Thymoma in the United States (2000-2023): Population-Based Incidence, Treatment Patterns, and Survival

Thymoma is a rare anterior mediastinal tumor for which contemporary US population-based data remain limited. The primary objectives of this study were to estimate age-adjusted thymoma incidence from 2000 to 2023 and to identify independent predictors of overall survival using multivariable Cox regression. Secondary objectives included characterizing demographic distributions, treatment patterns, and subgroup survival differences in a contemporary national cohort.

We conducted a retrospective population-based study using the Surveillance, Epidemiology, and End Results (SEER) 17 Registries Research Data, November 2025 submission, including patients diagnosed with thymoma between 2000 and 2023. Cases were identified using primary site C37.9 and ICD-O-3 histology codes 8580-8585. Age-adjusted incidence rates were calculated per 100,000 population. Overall survival was assessed with Kaplan-Meier methods, and multivariable Cox proportional hazards regression was used to identify factors associated with mortality.

The SEER incidence dataset included 5,706 thymoma cases, of which 5,697 had corresponding case-level data for descriptive analyses. The median age at diagnosis was 61 years, and 47.9% of patients were 50-69 years old. The overall age-adjusted incidence rate was 0.271 per 100,000 population. Incidence remained relatively stable through 2020, then increased to 0.570 in 2021 and 0.674 in 2023. Rates were slightly higher in men than in women (0.285 versus 0.261 per 100,000) and highest among Asian or Pacific Islander patients (0.473 per 100,000). Surgery was the most common treatment and was independently associated with better survival (hazard ratio (HR): 0.34, 95% confidence interval (CI): 0.29-0.40). In contrast, age 70 years or older, tumor size greater than 5 cm, and type B3 histology were associated with increased mortality.

In this contemporary SEER-based analysis, thymoma incidence remained low and largely stable from 2000 through 2020; reported incidence increased after 2020, although this pattern may reflect changes in registry case ascertainment rather than a true epidemiologic shift. Surgery was the strongest modifiable predictor of survival, whereas older age, larger tumor size, and type B3 histology were associated with worse outcomes. Radiation therapy was not independently associated with survival in the adjusted model, but this finding reflects confounding by indication inherent to the observational study design and should not be interpreted as evidence against radiotherapy efficacy.