Description

• The thymus gland is located in the anterior portion of the mediastinum; its function is to induce T lymphocytes. It is largest in the teenage years and slowly regresses through adulthood.
• Primary neoplasms of the thymus include thymoma and thymic carcinoma, which arise from thymic epithelial cells.
• Thymomas are uncommon tumors that are benign but can be locally invasive.
• Thymic carcinomas are rare and aggressive malignant tumors with generally a much poorer prognosis.

Epidemiology

• Thymic cancers are uncommon: 0.2–1.5% of all malignancies.
• Majority of thymic tumors are thymomas; these account for 20% of mediastinal tumors and 50% of all anterior mediastinal masses in adults.
• Thymomas present between ages of 40 and 60 years.
• Thymomas are slightly more common in males and Asians/Pacific Islanders.
• Thymic carcinomas are rare: 0.06% of all neoplasms.

Incidence
Thymoma incidence is 0.15/100,000 person-years.

Etiology and Pathophysiology

• Embryologically, the thymus derives from the 3rd pharyngeal pouch, which gradually is replaced with adipose tissue in adulthood.
• Thymomas originate from either the cortical or medullary epithelial cells of the thymus. These tumors are considered histologically benign. They are slow-growing and typically have an indolent course. However, they can exhibit invasive and locally aggressive behavior. Tumor spread can occur by local extension or by regional pleural implants. Distant metastatic spread is not seen.
• Thymic carcinomas are also epithelial in origin but have a high degree of atypical and invasiveness.
• Thymic carcinomas are malignant tumors with potential for local, regional, and distant spread.
• At presentation, 80% of thymic carcinomas invade into adjacent structures and 40% have metastasis to mediastinal lymph nodes.
• For thymic carcinomas, histologic grade is an important prognostic factor.
• WHO pathologic classification for thymomas and thymic carcinoma (1)[A]
  • Type A: composed of spindle or oval cells, medullary thymoma
  • Type AB: composed of two components, mixed thymoma
  • Type B1: composed of predominantly cortical thymoma
  • Type B2: cortical thymoma
  • Type B3: atypical thymoma
  • Type C: thymic carcinoma

Risk Factors

None known

Commonly Associated Conditions

• Thymoma
  • Myasthenia gravis: 30–40% of patients with thymomas. 10–15% of those with myasthenia gravis have a thymoma; usually have less advanced disease because the symptoms favor earlier detection
  • Pure red cell aplasia (5–10%)
  • Hypogammaglobulinemia (3–6%)
  • Epstein-Barr virus
• Thymic carcinoma
  • Occasionally associated with paraneoplastic syndromes