A 52-year-old man with a 15-year history of myasthenia gravis presents with progressive dyspnea and chest discomfort. Chest X-ray shows a well-defined, homogeneous anterior mediastinal mass measuring 8 cm. CT chest reveals a mass with a smooth border, mild heterogeneity, and no evidence of invasion into adjacent structures. The mass shows mild enhancement on contrast administration. Serum tumor markers (AFP, beta-hCG) are negative. What is the most likely diagnosis?

## Diagnosis: Thymoma ### Clinical Context and Associations **Key Point:** The strong association with myasthenia gravis (MG) is the most important clinical clue. Approximately 30-50% of thymoma patients have MG, and 10-15% of MG patients have thymoma. This patient's 15-year history of MG makes thymoma highly likely. ### Radiological Features of Thymoma | Feature | Thymoma | Lymphoma | Thymic Hyperplasia | Germ Cell Tumor | |---------|---------|----------|-------------------|----------------| | Age of onset | 40-60 years | Variable | Young adults | 20-40 years | | Border | Well-defined, smooth | Often infiltrative | Ill-defined | May be irregular | | Homogeneity | Mild heterogeneity common | Variable | Homogeneous | Heterogeneous with necrosis | | MG association | 30-50% | Rare | Rare | No | | Tumor markers | Negative | Negative | Negative | AFP/beta-hCG elevated | | Enhancement | Mild to moderate | Variable | Homogeneous | Heterogeneous | **High-Yield:** A well-defined anterior mediastinal mass in a patient with myasthenia gravis is thymoma until proven otherwise. ### Pathogenesis Thymomas arise from the epithelial cells of the thymus gland. The thymus is a primary lymphoid organ involved in T-cell maturation. Thymomas are the most common primary anterior mediastinal mass in adults aged 40-60 years. ### WHO Classification and Staging Thymomas are classified histologically (A, AB, B1, B2, B3) and staged clinically (Masaoka-Koga stages I-IV based on invasion). This patient's smooth border and lack of invasion suggest early-stage disease. ### Clinical Pearl Negative tumor markers (AFP, beta-hCG) effectively exclude germ cell tumors. The combination of MG association, patient age (52 years), well-defined mass, and negative markers is diagnostic of thymoma. ### Management Thymectomy is the treatment of choice for thymoma, especially in patients with MG. Chemotherapy and radiation may be used for advanced stages or incomplete resection.