoses were rendered in 18 (50%) SqC and 49 (68%) AC; 26 of 54 cases with a diagnosis of NSCC had adequate cell block material for ICC. TTF-1 and p63 accurately classified 8 of 9 (89%) SqC and 8 of 8 (100%) AC. One SqC, 2 ASC and 3 LCC had equivocal staining, while 1 ASC and 3 LCC stained as AC. Conclusions: The majority of SqC and AC (56%) can be classified by cytomorphology alone. TTF-1 and p63 ICC on cell blocks can provide accurate subclassification for NSCC in the vast majority of cases.

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