Computed tomography (CT) with intravenous contrast may be useful in the diagnosis and clinical staging of thymoma, especially for noninvasive tumors. CT is usually accurate in predicting the following:
- Tumor size.
- Invasion into vessels, the pericardium, and the lungs.
However, CT cannot predict invasion or resectability with accuracy.[1,2] Appearance of the tumor on CT may be related to the World Health Organization (WHO) histologic type. A retrospective study involving 53 patients who underwent thymectomy for thymic epithelial tumors indicated that smooth contours with a round shape were most suggestive of type A thymomas, and irregular contours were most suggestive of thymic carcinomas. Calcification was suggestive of type B thymomas. In this study, however, CT was found to be of limited value differentiating type AB, B1, B2, and B3 thymomas.
Most patients with thymic carcinomas present initially with any of the following:
- Chest pain.
- Phrenic nerve palsy.
- Superior vena cava syndrome.
Patients may have evidence of invasion of contiguous mediastinal structures at presentation. Thymic carcinoma can metastasize to any of the following:
- Regional lymph nodes.
An evaluation for sites of metastases may be warranted for these patients.
Positron emission tomography of 18-flouro-deoxyglucose (FDG-PET) as well as thallium single-photon emission computed tomography have been reported in small series for diagnosis and evaluation of therapeutic outcomes in thymic carcinoma.[5,6,7,8] Two small series reported that FDG uptake was related to the invasiveness of thymic carcinoma.[7,8] This raises the possibility of FDG-PET utilization for diagnosis, treatment planning, and monitoring for recurrence. Sensitivity, specificity impact on clinical therapeutic decisions, remains to be defined.
Histologic classification of thymoma is not sufficient to distinguish biologically benign thymomas from malignant thymomas. The degree of invasion or tumor stage is generally thought to be a more important indicator of overall survival.[1,9,10]
Evaluating the invasiveness of a thymoma involves the use of staging criteria that indicate the presence and degree of contiguous invasion, the presence of implants, and lymph node or distant metastases regardless of histologic type. Although no standardized staging system exists, the one proposed by Masaoka in 1981 is commonly employed. It was revised in 1994 and is shown below.
Thymoma Staging System of Masaoka 1994a
|I||Macroscopically, completely encapsulated; microscopically, no capsular invasion.|
|II||Macroscopic invasion into surrounding fatty tissue or mediastinal pleura; microscopic invasion into capsule.|
|III||Macroscopic invasion into neighboring organs (pericardium, lung, and great vessels).|
|IVa||Pleural or pericardial dissemination.|
|IVb||Lymphogenous or hematogenous metastases.|
Application of this staging system to a series of 85 surgically treated patients confirmed its value in determining prognosis, with 5-year survival rates of 96% for stage I disease, 86% for stage II disease, 69% for stage III disease, and 50% for stage IV disease.[11,13] In a large, retrospective study involving 273 patients with thymoma, 20-year survival rates (as defined by freedom from tumor death) according to the Masaoka staging system were reported to be 89% for stage I disease, 91% for stage II disease, 49% for stage III disease, and 0% for stage IV disease.