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

Anterior Mediastinal Masses – 3 T’s and an L



Thyroid enlargement



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


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


  • 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%

Thymoma. Contrast-enhanced coronal (left) and sagittal (right) reformatted CT images of the thorax demonstrate a lobulated mass in the anterior mediastinum (white and red arrows) with mixed attenuation consistent with a thymoma. The aorta is labeled Ao.
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Thymus Lesions eMedicine Ramachandran, T; Raghunathan, U; Patel, D: Kohman, L;  Strohl, J; Swarnkar, A.