Prognostic Impact of Primary Tumor Size in Thymic Epithelial Tumor: An NCDB-Based Study

Background. Thymic epithelial tumor (TET) staging has been based on Masaoka-Koga systems or the 8th edition of the TNM classification, which do not use tumor size as a T descriptor. The 9th edition of the TNM classification incorporates tumor size; however, the study on which this classification is based included only 4.4% of patients from North America. This study investigated the prognostic impact of primary tumor size in TET patients in the US population. Methods. Using data from the National Cancer Database (NCDB), we analyzed patients with surgically resected TETs diagnosed in 2004-2020. Survival analysis was performed by using the Kaplan-Meier method and multivariate Cox regression analyses; propensity score matching (PSM) analyses were performed. Thymoma data from our facilities (n = 166) were used for validation. Results. Of 4,151 and 647 thymoma and thymic-carcinoma patients, respectively, we classified 1,618 and 268 patients into small-tumor (primary tumor size <= 5 cm) and large-tumor groups, respectively. Thymoma patients in the small-tumor group had a significantly longer overall survival (OS) than those in the large-tumor group (>5 cm) (median OS 193.2 vs. 161.4 months, respectively; log-rank P < 0.0001; hazard ratio 0.72; 95% confidence interval 0.64-0.82). After PSM, multivariate analysis showed that tumor size was an independent prognostic factor for OS (P < 0.0001). Validation cohort analysis supported these results. Tumor size did not have a significant impact on OS (P = 0.0994) in thymic-carcinoma patients. Conclusions. Tumor size in thymoma, but not in thymic carcinoma, was an important prognostic factor in the U.S. population.