Atypical Thymoma (WHO Type B3) with Prominent Microcystic and Mucoid Changes Mimicking Thymic Mucoepidermoid Carcinoma: A Clinicopathological and Immunohistochemical Study of Five Cases

Five atypical thymomas (WHO type B3) with prominent microcystic and mucoid changes are presented. The patients were four men and one woman between the ages of 57 and 72 years. The patients presented with non-specific symptoms of cough, chest pain, and dyspnea. None of the patients had a history of myasthenia gravis. Diagnostic imaging revealed the presence of anterior mediastinal masses and surgical resection was accomplished in all patients. Macroscopically, the tumors ranged in size from 3.5 to 5.0 cm in greatest diameter; four of these were well circumscribed but unencapsuled, tan colored tumors without evidence of necrosis, hemorrhage, or gross cystic changes. One tumor had more infiltrative borders and was involving the mediastinal pleura. Microscopically, the low power view was characterized by prominent microcysts that were filled with a mucoid granular material. Higher magnification demonstrated a homogenous epithelial proliferation with mild cytologic atypia but lacking mitotic activity. Focal areas of squamoid differentiation were identified but perivascular spaces were absent. Histochemical staining confirmed mucinous material in the microcysts but no intracytoplasmic mucin. Immunohistochemical stains showed positive staining of the tumor cells with keratin AE1/AE3, keratin 5/6, p63, and p40. No terminal deoxynucleotidyl transferase+/CD3 + immature lymphocytes were identified. Clinical follow-up demonstrated that four patients have remained alive without recurrence while one patient was lost to follow-up. This report highlights histological features in atypical thymoma that may be confused with other tumors, especially thymic mucoepidermoid carcinoma. Separation of these tumors may be important for patient management and prognosis.