General Considerations

• Most common anterior mediastinal mass
• Accounts for 50% of anterior mediastinal masses and 25% of all mediastinal tumors
• Most are solid lymphoepithelial tumors of the thymus, some are cystic
• About 1/3 are malignant under 20 and over 40 years of age
• About half are malignant in those 20-40
• Rare in children — most common around 5th or 6th decade
• Mean age of 52
• They can be classified into four types which occur in about equal frequency
• Lymphocytic
• Epithelial
• Mixed
• Spindle cell (Hassall’s corpuscles in this type)
• There are World Health Organization classifications and surgical staging classifications as well

Clinical Findings

• Most benign thymomas are asymptomatic
  • Most with malignant thymomas are symptomatic
• Symptoms include
  • Cough
• Chest pain
• Dyspnea
• Dysphagia
• Superior vena caval syndrome
• Red cell aplasia, hypogammaglobulinemia or collagen vascular disease such as dermatomyositis and lupus

Imaging Findings

• Conventional radiographs of the chest may show
• Oval round or lobulated soft tissue mass, sharply demarcated, usually smaller than teratomas
• Superior aspect of anterior mediastinum
• Project predominantly to one side or the other
• May displace heart and great vessels posteriorly
• On CT
• Normal thymic tissue may be seen as a triangular density in the anterior mediastinum up to 30 years of age at which time fatty involution occurs
• Thymus should be < 1.8 cm up to 20 years and < 1 cm after 20 years
• A small percentage (5%) may contain curvilinear or amorphous calcification
• Absence of fat planes and invasion of adjacent structures favors malignancy
• A homogeneously enhancing capsule favors benignancy
• MRI
• May be more sensitive to small thymic masses than CT
• Hypointense to mediastinal fat on T1
• On T2, signal is isointense or hyperintense ro surrounding fat

Differential Diagnosis

• Non-Hodgkin’s lymphoma can occur in thymus
• Thymolipomas are rare, fatty tumors of the thymus that have been associated with
  • Aplastic anemia
• Hypogammaglobulinemia
• Grave’s disease
• Hodgkin’s disease
• Chronic lymphocytic leukemia

Treatment

• Most thymomas are treated surgically
• Degree of invasiveness rather than histopathology is best determinant of malignancy versus benignancy

Complications

• About 15% of patients with myasthenia gravis have thymomas and about 33-50% of patients with thymomas have myasthenia
• Thymomas are associated with leukemia

Prognosis

• Surgical evaluation of encapsulation or invasion is better indicator of prognosis than actual histology
• In patients with myasthenia, about 50% improve following removal of the thymoma
• For those with invasive thymoma, 15 year survival is 12.5%
• For those with non-invasive thymoma, 15 year survival is 47%