ACR Codes: 67.3154
The anterior compartment of the mediastinum includes the aorta and its branches, the great veins, thymus, lymphatic tissue, and lung. While many patients are asymptomatic, some patients may have signs or symptoms of mass effect on the surrounding organs, such as cough, chest pain, shortness of breath, or superior vena cava syndrome. They may also have "B" symptoms of fever, night sweats, or weight loss, or they may have symptoms of conditions that are associated with anterior mediastinal tumors, such as myasthenia gravis (thymoma), pure red cell aplasia (thymoma), thyrotoxicosis (thyroid abnormality), or hyperparathyroidism (parathyroid abnormality).
The differential diagnosis offered above is often referred to as the 4 "T's" - teratoma, thymoma, thyroid abnormality, or terrible lymphoma. This differential can be much extended, to include cardiovascular masses such as aortic aneurysm, cardiac tumor, paricardial cyst, etc., or metastases of other primary tumors, or tumors of phrenic or vagus nerves, or a number of other lesions. The radiographic appearance of a mediastinal mass can offer some clues to the diagnosis. The radiographs in this case are suggestive of thymoma - the mass is characteristically located between the sternum and heart/great vessels, it is well-circumscribed, and it demonstrates the "sulcus sign." The sulcus sign indicates that the mass is firm, creating a sulcus where the mass meets the sternum or other firm structure, rather than being flattened against the structure and obliterating the sulcus. The well-encapsulated appearance also suggests benign thymoma. Malignancy of thymoma is defined by invasive spread, although thymomas in general are considered slow-growing. Malignant thymomas spread contiguously, and can spread around the pleura and cause multiple pleural masses resembling mesothelioma. Less extensive spread is difficult to rule out. Chest CT may be performed to more sensitively identify invasiveness, but even intraoperatively invasiveness may be difficult to identify. Staging is usually performed with the Masaoka system, but the GETT (French Study Group on Thymic Tumors) is also used. Invasion is the most important prognostic factor with thymoma, with 5-year survival of 70% (50 with local invasion, 75% without invasion), and 10-year survival of 50% (30% with invasion, 60% without invasion).
The age of the patient may be used to predict the character of a mass. Thymoma occur most frequently in patients aged 45-50, rarely in patients less than 20, so this patients age of 28 would not have predicted thymoma as being the most likely lesion. Over all age groups, thymoma is the most common primary tumor of the anterior mediastinum, with an incidence of 0.13 in 100,000. In patients aged 20-40, Hodgkin's and non-Hodgkin's lymphoma and germ cell tumors are most common. These lesions are also most likely to be malignant.
Thymoma has a significant association with myasthenia gravis - 40% of patients with a thymoma have myasthenia gravis, and 15% of patients with myesthenia gravis have a thymoma. Thymectomy usually achieves some attenuation of the severity of myesthenia gravis of these patients. 5-15% of patients with thymoma also have pure red cell aplasia, usually occurring in older women - thymectomy resolves this condition in up to 40% of patients. Immunodeficiency resulting from hypogammaglobulinemia and white cell aplasia are much less common, occurring in less than 5% of thymoma patients.
Primary treatment for thymoma is surgical resection. Difficulties that may be encountered intraoperatively may include invasion, adhesion of tumor to adjacent structures, or extension of tumor along phrenic nerves which increases the risk of repiratory impairment if these nerves are damaged during dissection. Adjuvant radiation therapy has not been shown to be of benefit after resection of a well-encapsulated, non-invasive tumor. However, adjuvant radiation is often beneficial when used after incomplete resection, after complete resection of invasive tumor, or in unresectable disease. It is sometimes used after total resection in which adhesions were noted. Thymoma is a chemotherapy-sensitive tumor, and chemotherapy may be used in patients with locally invasive or bulky tumors prior to resection. Chemotherapy is also traditionally used for patients with unresectable or metastatic thymoma, or with thymic carcinoma.
Reference(s): Brant, W. E. and Helms, C.A. eds., Fundamentals of Diagnostic Radiology, 2nd ed. 1999. Williams and Wilkins. pp.320-326.
Reed, J.C. Chest Radiology - Plain Film Patterns and Differential Diagnoses, 4th ed. 1997. Mosby-Year Book, Inc. pp. 107-124.
Salgia, R. "Clinical presentation and management of thymoma." In 2002 UpToDate online, topic last updated 12aug2002.
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