Thymoma
Classification and external resources
An encapsulated thymoma (mixed lymphocytic and epithelial type).
ICD-10	C37.9, D60
ICD-9	164.0, 212.6
ICD-O:	8580
DiseasesDB	13067
MedlinePlus	001086
eMedicine	med/2752 med/3448 ped/2246
MeSH	D013945

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: distant metastasis are 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 an uncommon tumor, best known for its association with the neuromuscular disorder myasthenia gravis.^[1] Thymoma is found in 15% of patients with myasthenia gravis.^[2]

[edit] 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), dysphagia (difficulty swallowing), cough or chest pain.^[1]

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.^[1]

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 or CT/CAT scan performed for an unrelated problem.^[1]

[edit] Diagnosis

CT scan of the chest revealing a large necrotic mass in the left anterior mediastinum (indicated by the red line). Histology later proved the diagnosis of a thymoma.

Another axial slice of a CT scan of the chest showing a small thymoma just in front of the heart (marked with the red line).

When a thymic mass is identified, the diagnosis is confirmed by histologic exam (after obtaining a tissue sample of the mass). When a thymoma is suspected, a CT/CAT scan is generally performed to estimate the size and extent of the tumor, and can be biopsied with a CT-guided needle. There is a very small risk of pneumomediastinum, mediastinitis and the extremely low risk of damaging the heart or large blood vessels. The final diagnosis and staging is made by removing the thymus, tumor and adjacent structures and then examining it microscopically to determine its extent and invasion. 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.^[1]

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.^[1]

[edit] Pathophysiology

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.^[1] There are two major types of thymoma, depending on the appearance of the cells on microscopy:

• Type A if the epithelial cells have an oval shape
• Type B if they have an epithelioid shape. (Type B has three subtypes B1, B2 and B3.^{[citation needed]})
• If the cells have a combination of both types, they are designated type AB.

[edit] Staging

The Masaoka Staging System is used widely and based on anatomic extent of disease at the time of surgery:^[3]

• 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

[edit] Treatment

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, doxorubicin and cisplatin)^[1] if a lifetime dose of radiation was delivered to the tumor area.^{[citation needed]}

[edit] Prognosis

Thymomas associated with autoimmune disorders usually are benign.^{[citation needed]} Malignant thymomas can metastasize, generally to pleura, kidney, bone, liver or brain.^[1]

[edit] Epidemiology

Men and women are equally affected. The typical age at diagnosis is 30-40, although cases have been described in every age group.^[1]

[edit] See also

• Mediastinal tumor

[edit] Additional images

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

[edit] References

1. ^ ^{a b c d e f g h i j} Thomas CR, Wright CD, Loehrer PJ (1999). "Thymoma: state of the art". J. Clin. Oncol. 17 (7): 2280–9. PMID 10561285. 
2. ^ Mitchell, Richard Sheppard; Kumar, Vinay; Robbins, Stanley L.; Abbas, Abul K.; Fausto, Nelson (2007). Robbins basic pathology. Saunders/Elsevier. ISBN 1-4160-2973-7. 
3. ^ Masaoka A, Monden Y, Nakahara K, Tanioka T (1981). "Follow-up study of thymomas with special reference to their clinical stages" (PDF). Cancer 48 (11): 2485–92. doi:10.1002/1097-0142(19811201)48:11<2485::AID-CNCR2820481123>3.0.CO;2-R. PMID 7296496. http://www3.interscience.wiley.com/cgi-bin/fulltext/112678301/PDFSTART. 

[edit] External links

• A large resource about thymoma
• Foundation for Thymic Cancer Research