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Thymoma



General Considerations

  • Most common anterior mediastinal neoplasm
  • Mean age of presentation is 52, older than teratomas
  • Most are solid lymphoepithelial tumors of the thymus, some are cystic
  • About 1/3 are malignant
  • Rare in children — most common around 5th or 6th decade
  • Four types which occur in about equal frequency
    • Lymphocytic
    • Epithelial
    • Mixed
    • Spindle cell (Hassall’s corpuscles in this type)
  • About 15% of patients with myasthenia gravis have follicular thymic hyperplasia and about 50% of patients with follicular thymic hyperplasia have myasthenia
  • Other associated disorders include: red cell aplasia, dermatomyositis, systemic lupus erythematosis, Cushing syndrome, and syndrome of inappropriate antidiuretic hormone secretion

Clinical Findings

  • 1/3 to ½ are asymptomatic
  • About 1/3 present with local symptoms
    • Cough
    • Chest pain
    • Superior vena caval syndrome
    • Dysphagia
    • Hoarseness
  • About 1/3 are found during workup for myasthenia gravis

Imaging Findings

  • Oval round or lobulated soft tissue mass, sharply demarcated, usually smaller than teratomas
  • Usually occur in upper third of chest around the heart and great vessels, which they may displace posteriorly
  • Thymomas may spread along pleural reflections to posterior mediastinum, diaphragm and retroperitoneum
  • CT is the study of choice in detecting enlarged thymus in patients with myasthenia gravis
    • Small tumors may show homogeneous enhancement; larger tumors are usually heterogeneous
    • Larger tumors especially may show cystic areas and necrosis
    • Invasive thymoma may demonstrate unilateral pleural nodules or masses, mimicking pleural metastatic disease or mesothelioma
    • About 30% may contain curvilinear or, less commonly, amorphous calcification
    • Look for invasion of adjacent vessels or pericardium (thickening)

Differential Diagnosis

  • Teratoma – occurs in younger age group; may contain bone and fat
  • Lymphoma
  • Thymolipoma
    • Fatty tumors of the thymus which constitute 2-9% of thymomas
    • They can grow to huge sizes

Treatment

  • Surgical resection
  • If unresectable, chemotherapy is frequently used

Prognosis

  • Surgical evaluation of encapsulation or invasion is better indicator of malignancy than actual histology
  • In patients with myasthenia, about 50% improve following removal of the thymic tissue
  • Recurrence rate can be high (50% 10 year survival)
  • Thymomas are associated with second malignancies, especially non-Hodgkin lymphoma

Thymoma. Upper: Frontal and lateral views of the chest show a well-circumscribed anterior mediastinal mass without apparent calcifications (white and black arrows). Lower:Axial CT scan of the upper chest shows a non-enhancing lobulated but sharply marginated
anterior mediastinal mass (yellow arrow)..

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Thymoma. KJ Evans,  Q Miller, and AL Kline. eMedicine