Study objectives: It is sometimes very difficult both clinically and pathologically to distinguish thymic epithelial tumors from primary lung carcinoma with massive anterior mediastinal involvement. The expression of KIT (CD117) in thymic epithelial tumors was investigated in order to evaluate its usefulness as a marker supporting differential diagnosis and choice of therapy.
Methods: We examined the immunohistochemical expression of KIT in 70 resected thymic epithelial tumors (thymomas, 50; thymic carcinomas, 20) that had been reclassified on the basis of the World Health Organization histologic classification system. We also compared the expression of KIT and CD5 in 20 thymic carcinomas with their expression in 20 resected pulmonary squamous cell carcinomas that were spreading directly into the mediastinum.
Results: Of the 50 thymomas, only 2 (4%) showed positive immunoreactivity for KIT (type A thymoma, 1; type B3 thymoma, 1), whereas 16 of the 20 thymic carcinomas (80%) showed positive immunoreactivity. Testing was positive for CD5 in 14 of the 20 thymic carcinomas (70%). In the pulmonary squamous cell carcinomas, in contrast, the immunohistochemical expression of KIT and CD5 was found in only 4 of 20 carcinomas (20%) and 3 of 20 carcinomas (15%), respectively. Furthermore, of the 40 specimens examined (either thymic or lung carcinoma) all 13 that were positive for both KIT and CD5 were thymic carcinomas, and 13 of the 16 that were negative for both were lung carcinomas.
Conclusion: KIT expression is a useful immunohistochemical marker for the diagnosis of thymic carcinoma, and its examination in combination with CD5 immunohistochemistry would greatly help in the differential diagnosis of primary thymic carcinoma from pulmonary squamous cell carcinoma. Further investigations at a genetic level should be encouraged, not only to define the role of KIT in the oncogenesis of thymic epithelial tumors, but also to establish target-based therapy.