A 49-year-old woman has experienced increasing weakness and chest pain over the past 6 months. On physical examination, she is afebrile and normotensive. Motor strength is 5/5 in all extremities, but diminishes to 4/5 with repetitive movement. There is no muscle pain or tenderness. Laboratory studies show hemoglobin, 14 g/dL; hematocrit, 42%; platelet count, 246,000/mm3; and WBC count, 6480/mm3. A chest CT scan shows an irregular 10x12 cm anterior mediastinal mass. The surgeon has difficulty removing the mass because it infiltrates surrounding structures. Microscopically, the mass is composed of large, spindled, atypical epithelial cells mixed with lymphoid cells. Which of the following is the most likely cause of this mass lesion?

Thymomas are rare neoplasms that can be benign or malignant. In one third to one half of cases, thymomas are associated with myasthenia gravis as an initial presentation (as in this case). Benign thymomas have a mixed population of lymphocytes and epithelial cells and are circumscribed, whereas malignant thymomas are invasive and have atypical cells. Thymic carcinomas resemble squamous cell carcinomas. Granulomas can have epithelioid macrophages and lymphocytes, but the thymus is an unusual location for them. Hodgkin lymphoma involves lymph nodes in the middle or posterior mediastinum, with a component of Reed-Sternberg cells. Lymphoblastic lymphoma of the T-cell variety is seen in the mediastinal region, including thymus, in children, but it has no epithelial component. Metastases to the thymus are quite unusual. An organizing abscess could have granulation tissue at its edge, with a mixture of inflammatory cell types, but not atypical cells.