Utility of Cluster of Differentiation 5 and Cluster of Differentiation 117 Immunoprofile in Distinguishing Thymic Carcinoma from Pulmonary Squamous Cell Carcinoma: A Study on 1800 Nonsmall Cell Lung Cancer Cases

Background Mediastinum harbors a mixed bag of neoplastic lesions with varied therapeutic and prognostic implications. Distinguishing pulmonary squamous cell carcinoma from thymic carcinoma is a challenging task owing to rather nonspecific clinicoradiological features and a considerable overlap in histomorphological features. Thus, we evaluated the diagnostic utility of cluster of differentiation 5 (CD5) and CD117 immunohistochemical markers in distinguishing these tumors. Materials and Methods The archives of department of histopathology were retrospectively reviewed from June 2012 to May 2016. Formalin-fixed paraffin-embedded blocks of 1800 cases diagnosed as nonsmall cell lung carcinoma were retrieved, and immunohistochemical expression of CD5 and CD117 was evaluated in these cases. Results Adenocarcinoma (980; 54.44%) was the most common histological subtype of lung carcinoma observed in our study. CD117 was positive in 171 out of 1800 cases (9.5%) of which 120 cases (70.17%) were adenocarcinoma, followed by 40 cases (23.4%) of squamous cell carcinoma. Immunoreactivity for CD5 was observed in 209 cases (11.61%), 200 (95.7%) cases of which were diagnosed as adenocarcinoma. None of the cases diagnosed as squamous cell carcinoma on histomorphology showed CD5 immunoexpression. Conclusion While thymic squamous cell carcinomas are well known for CD5 and CD117 coexpression, none of the cases of squamous cell carcinoma arising in lung express CD5. These markers are a diagnostic tool to distinguish a primary lung squamous cell carcinoma from thymic carcinoma, particularly in the setting of a central pulmonary lesion with mediastinal involvement.