Thymoma/Invasive Thymoma


Thymomas generally occur in patients over the age of 30 years abd are rare in children (70% of lesions are found in adults between the 5th and 6th decades) [12,21]. Thymomas occur with equal frequency in men and women [20,21]. Approximately 35% of thymomas are invasive, but they are usually indistinguishable from benign tumors radiographically. Most patients are asymptomatic (20-50% of patients [12]), however, thymomas are associated with myasthenia gravis. About 35-40% of patients with thymomas have myasthenia, whereas 10-15% of myasthenia patients have a thymoma [21]. Thymomas associated with myasthenia are typically less aggressive and have a better prognosis. Thymomas are also associated with other hematologic disorders such as hypogammaglobulinemia (Good's Syndrome-in 5% of cases) and red cell aplasia (in about 5%- particularly spindle cell thymoma). Conversely, about 10% of patients with hypogammaglobulinemia are discovered to have a thymoma, while about 50% of patients with red cell aplasia have a thymoma. Thymectomy for thymoma frequently alleviates the symptoms of myesthenia gravis, but only occasionally results in remission of red cell aplasia and it never improves hypogammaglobulinemia [6,8]. 
Thymomas rarely metastasize and more commonly they extend locally or seed the pleural or pericardial surfaces. Thymic epithelial tumors consist of several histologic subtypes in order of increasing malignancy- A, AB, B1, B2, B3, and thymic carcinoma [16]. Thymoma types A and AB generally behave like benign tumors, B1 is a low-grade malignant tumor (over 90% ten year survival), B2 has a higher degree of malignancy, and B3 has a poor prognosis [16]. In general, A, AB, and B1 are considered low risk lesions, while B2 and B3 are high risk [16]. 
Histologic differentiation of benign from invasive lesions is not possible and lesions are termed "invasive" if there is evidence of extension of the tumor beyond its surrounding fibrous capsule at histologic analysis. Although patients with encapsulated thymomas have the best prognosis and are almost always cured with complete excision, 2-12% of resected encapsulated thymomas recur, sometimes months or years later [5]. The five year survival for patients with encapsulated thymoma is about 75%, while those with invasive thymomas have a 50% 5 year survival [5]. After surgery, radiation therapy is recommended to reduce the risk of recurrences in patients with invasive thymomas (typically 40-60 Gy) [12]. About 5% of patients with invasive thymomas have distant metastases [20]. Patients with invasive thymomas who develop metastases and all patients with stage IV disease are candidates for chemotherapy (cisplatin- overall response rate 40-90%) [12]. Because of the risk of recurrence, all thymomas warrant long-term follow-up. The presence of coexistent myasthenia does not adversely affect patient survival 

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