In medicine (oncology), thymoma is a tumor originating from the thymus. It consists of any type of thymic epithelial cell as well as lymphocytes that are usually abundant and probably not neoplastic. Thymoma usually is benign, and frequently encapsulated; when occasionally malignant, then it is invasive: metastasis is extremely rare. Malignant lymphomas that involve the thymus, e.g., lymphosarcoma, Hodgkin's disease (termed "granulomatous thymoma" in the past), should not be regarded as thymoma. Thymoma is a rare disease, best known for its enigmatic association with the neuromuscular disorder myasthenia gravis.
Signs and symptoms
A third of all people with a thymoma have symptoms due to compression of the surrounding organs by an expansive mass. This may take the form of superior vena cava syndrome
(compression of the upper caval vein
(difficulty swallowing), cough
or chest pain
A third have a thymoma detected because they have an associated autoimmune disorder. The most common condition in this group is myasthenia gravis (of which 10-15% are associated with a thymoma and 30-45% of patients with thymomas have MG); patients with myasthenia are routinely screened for thymoma. Other associated autoimmune conditions are pure red cell aplasia and Good's syndrome (thymoma with combined immunodeficiency and hypogammaglobulinemia). Rare associations that have been reported are: acute pericarditis, Addison's disease, agranulocytosis, alopecia areata, ulcerative colitis, Cushing's disease, hemolytic anemia, limbic encephalopathy, myocarditis, nephrotic syndrome, panhypopituitarism, pernicious anemia, polymyositis, rheumatoid arthritis, sarcoidosis, scleroderma, sensorimotor radiculopathy, stiff person syndrome, systemic lupus erythematosus and thyroiditis.
A third to half of all people with a thymoma have no symptoms at all, and the mass is identified on a chest X-ray performed for an unrelated problem.
When a thymic mass is identified, the diagnosis is confrmed with a histology
(obtaining a tissue sample of the mass) study. When a thymoma is suspected, a CT/CAT scan
is generally performed to estimate the size of the tumor, and can be biopsied
with a CT-guided needle. There is a small risk of pneumomediastinum
and the risk of damaging the heart
or large blood vessels. The final diagnosis is made by removing the thymus. Pathological
investigation of the specimen will reveal if the tumor was benign or malignant, although the initial biopsy is usually indicative.
The tumor is generally located inside the thymus, and can be calcified. Increased vascular enhancement can be indicative of malignancy, as can be pleural deposits.
If the suspicion is high, some blood tests are often performed to look for associated problems or possible spread. These include: full blood count, protein electrophoresis, antibodies to the acetylcholine receptor (indicative of myasthenia), electrolytes, liver enzymes and renal function.
Thymoma originates from the epithelial
cell population in the thymus. Many subtypes are recognized, some of which have a better- or worse-than-general prognosis. There are two major types of thymoma. Type A if the epithelial cells have an oval shape or Type B if they have an epithelioid shape. If the cells have a combination of both types, they are designated type AB. Type B has three subtypes B1, B2 and B3.
The Masaoka Staging System is used widely and based on anatomic extent of disease at the time of surgery:
- I: Completely encapsulated
- II-1: Macroscopic invasion into surrounding fatty tissue
- II-2: Microscopic invasion into capsule
- III: Macroscopic invasion into adjacent organs
- IVA: Pleural or pericardial implants
- IVB: Lymphatogenous or hematogenous metastasis
Surgery is the mainstay of treatment. If the tumor is malignant and very large, chemotherapy may be required to shrink the tumor before surgery is attempted. If the tumor was benign and was removed completely, no further therapy is necessary. Removal of the thymus in adults does not appear to induce a severe immune deficiency
. In children, however, added care and scrupulous vaccination are necessary to protect from infections. Malignant tumors may need additional treatment with radiotherapy
, and for recurrence with chemotherapy (cyclophosphamide
) if a lifetime dose of radiation was delivered to the tumor area.
Thymomas associated with autoimmune disorders usually are benign. Malignant thymomas can metastasize
, generally to pleura
Men and women are equally affected. The typical age at diagnosis is 30-40, although cases have been described in every age group.