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Thymoma

A thymoma is a rare malignancy that originates from the epithelial cells of the thymus. It is often associated with neuromuscular disorders such as myasthenia gravis (MG), with 20% of MG patients having a thymoma. Thymomas primarily affect adults and are treated through surgical removal. Chemotherapy is reserved for malignant cases.

An encapsulated thymoma (mixed lymphocytic and epithelial type)
An encapsulated thymoma (mixed lymphocytic and epithelial type)

Signs and Symptoms

Approximately one-third of patients with thymoma exhibit symptoms caused by the compression of surrounding organs by the tumour mass. These symptoms can include superior vena cava syndrome, dysphagia (difficulty swallowing), cough, or chest pain. Another third of patients discover their tumour due to associated autoimmune disorders, the most common being myasthenia gravis.

Other autoimmune conditions linked with thymoma include pure red cell aplasia, Good syndrome, and thymoma-associated multiorgan autoimmunity.

Additional associations include acute pericarditis, agranulocytosis, alopecia areata, ulcerative colitis, Cushing's disease, hemolytic anaemia, limbic encephalopathy, myocarditis, nephrotic syndrome, panhypopituitarism, pernicious anaemia, polymyositis, rheumatoid arthritis, sarcoidosis, scleroderma, stiff person syndrome, systemic lupus erythematosus, and thyroiditis.

A significant proportion of patients (one-third to one-half) are asymptomatic, and the tumour is discovered incidentally through imaging performed for other reasons.

Pathology

Thymomas originate from thymic epithelial cells. They are classified into three principal histological types based on microscopic appearance:

  • Type A: Epithelial cells have an oval or fusiform shape with fewer lymphocytes.
  • Type B: Epithelial cells have an epithelioid shape and are subdivided into:
    • B1: Lymphocyte-rich
    • B2: Cortical
    • B3: Epithelial
  • Type AB: Contains a combination of both cell types.

Thymomas are further characterised by the features of thymic cortical or medullary epithelial cells, influencing their benign or malignant nature.

Diagnosis

CT scan of the chest revealing a large necrotic mass in the left anterior mediastinum
CT scan of the chest revealing a large necrotic mass in the left anterior mediastinum (indicated by the red line). Histology later established the diagnosis of a thymoma.
Another axial slice of a CT scan showing a small thymoma anterior to the heart
Another axial slice of a CT scan of the chest showing a small thymoma anterior to the heart (marked with the red line).

When thymoma is suspected, a CT/CAT scan is performed to determine the size and extent of the tumour. A CT-guided needle biopsy is typically used for sampling. Increased vascular enhancement and pleural deposits on CT scans can indicate malignancy. The diagnosis is confirmed through histologic examination by a pathologist using a tissue sample. The Masaoka Staging System is used to classify the tumour based on the extent of disease at the time of surgery:

  • I: Completely encapsulated
  • IIA: Microscopic invasion through the capsule into surrounding fatty tissue
  • IIB: Macroscopic invasion into the capsule
  • III: Macroscopic invasion into adjacent organs
  • IVA: Pleural or pericardial implants
  • IVB: Lymphogenous or hematogenous metastasis to distant sites

Treatment

Surgical removal is the primary treatment for thymoma. In cases of invasive and large tumours, neoadjuvant chemotherapy and/or radiotherapy may be employed to reduce tumour size before surgery. For early-stage tumours (Masaoka I through IIB), no additional therapy is required. Invasive thymomas may necessitate further treatment with radiotherapy and chemotherapy. Recurrences can occur in 10-30% of cases, and pleural recurrences may be surgically removed followed by hyperthermic intrathoracic perfusion chemotherapy.

Prognosis

Prognosis varies significantly with the stage of the tumour. Stage III or IV thymomas have a much worse prognosis compared to stage I and II tumours. Thymomas rarely metastasize, but when they do, common sites include the pleura, bones, liver, and brain. Approximately 40% of patients with stage III and IV thymomas survive for at least 10 years after diagnosis, with a median diagnosis age of 57 years.

Epidemiology

Thymomas have an incidence of about 0.13-0.26 per 100,000 people per year, with a slightly higher prevalence in females. The typical age at diagnosis ranges from the 40s to 50s, although cases have been reported from six to 83 years.

Gallery

An encapsulated cystic thymoma
An encapsulated cystic thymoma.
A locally invasive circumscribed thymoma
A locally invasive circumscribed thymoma (mixed lymphocytic and epithelial, mixed polygonal and spindle).
Histopathological image of thymoma type B1
Histopathological image of thymoma type B1. Anterior mediastinal mass surgically resected. Hematoxylin & eosin stain.
Cytokeratin CAM5.2 immunostain of thymoma type B1
Histopathological image of thymoma type B1. Anterior mediastinal mass surgically resected. Cytokeratin CAM5.2 immunostain.
Histopathological image of noninvasive thymoma type B1
Histopathological image representing a noninvasive thymoma type B1, surgically resected. Hematoxylin & eosin.
Field stain of thymoma FNA specimen
Thymoma. FNA specimen. Field stain.

Self-assessment MCQs (single best answer)

What is the primary treatment for thymoma?

Which autoimmune disorder is most commonly associated with thymoma?

What is the typical age range for thymoma diagnosis?

Which histological type of thymoma is characterised by oval or fusiform epithelial cells with fewer lymphocytes?

In the Masaoka Staging System, what stage is characterised by microscopic invasion through the capsule into surrounding fatty tissue?

What imaging modality is primarily used to determine the size and extent of a suspected thymoma?

Which of the following symptoms is NOT typically caused by the compression of surrounding organs by a thymoma?

What percentage of myasthenia gravis patients have a thymoma?

Which of the following is NOT a histological subtype of Type B thymoma?

What is the incidence of thymomas per 100,000 people per year?

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