Did the 2004 World Health Organization (WHO) histologic classification result in better prognostic categories for resected thymomas as compared to 1999 WHO classification scheme?

e18552 Background: The classification of thymic epithelial neoplasms is subject of controversy. The purpose of our investigation is to see whether the 2004 histologic classification results in better prognostic categories than that of the 1999 classification. Methods: The SEER 18 database was used to investigate incidence and overall survival/cause-specific survival (OS/CSS) of resected thymomas during 2000-2009. Incidence was examined by frequency and trend analyses. Patients diagnosed with first primary localized thymoma/thymic carcinoma were selected in two time periods (2000-2003: N=201; 2005-2009: N = 497). The median follow up is 79 months and 27 months respectively. OS/CSS in two patients group were analyzed by Kaplan-Meier estimation, log-rank tests, and multivariate proportional hazards modeling. Results: The proportion of six histology categories (A, AB, B1-B3,C) did not change significantly during the two time periods. Compared to patients diagnosed in 2000-2003, OS was not significantly different in patients diagnosed in 2005-2009 in the log rank tests or multivariate analysis after accounting for treatment, tumor factors, and patient characteristics (OS, HR=0.737, p=0.1580; CSS, HR=0.731, p=0.3711). Histology as 6 categories is a significant predictor for OS in the multivariate analysis in 2005-2009, but not in 2000-2003. However, the predictive role of histology is similar in both time periods in the multivariate analysis of CSS. Complete resection, classification C, and tumor stage are significantly linked to OS and CSS. Conclusions: Patients diagnosed after 2004 did not have better survival outcomes than earlier patients. The WHO classification of 2004 may be a better predictor of OS than that of 1999 as shown by the wide-spectrum of pathologists reporting to SEER.