Conditional Survival of Malignant Thymoma Using National Population-Based Surveillance, Epidemiology, and End Results (SEER) Registry (1973-2011)

Introduction: Thymoma is a rare and unique tumor with a long natural history that makes it difficult to study. Consequently, there is a dearth of prospective diagnostic or therapeutic clinical trials. To our knowledge, there has not been an analysis of conditional survival of thymoma in the literature. The specific aim of this study was to study the 5-year conditional survivals of a large population of thymoma patients. Methods: Cases of thymoma were extracted from the Surveillance, Epidemiology, and End Results registry (1973-2011) and categorized into Masaoka-Koga stage groupings. The primary outcomes compared overall survival (OS), cause specific survival (CSS), and 5-year conditional OS and CSS, by stage. OS and CSS were calculated using the Kaplan-Meier method with the log-rank test for significance using SAS v9.3. Conditional survival was the probability of surviving an additional 5 years at any point in follow-up, and used analysis of variance to test significance. Results: A total of 2182 patients met inclusion criteria and were categorized as Masaoka-Koga stage groupings of I and IIA ("localized," 24%), IIB ("regional," 16%), III and IV ("distant," 50%), and unknown (10%). Median age was 56 (18-91), and 53% were male. Earlier stages had better OS (p < 0.0001) and CSS (p < 0.0001). Twenty-year OS for local, regional, and distant stages were 42%, 30%, and 18%, respectively. Conditional survivals remained largely unchanged throughout follow-up. Conclusions: Conditional survival provides more relevant survival estimates for patients during follow-up. Further studies should investigate the possibility that thymoma should be considered a chronic disease.