Differential diagnostic value of CD5 and CD117 expression in thoracic tumors: a large scale study of 1465 non-small cell lung cancer cases.

Background: Thoracic pathologists are frequently faced with tissue specimens from intrathoracic/mediastinal tumors. Specifically the differentiation between thymic and pulmonary squamous cell carcinomas (SqCC) can be challenging. In order to clarify the differential diagnostic value of CD5 and CD117 in this setting, we performed a large scale expression study of both markers in 1465 non-small cell lung cancer (NSCLC) cases.

Methods: Tissue microarrays of formalin-fixed paraffin-embedded resection specimens of 1465 NSCLC were stained with antibodies against CD117 and CD5. Positivity of both markers was correlated with clinicopathological variables.

Results: CD117 was positive in 145 out of 1457 evaluable cases (9.9 %) and CD5 was positive in 133 out of 1427 evaluable cases (9.3 %). 28 cases (1.9 %) showed coexpression of CD117 and CD5. Among the 145 cases that were positive for CD117, 97 (66.8 %) were adenocarcinomas (ADC), 34 (23.4 %) were SqCC, 5 (3.4 %) were adenosquamous carcinomas (ADSqCC), 8 (5.5 %) were large cell carcinomas (LC), and one (0.6 %) was a pleomorphic carcinoma (PC). In the CD5 positive group consisting of 133 cases, 123 (92.4 %) were ADC, 0 (0 %) were SqCC, 4 (3.0 %) were ADSqCC, 3 (2.2 %) LC and 3 (2.2 %) were PC. None of the 586 SqCC showed expression of CD5. No association of CD117- or CD5 positivity to patients' age, pathological stages or to T-, N-, or M- categories was observed.

Conclusions: A substantial subset of NSCLC exhibit positivity of CD117 and CD5. Since CD5 expression was not observed in pulmonary SqCC, but is expressed in the majority of thymic squamous cell carcinomas, the application of this immunomarker is a valuable tool in the differential diagnosis of thoracic neoplasms.