Cystic and Encapsulated Atypical Thymoma (World Health Organization Type B3)

Objectives: Eight cases of encapsulated and cystic atypical thymomas (World Health Organization type B3) are presented. Methods: Cases were identified after review of more than 500 thymomas. Histologic material from surgical resections was evaluated. Results: The patients are seven men and one woman aged 43 to 71 years. Seven patients presented with nonspecific symptoms while one mass was found on autopsy. The tumors were 4 to 6 cm in greatest dimension and showed prominent cystic changes with an epithelial proliferation arranged in sheets and cords. Increased mitotic activity and nuclear pleomorphism were not identified. All tumors were encapsulated. Immunohistochemical studies showed the tumor cells positive for keratin cocktail, keratin 5/6, and p63. Clinical follow-up was available in five patients. All were alive without recurrence. Conclusions: These cases represent an unusual variant of atypical thymoma that can be misdiagnosed as thymic carcinoma or multilocular thymic cyst. Awareness of this histologic growth pattern is important for accurate diagnosis.