A clinicopathologic and immunohistochemical study of nine cases

Objectives: Nine cases of thymomas with an extensive clear cell component are presented. Methods: The patients were six men and three women aged between 45 and 62 years (mean, 52 years). Presenting symptoms included shortness of breath, chest pain, and cough. Diagnostic imaging revealed anterior mediastinal masses in all patients, and all underwent thymectomy. Results: Grossly, the tumors varied from 3 to 9 cm in the greatest dimension. Four cases were invasive and five encapsulated. Histologically, the tumors were characterized by an epithelial cell component with extensive clear cell change, admixed with lymphocytes in varying proportions. Immunohistochemically, the tumors demonstrated the typical immunophenotype of thymomas characterized by positive staining of the epithelial cells for pancytokeratin and cytokeratin 5/6, absent expression of CD5 and c-kit, and reactivity of the lymphocytes for terminal deoxynucleotidyl transferase. Clinical follow-up available for six patients showed that all were alive and well 12 to 24 months after surgical resection. Conclusions: The cases herein presented highlight an unusual feature in thymoma that has more commonly been ascribed to thymic carcinoma. They also emphasize the importance of correct diagnosis to determine the appropriate treatment strategy and to accurately predict prognosis.