Thymoma: Inter-relationships among World Health Organization histology, Masaoka staging and myasthenia gravis and their independent prognostic significance: A single-centre experience

Objective: In thymomas, World Health Organization (WHO) histology, Masaoka stage and myasthenia gravis (MG) have long been considered important for patient management and outcome. Their role has been independently investigated in the past. Few studies, however, focussed on the correlations among these variables. The aim of the present study was to retrospectively evaluate, in our patient population of resected thymomas, the inter-relationships among MG, WHO histology and Masaoka stage, and to look at how and to what extent one variable is associated with the other two in terms of clinical presentation and survival. Methods: From January 1990 to October 2008, 255 patients received resection of thymoma. MG was present in 105 cases (41%). Histology by WHO was: 25 A (10%), 72 AB (28%), 65 B1 (25%), 69 B2 (27%) and 24 B (9%). Masaoka staging was stage I, 54 cases (21%), stage II, 86(34%), stage III 79 (31%), and stage IVA 36 (14%). Ordinal and logistic regression models were undertaken to analyse correlations among ordinal (WHO histology and Masaoka stage) and categorical (MG, A vs B WHO types) variables. Univariate and multivariate survival analysis were also performed using the same covariates. Overall survival (OS) and disease-free survival (DFS) were calculated. Results: MG was associated with early Masaoka stages (odds ratio (OR) 0.45, 95% confidence interval (CI) 0.33-0.62) and B-type thymomas (OR 1.59, 95% CI 1.23-2.05). B-type thymomas were associated with high Masaoka stage (OR 0.46, 95% CI 0.36-0.60). High Masaoka stage was associated with non-MG (OR 3.27; 95% CI 2.00-5.34). In univariate survival analysis, MG (p=0.01) and Masaoka stage (p=0.0001) were significant prognostic indicators using OS. Using DFS, WHO histology (A/AB vs B1/B2/B3 types) (p=0.05) and Masaoka stage (p=0.0001) had a prognostic significance. In multivariate analysis, only Masaoka stage was an independent prognostic covariate using OS (hazard ratio (HR) 2.57, 95% CI 1.46-4.52, p=0.001) and DFS (HR 3.18, 95% CI 1.56-6.52, p=0.001). Conclusions: In thymomas, MG, WHO histology and Masaoka stage are inter-related. MG has an influence on histology and stage at presentation, while two clinical/histologic patterns are more likely: early Masaoka stage A/AB WHO type and high Masaoka stage/B WHO type. Among the three factors, only Masaoka stage had a prognostic significance on OS and DFS. Our results suggest that a consistent staging system for thymomas should take into account all three variables.