Type B thymoma: Is prognosis predicted only by World Health Organization classification?

Objective: The prognostic relevance of subtypes within type B thymomas is controversial. The objective of this study was to evaluate the utility of World Health Organization (WHO) classification in patients with type B thymoma. Methods: This was a retrospective review of 100 patients who underwent thymectomy for WHO type B thymoma. Recurrence patterns and survival were compared among subtypes. Results: There were 22 type B1 tumors, 43 type B2 tumors, and 35 type B3 tumors. Incomplete resection occurred in 5 patients with type B1 thymoma, 8 with type B2 thymoma, and 8 with type B3 thymoma (P = .87). Of the 79 patients with complete resection, tumor recurrence occurred in 1 (5.9%) patient with type B1 thymoma, 2 (5.7%) with type B2 thymoma, and 2 (7.4%) with type B3 thymoma, and all of these patients had Masaoka stage III disease. Disease-free survival at 5 years was 93%, 85%, and 82% in type B1, B2, and B3, respectively (B1 vs B2; P = .79; B2 vs B3; P = 0.6). Disease-free survival at 5 years was 94%, 100%, 61%, and 50% in Masaoka stages I, II, III, and IV, respectively (I vs II; P = .26; II vs III; P = .028; III vs IV; P = .002). Conclusions: Tumor recurrence was significantly associated with advanced Masaoka stage regardless of the WHO subtype of type B thymomas. Given the heterogeneity of WHO type B thymomas, Masaoka stage should always be considered when predicting prognosis and planning adjuvant treatment for patients with type B thymomas.