Introduction: The prognostic value of histopathologic classifications of thymoma is debated. Problematic reproducibility might cause this controversy. We studied the prognostic significance of three histopathologic classifications of thymomas after three thoracic pathologists agreed upon thymoma subtype and invasion. We also compared the outcome to established prognostic parameters.
Methods: Patients, surgically treated for thymic epithelial neoplasm at Mayo Clinic (1942-2008), were staged according to the modified Masaoka staging and the recently proposed staging by Moran. Three thoracic pathologists independently classified all cases according to the World Health Organization, Bernatz, and proposed Suster and Moran classification. Only thymoma that all three pathologists diagnosed as the same histopathologic subtype and extent of invasion were included in outcome analysis.
Results: In 214 (proposed Suster and Moran classification), 145 (World Health Organization classification), and 120 cases (Bernatz classification), reviewers agreed upon subtype of thymoma and invasion and follow-up was available. Median follow-up time was 7.5-7.7 years (range between classifications). All histopathologic classifications were associated with overall survival (OS) and disease-free survival (p ≤ 0.0001 to p = 0.048); only Bernatz classification was independent of modified Masaoka staging associated with OS (p = 0.04). Modified Masaoka stage predicted outcome independent of all histopathologic classifications and resection status and strongly correlated with the proposed Moran stage (correlation coefficient, 0.95). Thymoma size and age were prognostic parameters for OS independent of any histopathologic classification.
Conclusions: Histopathologic classifications of thymomas are associated with prognosis but are in general not independent predictors of outcome. Modified Masaoka stage and proposed Moran staging are independent prognostic parameters for thymoma and superior to histopathologic classifications.