Anatomical and Clinical Features of Thymoma
Thymoma is a rare neoplasm of the thymus gland that originates from epithelial tissue, characterized by slow growth, local extension, and association with various paraneoplastic syndromes. 1
Anatomical Features
Location and Structure
- Located in the anterior mediastinum (most common primary tumor of this region) 1, 2
- Originates from thymic epithelial cells 1
- Usually presents as a well-defined, encapsulated mass (65% of cases) or as an invasive tumor (35% of cases) 1
- Typically grows slowly and spreads by local extension 1
Radiological Appearance
- Most commonly appears as a rounded, soft-tissue mass in the anterior superior mediastinum 2
- CT and MRI findings may include:
- Variable size
- Lobulated or irregular contours
- Calcifications
- Infiltration of surrounding fat
- Lung infiltration
- Great vessel invasion or encirclement 1
Clinical Features
Epidemiology
- Incidence: approximately 0.15 per 100,000 person-years in the United States 1
- More common than thymic carcinomas, which have an estimated annual incidence of 0.48 per million 1
- 5-year survival rate: approximately 90% for thymomas vs. 60% for thymic carcinomas 1
Presentation
- About 30% of patients are asymptomatic, with tumors discovered incidentally on routine chest imaging 3
- Local symptoms may include:
- Cough
- Chest pain
- Hoarseness
- Dyspnea 3
- Metastases at diagnosis are uncommon 1, 3
- Metastases, when present, are usually confined to the pleura, pericardium, or diaphragm 1
- Extrathoracic metastases are rare 1
Paraneoplastic Syndromes
- Approximately 40% of patients with thymoma have one or more paraneoplastic syndromes 4
- Most common associated condition is myasthenia gravis (30-65% of cases) 4, 5
- Other associated conditions include:
- Autoimmune disorders have been reported as the cause of death in 25% of thymoma patients, especially those with early-stage tumors 1
Classification and Staging
Histological Classification
- WHO classification (1999) is widely used and includes:
- Type A: Neoplastic oval or spindle-shaped epithelial cells without atypia or lymphocytes
- Type AB: Type A features with foci of lymphocytes
- Type B (1-3): Characterized by plump epithelial cells with increasing proportions of epithelial cells and atypia
- Type C: Thymic carcinomas 1
Staging Systems
- Masaoka-Koga staging system is most widely used 1, 6:
- Stage I: Completely encapsulated tumor
- Stage II: Microscopic or macroscopic invasion into thymic or surrounding fatty tissue
- Stage III: Macroscopic invasion into neighboring organs
- Stage IVA: Pleural or pericardial metastasis
- Stage IVB: Lymphogenous or hematogenous metastasis 1
Prognostic Factors
Key Determinants of Prognosis
- Complete surgical resection is the most significant prognostic factor 1, 6
- Disease stage strongly correlates with prognosis 1:
- Histologic subtype has prognostic value but is less important than resection status and stage 6
- Vascular invasion is a negative prognostic indicator 6
Clinical Pitfalls and Considerations
- Thymomas have a tendency for late recurrence, even after complete resection, necessitating long-term follow-up 1
- Up to 50-60% of patients do not die from progression of the thymic tumor but from other causes, including associated autoimmune disorders 1
- The evolution of autoimmune disorders associated with thymoma does not parallel the progression of the tumor itself 1
- Extrathymic malignancies develop in up to 20% of patients with thymoma 4
- Distinguishing between thymoma and thymic carcinoma is crucial, as thymic carcinomas have markedly worse survival rates 1
- The term "epithelial tumor of the thymus" is preferred over "thymoma" as it encompasses both thymomas and thymic carcinomas 1