Laboratory Studies
The diagnosis of a thymoma usually is clinically based on radiological findings. Laboratory studies generally are not indicated.
Imaging Studies
Posteroanterior (PA) and lateral chest radiographs can detect most thymomas. On the PA view, the lesion typically appears as a smooth mass in the upper half of the chest, overlying the superior portion of the cardiac shadow near the junction of the heart and great vessels. The mass usually projects predominantly into one of the hemithoraces. On the right, the silhouette sign is present and the ascending portion of the aortic arch is obliterated. Conversely, if the thymoma is on the left, the silhouette sign is obscured and the aortic knob is identified behind the mass.
Computed tomography (CT) scanning may delineate a mass further or detect a smaller tumor missed on radiograph. Chest CT scanning is the imaging procedure of choice in patients with MG. Thymic enlargement should be determined because most enlarged thymus glands on CT scan represent a thymoma. CT scanning with intravenous contrast dye is preferred to show the relationship between the thymoma and surrounding vascular structures, to define the degree of its vascularity, and to guide the surgeon in removal of a large tumor, possibly involving other mediastinal structures. An example of a CT scan is shown below.
This computed tomography (CT) scan clearly illustrates the mass in the right anterolateral mediastinum. A case report revealed that positron emission tomography (PET) scanning proved to be invaluable in confirming the diagnosis of an invasive malignant thymoma. Although CT scanning revealed evidence of an anterior mediastinal mass, the PET scan showed a hypermetabolic mass consistent with this location, thereby raising suspicion of malignancy. Subsequent resection of the mass revealed a minimally invasive thymoma due to capsular invasion. PET scanning should be added to the armamentarium as an available diagnostic modality to aid in staging and excluding extramediastinal involvement.[3]
Diagnostic Procedures
- Biopsy: If a patient presents with atypical features or is found to have an invasive tumor and is under consideration for induction therapy, obtaining preoperative biopsy is indicated. The limited anterior mediastinotomy (Chamberlain approach) is the standard approach that typically is performed over the projection of the tumor. A thoracoscopic approach for biopsy also can be used.
- Fine-needle aspiration: Controversy exits over the efficacy of fine-needle aspiration (FNA). FNA has been reported by some to be beneficial in making the diagnosis of a thymoma. Performing a core biopsy in conjunction with FNA is a modality that can increase the accuracy in differentiating thymomas from other neoplasms, such as lymphomas and germ cell tumors.
Histologic Findings
Traditionally, thymomas are classified into 3 histologic types based on the predominant cell type—lymphocytic, epithelial, and lymphoepithelial. A World Health Organization (WHO) classification has been developed. In a study conducted in Japan between 1973 and 2001 of a series of 100 resected thymomas, prognostic categories were distinguished using this WHO classification (Table 1).[4]
Table 1. World Health Organization Pathologic Classification and Associated Prognostic Categories (Open Table in a new window)
| Type | Histologic Description | Disease-Free Survival at 10 years*, % |
| A | Medullary thymoma | 100 |
| AB | Mixed thymoma | 100 |
| B1 | Predominantly cortical thymoma | 83 |
| B2 | Cortical thymoma | 83 |
| B3 | Well-differentiated thymic carcinoma | 35 |
| C | Thymic carcinoma | 28 |
| *Series of 100 thymomas resected in Japan between 1973 and 2001 using the WHO classification. | ||
Staging
The Masaoka staging system of thymomas is the most commonly accepted system.
Table 2. Masaoka Staging System of Thymomas and Corresponding Therapy (Open Table in a new window)
| Stage | Definition | Treatment |
| I | Encapsulated tumor with no gross or microscopic invasion | Complete surgical excision |
| II | Macroscopic invasion into the mediastinal fat or pleura or microscopic invasion into the capsule | Complete surgical excision and postoperative radiotherapy to decrease the incidence of local recurrence |
| III | Invasion of the pericardium, great vessels, or lung | Complete surgical excision and postoperative radiotherapy to decrease the incidence of local recurrence |
| IVA | Pleural or pericardial metastatic spread | Surgical debulking, radiotherapy, and chemotherapy |
| IVB | Pleural or pericardial metastatic spread | Surgical debulking, radiotherapy, and chemotherapy |
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| Type | Histologic Description | Disease-Free Survival at 10 years*, % |
| A | Medullary thymoma | 100 |
| AB | Mixed thymoma | 100 |
| B1 | Predominantly cortical thymoma | 83 |
| B2 | Cortical thymoma | 83 |
| B3 | Well-differentiated thymic carcinoma | 35 |
| C | Thymic carcinoma | 28 |
| *Series of 100 thymomas resected in Japan between 1973 and 2001 using the WHO classification. | ||
| Stage | Definition | Treatment |
| I | Encapsulated tumor with no gross or microscopic invasion | Complete surgical excision |
| II | Macroscopic invasion into the mediastinal fat or pleura or microscopic invasion into the capsule | Complete surgical excision and postoperative radiotherapy to decrease the incidence of local recurrence |
| III | Invasion of the pericardium, great vessels, or lung | Complete surgical excision and postoperative radiotherapy to decrease the incidence of local recurrence |
| IVA | Pleural or pericardial metastatic spread | Surgical debulking, radiotherapy, and chemotherapy |
| IVB | Pleural or pericardial metastatic spread | Surgical debulking, radiotherapy, and chemotherapy |
| Stage | 5-Year Survival | 10-Year Survival |
| I | 90% | 80% |
| II | 90% | 80% |
| III | 60% | 30% |
| IV | Less than 25% | N/A |
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