Thymus and Mediastinal Tumors

I

Anatomy and Embryology

1.
The thymus gland originates from the third and fourth pharyngeal pouches (along with the lower parathyroid glands) and typically descends into the anterior mediastinum.
2.
The Shields three-compartment model is the most anatomic model and widely used by thoracic surgeons.
3.
Anterior compartment is bordered by sternum (anterior), pericardium (posterior), and pleura (lateral).
- a.
  Contains thymus, internal mammary arteries, connective tissue, lymph nodes, fat
4.
Visceral compartment is bordered by thoracic inlet (superior), pericardium (anterior), and anterior surface of vertebrae (posterior).
- a.
  Contains heart, pericardium, great vessels, airway, esophagus
5.
Posterior compartment/paravertebral sulci are bordered by anterior aspect of vertebral bodies (posterior) and costophrenic angles (laterally).
- a.
  Contains spine, proximal intercostal neurovascular bundles, sympathetic chain, lymphatic tissue, connective tissue

II

Assessment of Mediastinal Masses

Differential diagnosis for mediastinal masses depends on patient age, gender, the compartment of the mass, computed tomography (CT) characteristics, and symptoms. The appropriate evaluation for mediastinal masses is therefore highly variable and specific to the lesion in question.

III

Anterior Mediastinal Masses

Despite being the smallest compartment, 50% of mediastinal tumors are located in the anterior mediastinum.

A

Thymomas

1.
Epidemiology
- a.
  Most common mediastinal neoplasm (20%) in adults, most common neoplasm in anterior mediastinal compartment
- b.
  Most common in the 40–60-year-old age group
2.
Presentation
- a.
  Symptoms are typically secondary to mass effect and may include dyspnea, chest pain, and cough.
  - (1)
    At time of presentation, 1/3 – 1/2 are asymptomatic.
  - (2)
    A total of 40% have symptoms related to mass effect.
  - (3)
    A total of 30% have systemic symptoms.
  - (4)
    Locally invasive tumors may present with phrenic nerve paralysis or superior vena cava (SVC) syndrome.
- b.
  Several autoimmune disorders are associated with thymoma.
  - (1)
    Myasthenia gravis (MG) is the most common autoimmune disorder (45%).
    - (a)
      Etiology: Autoantibodies block acetylcholine receptors at neuromuscular junction.
    - (b)
      Presentation: diplopia, ptosis, dysphagia, weakness, fatigue
    - (c)
      A total of 30%–50% of patients with thymoma will have MG, so it is critical to rule out myasthenia when evaluating a patient with a suspected thymoma; only 10%–15% of patients with MG will have concomitant thymoma.
    - (d)
      Thymectomy improves symptoms in up to 60% of patients (symptoms improve slowly).
  - (2)
    Red cell aplasia (2%–5%)
  - (3)
    Hypogammaglobulinemia (2%–5%)
  - (4)
    Systemic lupus erythematosus, polymyositis, rheumatoid arthritis, and other syndromes are less commonly associated
3.
Evaluation
- a.
  Differential: intrathoracic thyroid, parathyroid tumors, lymphoma, germ cell tumors
- b.
  Imaging
  - (1)
    CT chest demonstrates a homogeneous mass with possible calcification. It is used to delineate the size and extent of the lesion, invasion of neighboring structures, and distant metastases.
  - (2)
    Chest MRI is typically not necessary but can be helpful to determine vascular or neural involvement.
  - (3)
    Fluorodeoxyglucose-positron emission tomography (FDG-PET) is used in some centers; however, most thymomas are slow growing and rarely metastasize, making its clinical utility unclear. It is more useful for thymic carcinoma.
- c.
  Labs: α-fetoprotein (AFP), β-human chorionic gonadotropin (β-hCG) to rule out germ cell tumors in appropriate situations
- d.
  Biopsy
  - (1)
    Diagnosis: truly needed for advanced tumors in which induction therapy or nonoperative approach is being considered OR when lymphoma is high on differential
  - (2)
    Small lesions (stage I/II): complete excision for diagnosis
  - (3)
    Large lesions: CT-guided core needle biopsy, anterior mediastinotomy (Chamberlain procedure), thoracoscopic biopsy
4.
Staging: The most common staging systems for thymoma is the Koga modification of the Masaoka staging system.
- a.
  Stage I: completely encapsulated
- b.
  Stage IIa: microscopic extracapsular extension
- c.
  Stage IIb: macroscopic invasion into surrounding fatty tissue
- d.
  Stage III: macroscopic invasion into neighboring organs
- e.
  Stage IVa: pleural or pericardial metastasis
- f.
  Stage IVb: lymphogenous or hematogenous metastasis
5.
Treatment
- a.
  Stage I: complete resection, adjuvant therapy not recommended
- b.
  Stage II: complete resection, adjuvant radiation therapy controversial
  - (1)
    Radiation therapy may be recommended for close or positive margins or for higher-risk pathology (World Health Organization [WHO] type B2, B3, or C).
- c.
  Stage III: patients typically treated with multimodality therapy
  - (1)
    Induction chemotherapy (cisplatin based) typically given if suspected preoperative
  - (2)
    Complete resection 4–6 weeks after completion of chemotherapy
  - (3)
    Adjuvant radiation therapy
- d.
  Stage IV
  - (1)
    In patients with limited stage IVa disease, a combination of induction chemotherapy followed by complete resection and adjuvant radiation can be considered.
  - (2)
    Unresectable patients are typically treated with chemotherapy.
- e.
  Prognostic factors for overall survival: completeness of resection, Masaoka stage, and WHO classification.
- f.
  Ten-year survival by stage
  - (1)
    Stage I: 100%
  - (2)
    Stage II: 98%
  - (3)
    Stage III: 78%
  - (4)
    Stage IV: 47%
6.
Surgical approach
- a.
  Complete resection through a median sternotomy is the most common surgical approach.
- b.
  Resection should be from the phrenic nerves laterally to the thoracic inlet (including the thymic horns) superiorly and down to the diaphragm inferiorly.
- c.
  En bloc resection of locally invaded structures should be done to get a complete resection: pericardium, lung, unilateral phrenic nerve, SVC, innominate vein.
- d.
  Phrenic nerve resection should be avoided in patients with myasthenia.
- e.
  Minimally invasive techniques (i.e., thoracoscopic, robotic, transcervical) are being increasingly used for smaller tumors.

B

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I

Anatomy and Embryology

II

Assessment of Mediastinal Masses

III

Anterior Mediastinal Masses

A

Thymomas

B

You're Reading a Preview

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