4.1 Clinico-pathologic Considerations
Thymomas are epithelial tumours that may or may not be extensively infiltrated by lymphocytes. True thymomas contain cytologically bland thymic epithelial cells and should be distinguished from thymic carcinomas that have malignant cytological characteristics and carry a poor prognosis. The terms invasive and noninvasive should be used to describe the behaviour of these tumours. Noninvasive thymomas have an intact fibrous capsule, are mobile and are easily resected. Invasive thymomas are malignant as evidenced by the finding of invasion of the tumour capsule or surrounding organs, or by the presence of a metastasis. About 30-40% of all thymomas are invasive.
Other anterior mediastinal tumours that need to be differentiated from thymoma include thymic carcinomas, germ cell tumours, lymphomas, carcinoids, and T-cell leukemias. Some mediastinal tumours can be difficult to categorise with standard pathology and it is crucial to distinguish potentially curable tumours such as thymoma, lymphoma and germ cell neoplasms. Special stains and tumour markers usually suffice but cytogenetic analysis may be required in some cases. Germ cell tumours characteristically have an isochromosome of the short arm of chromosome 12 (i[12p]) and lymphomas may have typical cytogenetic abnormalities.
Kornstein MJ. Controversies regarding the pathology of thymomas. Path Ann 1992;27:1
4.2 Diagnostic Procedures
Chest radiograph and CT of the chest are routine assessments. Magnetic resonance imaging may demonstrate invasion of pulmonary artery, innominate vein, or superior vena cava. The role of positon emission tomography scans has not been determined. Some cases are pathologically difficult and it is crucial to rule out germ cell tumours.
Tumour markers are not usually elevated in thymomas. Nevertheless, in view of the differential diagnosis of an anterior mediastinal mass (particularly in young males), it would be wise to check beta HCG, alpha fetoprotein, and CEA.
Histologic diagnosis is essential in planning treatment. Fine needle aspiration or needle biopsy usually provide sufficient information for the diagnosis. Mediastinoscopy, anterior mediastinotomy, and video-assisted thoracic surgery may have specific indications, but are rarely necessary in establishing the diagnosis. Consideration should be given to obtaining diagnostic material suitable for cytogenetic studies in appropriate cases.
Graeber GM, Shriver CD, Albus RA, et al. The use of computed tomography in the evaluation of mediastinal masses. J Thorac Cardiovasc Surg 1986:91:662
Pearson FG. Mediastinal masses diagnosis: invasive techniques. Semin Thorac Cardiovasc Surg 1992;4:23.