Invited commentary

Abstract

The idea of clinical staging of thymoma was introduced by Bergh and colleagues [1Bergh N.P. Gatzinsky P. Larsson S. Lundin P. Ridell B. Tumors of the thymus and thymic region: I. Clinicopathological studies on thymomas.Ann Thorac Surg. 1978; 25: 91-98Abstract Full Text PDF PubMed Scopus (166) Google Scholar], which was later modified by Wilkins and Castleman [2Wilkins Jr, E.W. Castleman B. Thymoma a continuing survey at the Massachusetts General Hospital.Ann Thorac Surg. 1979; 28: 252-256Abstract Full Text PDF PubMed Scopus (114) Google Scholar], and was advanced by Masaoka and associates [3Masaoka A. Monden Y. Nakahara K. Tanioka T. Follow-up study of thymomas with special reference to their clinical stages.Cancer. 1981; 48: 2485-2492Crossref PubMed Scopus (1345) Google Scholar] in 1981. Masaoka’s system has been widely adopted and is an excellent system for predicting prognosis for thymomas. Several articles including the article by Bedini and associates [4Bedini A.V. Andreani S.M. Tavecchio L. et al.Proposal of a novel system for the staging of thymic epithelial tumors.Ann Thorac Surg. 2005; 80: 1994-2001Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar] have pointed out problems and have suggested that an update of the system is desirable. The authors identified seven major criticisms of the Masaoka system. I think that some criticisms are important (1 through 4), and I would like to add two additional staging issues (5 and 6): 1The classification system does not provide appreciable prognostic separation between stages I and II.2Some definitions are not clinically applicable because surgical or pathological assessment is required. In particular, the definition of stage II is unclear. Some pathologists propose that microscopic invasion into the capsule in stage II should be replaced by microscopic transcapsular invasion. The most recent World Health Organization classification of thymic epithelial tumors in 2004 defined T2 thymoma as “tumor invades pericapsular connective tissue” [5Müller-Hermelink H.K. Ströbel P. Zettl A. et al.Combined thymic epithelial tumours.in: Travis W.D. Brambilla E. Müller-Hermelink H.K. Harris C.C. Pathology and genetics of tumours of the lung, pleura, thymus and heart (WHO classification of tumours series). IARC Press, Lyon, France2004: 196-198Google Scholar].3The system is not well suited for staging thymic carcinomas.4Presence residual tumor is classified by use of the “R” category. Many reports demonstrate completeness of thymoma resection is the most important predictor of survival, although some reports show a value of so-called debulking procedures.5As stage III thymoma is highly heterogenous in terms of involved organs, classification should divide the subgroups according to prognosis. Okumura and associates [6Okumura M. Miyoshi S. Takeuchi Y. et al.Results of surgical treatment of thymomas with special reference to the involved organs.J Thorac Cardiovasc Surg. 1999; 117: 605-613Abstract Full Text Full Text PDF PubMed Google Scholar] report that involvement of the great vessels is an independent prognostic factor in patients with stage III thymoma.6The TNM system classification of thymic epithelial tumors has not been established. Yamakawa and Masaoka [7Yamakawa Y. Masaoka A. Hashimoto T. et al.A tentative tumor-node-metastasis classification of thymoma.Cancer. 1991; 68: 1984-1987Crossref PubMed Scopus (147) Google Scholar] presented a tentative TNM system classification of thymoma in 1991, which some reports subsequently supported. In Masaoka’s system, the presence of local invasion (T factor) is strongly emphasized in comparison with lymphogenous and hematogenous metastasis (N and M factors) because of the rarity of lymphogenous and hematogenous metastasis in thymoma. However, it is necessary to determine how N or M factors influence prognosis to establish a TNM system classification of thymic epithelial tumors including thymic cancer and carcinoid. The World Health Organization’s histologic classification of 1999, which can distinguish thymic carcinoma and carcinoid from thymoma, has been widely adopted, and large-scale clinicopathologic studies of thymic carcinoma and carcinoid may provide sufficient prognostic information to include N or M factors in a TNM system of thymic epithelial tumor.