A Population-Based Study of Incidence and Survival of 1588 Thymic Malignancies: Results From the California Cancer Registry

Because of limited population-based epidemiological studies on thymic malignancies in the current literature, we sought to evaluate incidence and survival trends in thymic malignancies in the California Cancer Registry. Among 1588 adult cases of thymic malignancy diagnosed between 1988 and 2015, we found that thymic malignancy incidence is rising and there is a variation in incidence according to race/ethnicity. As in previous studies, advanced stage and thymic carcinoma were found to be associated with worsened survival. There also appears to be a trend toward detecting more localized stage disease over time, possibly because of the increased use of thoracic imaging studies. Treatment with surgery was associated with improved overall survival in all stages of disease and improved cause-specific survival in local and regional disease. Further research is required to evaluate and better understand contemporary incidence and prognostic factors in thymic malignancies. Background: Thymic malignancies are rare and there are limited contemporary population-based epidemiological studies for this uncommon cancer. Patients and Methods: Adults aged 20 years and older diagnosed with thymic malignancies between 1988 and 2015 were identified from the California Cancer Registry (n = 1588). Trends in age-adjusted incidence rates were examined overall and according to race/ethnicity, and the proportion diagnosed according to stage was evaluated over time. Cox proportional hazards regression was used to estimate hazard ratios (HRs) for overall survival (OS), and Fine and Gray competing risks regression for cause-specific survival (CSS). Results: Age-adjusted incidence increased on average 2.08% per year over the study period (95% confidence interval [CI], 1.30%-2.86%; P<.0001), with an incidence of 0.277 cases per 100,000 in 2015. Incidence was highest among Asian/Pacific Islander and non-Hispanic black individuals. The proportion of unknown stage at diagnosis declined as localized diagnoses increased over time. Compared with patients with thymoma, those with thymic carcinoma had significantly worse OS (HR, 1.63; 95% CI, 1.33-2.01; P<.0001) and CSS (subdistribution HR, 2.99; 95% CI, 2.29-3.91; P<.0001). Advanced stage at diagnosis was also associated with worse survival. Surgical intervention was associated with better prognosis for patients with localized (HR, 0.08; 95% CI, 0.02-0.30; P=.0002) or regional disease (HR, 0.14; 95% CI, 0.06-0.34; P<.0001). Conclusion: Thymic malignancy incidence is increasing in California. There was incidence variation across race/ethnicity, which warrants future study. These findings provide contemporary insight into the incidence and prognostic factors of thymic malignancies.