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.[1-3]

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 and is shown below.[4]

For the purposes of discussion of treatment in this summary, these stages are grouped as either noninvasive or invasive.

Noninvasive

In noninvasive (stage I) disease, the tumor is limited to the thymus gland and has not involved other tissues. All of the tumor cells remain within a fibrous capsule that surrounds the tumor.

Invasive

In locally invasive (stage II) disease, the tumor has broken through the capsule and invaded the fat or pleura. In extensively invasive (stages III and IVa) disease, the tumor has spread contiguously from the thymus gland to involve other organs in the chest. Spread to organs in the abdomen or metastatic embolic spread (stage IVb) is unusual at the time of presentation.

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.[4,5] 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.[1]

Some investigators maintain that the Masaoka staging system does not accurately predict outcome for thymic carcinoma.[6,7] In one retrospective study evaluating 43 cases of thymic carcinoma, prognosis was found to be dependent solely on tumor invasion of the innominate artery.[7]

Computed tomography (CT) may be useful in the diagnosis and clinical staging of thymoma, especially for noninvasive tumors. CT is usually accurate in predicting tumor size, location, and invasion into vessels, the pericardium, and the lung. CT cannot predict, however, invasion or resectability with accuracy.[3,8] Appearance of the tumor on CT may be related to the World Health Organization histologic type.[9] A retrospective study involving 53 patients who underwent thymectomy for thymic epithelial tumors indicates that smooth contours and a round shape are most suggestive of type A thymomas, whereas irregular contours are most suggestive of thymic carcinomas. Calcification is 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.[10]

References

1. Okumura M, Ohta M, Tateyama H, et al.: The World Health Organization histologic classification system reflects the oncologic behavior of thymoma: a clinical study of 273 patients. Cancer 94 (3): 624-32, 2002.  [PUBMED Abstract]
   
   
2. Chen G, Marx A, Wen-Hu C, et al.: New WHO histologic classification predicts prognosis of thymic epithelial tumors: a clinicopathologic study of 200 thymoma cases from China. Cancer 95 (2): 420-9, 2002.  [PUBMED Abstract]
   
   
3. Sperling B, Marschall J, Kennedy R, et al.: Thymoma: a review of the clinical and pathological findings in 65 cases. Can J Surg 46 (1): 37-42, 2003.  [PUBMED Abstract]
   
   
4. Masaoka A, Monden Y, Nakahara K, et al.: Follow-up study of thymomas with special reference to their clinical stages. Cancer 48 (11): 2485-92, 1981.  [PUBMED Abstract]
   
   
5. Cameron RB, Loehrer PJ Sr, Thomas CR Jr: Neoplasms of the mediastinum. In: DeVita VT Jr, Hellman S, Rosenberg SA, eds.: Cancer: Principles and Practice of Oncology. 7th ed. Philadelphia, Pa: Lippincott Williams & Wilkins, 2005, pp 845-58. 
   
   
6. Ritter JH, Wick MR: Primary carcinomas of the thymus gland. Semin Diagn Pathol 16 (1): 18-31, 1999.  [PUBMED Abstract]
   
   
7. Blumberg D, Burt ME, Bains MS, et al.: Thymic carcinoma: current staging does not predict prognosis. J Thorac Cardiovasc Surg 115 (2): 303-8; discussion 308-9, 1998.  [PUBMED Abstract]
   
   
8. Rendina EA, Venuta F, Ceroni L, et al.: Computed tomographic staging of anterior mediastinal neoplasms. Thorax 43 (6): 441-5, 1988.  [PUBMED Abstract]
   
   
9. Rosai J: Histological Typing of Tumours of the Thymus. New York, NY: Springer-Verlag, 2nd ed., 1999. 
   
   
10. Tomiyama N, Johkoh T, Mihara N, et al.: Using the World Health Organization Classification of thymic epithelial neoplasms to describe CT findings. AJR Am J Roentgenol 179 (4): 881-6, 2002.  [PUBMED Abstract]
    
    

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