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Anatomy of thymus gland

  • Thymus is lymphoid organ located in anterior mediastinum
  • Composed of epithelial cells and lymphocytes
  • Responsible for maturation of cell-mediated immunity
  • Reached maximum size at puberty
  • Regresses in later life


  • Thymoma is the most common neoplasm of anterior mediastinum
  • Accounts for 25% of all mediastinal tumours
  • Peak incidence is between 40 and 50 years of age
  • Originates from epithelial cells of thymus gland
  • No clear aetiological factors have been defined
  • Associated with development of myaesthenia gravis
  • No clear distinction between benign and malignant tumours
  • Malignant tumours can invade vasculature and adjacent structures
  • Death often occurs from cardiac tamponade or cardiorespiratory complications

Clinical features

  • Almost 50% of thymomas are asymptomatic
  • 30% present with local symptoms related to encroachment on adjacent structures
  • Present with cough, chest pain, superior vena cava compression
  • 20% identified during the investigation of myaesthenia gravis


  • Most thymomas are visible on chest x-ray
  • CT may be used to delineate mass further
  • Contrast-enhancement is useful in defining vascularity and extent of invasion
  • Diagnosis can be confirmed by biopsy taken at anterior mediastinoscopy

Thymoma on chest x-ray

Picture provided by Ian Hunt, Guy's and St Thomas' Hospital, London, United Kingdom

Thymoma on CT sacn

Picture provided by Ian Hunt, Guy's and St Thomas' Hospital, London, United Kingdom

Staging (Masaoka Staging System)

  • Stage 1 - Encapsulated tumour with no gross or microscopic invasion
  • Stage 2 - Macroscopic invasion into mediastinal fat or pleura
  • Stage 3 - Invasion of pericardium, great vessels or lung
  • Stage 4 - Pleural or pericardial metastatic spread
  • Stage 5 - Lymphatic or haematogenous spread


  • Treatment depends on the stage of the disease
  • Stage 1 disease can be managed by complete surgical excision
  • Stage 2 and 3 disease requires surgical excision and postoperative radiotherapy
  • Stage 4 and 5 disease requires surgical debulking, radiotherapy and chemotherapy


  • Initial management of most thymomas is surgery
  • Usually performed via a median sternotomy
  • Resection may require excision en-bloc of involved pericardium, pleura and phrenic nerves
  • Care should be taken to avoid damaging both phrenic nerves
  • Clips should be placed to aide radiotherapy planning


Picture provided by Ian Hunt, Guy's and St Thomas' Hospital, London, United Kingdom


  • Prognosis is worse for symptomatic thymomas
  • Most important factor that determines prognosis is invasion of adjacent structures
  • Stage 1 disease associated with greater than 90% 5-year survival
  • Stage 4 disease associated with less than 25% 5-year survival


Detterbeck F C,   Parsons A M.  Thymic tumors.  Ann Thorac Surg 2004;  77:  1860-1869.

Duwe B V,  Sternman D H,  Musani A I.  Tumors if the mediastinum.  Chest 2005;  128:  2893 - 2909.

Giaccone G.  Treatment of malignant thymoma.  Curr Opin Oncol 2005;  17:  140 - 146.

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