OBJECTIVETo study the prognostic and clinical relevance of histologic subtyping of thymoma according to the World Health Organization (WHO) classification.

METHODSThe clinicopathologic features of 108 patients with thymoma removed surgically were retrospectively reviewed. The histologic diagnosis of the tumors was made on the basis of 2004 WHO classification by two experienced pathologists. The correlation between Masaoka tumor stage, WHO histologic subtype, completeness of resection, presence of myasthenia gravis, other clinical parameters (including age, gender and tumor size) and survival was studied.

RESULTSAccording to WHO classification, there were 7 cases (6.5%) of type A thymoma, 19 cases (17.6%) of type AB thymoma, 23 cases (21.3%) of type B1 thymoma, 19 cases (17.6%) of type B2 thymoma, 27 cases (25.0%) of type B3 thymoma and 13 cases (12.0%) of type C thymoma. According to Masaoka tumor staging, 36 cases (33.3%) were in stage I, 34 cases (31.5%) in stage II, 27 cases (25.0%) in stage III and 11 cases (10.2%) in stage IV(a). The association between histologic subtype and Masaoka tumor stage was statistically significant (P = 0.000). The 5-year survival rates of type A, AB, B1, B2 and B3 thymoma cases were 100%, 100%, 93%, 83% and 43%, respectively; while the 10-year survival rates were 100%, 100%, 81%, 70% and 33%, respectively. The median survival time of type C thymoma was 62.5 months. Type B2 and B3 thymoma cases had an intermediate prognostic ranking in comparison with type C thymoma and other groups (P = 0.000). The 5-year survival rates of tumors in stage I, II and III were 100%, 77% and 54%, respectively; while the 10-year survival rates were 100%, 70% and 27%, respectively. The median survival time of patients in stage IV(a) was 14.0 months. Masaoka tumor stage was highly significant in predicting survival of patients (P = 0.000). On multivariate analysis, Masaoka tumor stage was an independent predictive factor for survival (P = 0.027). On the other hand, the WHO subtype (type A to B1 versus type B2 to B3 versus type C) and completeness of resection could predict the tumor-related survival.

CONCLUSIONSThe Masaoka tumor stage is the single most important prognostic factor of thymoma. The WHO histologic subtype and completeness of resection affect mainly the post-operative survival. The classification of thymoma may also reflect the clinical behavior of the tumor. Type A, AB and B1 thymomas belong to the low-risk group, while type B2 and B3 thymomas have an intermediate prognostic ranking. Type C thymoma carries the worst prognosis.