Role of PAX-8, CD5, and CD117 in Distinguishing Thymic Carcinoma From Poorly Differentiated Lung Carcinoma

Aim:To determine if PAX-8, CD5, and CD117 can differentiate thymic carcinoma from poorly differentiated lung carcinoma.Design:Archived cases of thymic (n=13) and poorly differentiated lung (n=15) carcinoma were analyzed for intensity and proportion of expression of PAX-8, CD117, and CD5.Results:PAX-8 was positive in 69.2% of thymic and 5.8% of lung carcinomas. CD117 was positive in 84% of thymic and 26.6% of lung carcinomas. A total of 53% of thymic and none of the lung carcinomas were positive for CD5. Forty-six percent, 53%, and 69% of thymic carcinomas were dual positive for combinations of CD5/PAX-8, CD117/CD5, and CD117/PAX-8, respectively. None of the lung carcinomas were dual positive. Positivity for any 2 of the 3 markers was seen in 84% of thymic and none of the lung carcinomas. Triple positivity was seen in 53% of thymic carcinomas.Conclusion:Adding PAX-8 to CD117 and CD5 increases the diagnostic yield for thymic carcinoma.