Background: Distinction of small cell lung carcinoma (SCLC) from non-small cell lung carcinoma (NSCLC) is critical because of the differences in prognosis and management. Patients with SCLC usually present with distant metastasis, and clinicians demand an accurate diagnosis in order to initiate appropriate therapy. Limited cytology material, occasionally with crush artifact, is not uncommon. Therefore, robust cytomorphologic features and a small immunostaining panel would be ideal to differentiate SCLC from NSCLC and other neuroendocrine neoplasms. We evaluated CD56 and the quantitative Ki-67 immunohistochemical panel in comparison to synaptophysin and chromogranin, along with cytomorphology to diagnose SCLC. Design: Eighty-eight cases of SCLC were retrieved from the cytology archives of The Johns Hopkins Hospital. Forty neuroendocrine neoplasms were used as control cases. Results: SCLCs included 33 lung cases and 55 metastatic lesions. The specimens were obtained by fine needle aspiration, thoracocentesis, bronchoalveolar lavage and abdominal paracentesis. CD56 was expressed in 98.9% of SCLCs, which is significantly more sensitive than synaptophysin and chromogranin. The Ki-67 labeling index was high (>70%) in all cases, which is a reliable marker to differentiate SCLC from other neuroendocrine neoplasms and NSCLC. Conclusion: CD56 and quantitative Ki-67 along with cytomorphology is a robust immunohistochemical panel to differentiate SCLC from other neuroendocrine neoplasms and NSCLC.

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