A thymoma is a tumor originating from the epithelial cells of the thymus that is considered a rare malignancy. Thymomas are frequently associated with neuromuscular disorders such as  myasthenia gravis; thymoma is found in 20% of patients with myasthenia gravis. Once diagnosed, thymomas may be removed surgically. In the rare case of a malignant tumor, chemotherapy may be used. Signs and symptoms
A third of all people with a thymoma have symptoms caused by compression of the surrounding organs by an expansive mass. These problems may take the form of superior vena cava syndrome, dysphagia (difficulty swallowing), cough, or chest pain.One-third of patients have their tumors discovered because they have an associated autoimmune disorder. As mentioned earlier, the most common of those conditions is myasthenia gravis (MG); 10–15% of patients with MG have a thymoma and, conversely, 30–45% of patients with thymomas have MG. Additional associated autoimmune conditions include thymoma-associated multiorgan autoimmunity, pure red cell aplasia and Good syndrome (thymoma with combined immunodeficiency and hypogammaglobulinemia). Other reported disease associations are with acute pericarditis, agranulocytosis, alopecia areata, ulcerative colitis, Cushings 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.One-third to one-half of all persons with 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. Pathology
Thymoma originates from the epithelial cell population in the thymus, and several microscopic subtypes are now recognized.
These include: full blood count, protein electrophoresis, antibodies to the acetylcholine receptor (indicative of myasthenia), electrolytes, liver enzymes and renal function. Staging
The Masaoka Staging System is used widely and is based on the anatomic 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 capsule
III: Macroscopic invasion into adjacent organs
IVA: Pleural or pericardial implants
IVB: Lymphogenous or hematogenous metastasis to distant (extrathoracic) sites

Treatment
Surgery is the mainstay of treatment for thymoma. If the tumor is apparently invasive and large, preoperative (neoadjuvant) chemotherapy and/or radiotherapy may be used to decrease the size and improve resectability, before surgery is attempted. When the tumor is an early stage (Masaoka I through IIB), no further therapy is necessary. Removal of the thymus in adults does not appear to induce immune deficiency. In children, however, postoperative immunity may be abnormal and vaccinations for several infectious agents are recommended. Invasive thymomas may require additional treatment with radiotherapy and chemotherapy (cyclophosphamide, doxorubicin and cisplatin).. Recurrences of thymoma are described in 10-30% of cases up to 10 years after surgical resection, and in the majority of cases also pleural recurrences can be removed. Recently, surgical removal of pleural recurrences can be followed by hyperthermic intrathoracic perfusion chemotherapy or intrathoracic hyperthermic perfused chemotherapy (ITH). Prognosis
Prognosis is much worse for stage III or IV thymomas as compared with stage I and II tumors. Invasive thymomas uncommonly can also metastasize, generally to pleura, bones, liver or brain in approximately 7% of cases.