Reassessing the NTCTCS staging systems for differentiated thyroid cancer, including age at diagnosis

Background: Thyroid cancer is unique for having age as a staging variable. Recently, the commonly used age cut-point of 45 years has been questioned. Objective: This study assessed alternate staging systems on the outcome of overall survival, and compared these with current National Thyroid Cancer Treatment Cooperative Study (NTCTCS) staging systems for papillary and follicular thyroid cancer. Methods: A total of 4721 patients with differentiated thyroid cancer were assessed. Five potential alternate staging systems were generated at age cut-points in five-year increments from 35 to 70 years, and tested for model discrimination (Harrell's C-statistic) and calibration (R²). The best five models for papillary and follicular cancer were further tested with bootstrap resampling and significance testing for discrimination. Results: The best five alternate papillary cancer systems had age cut-points of 45-50 years, with the highest scoring model using 50 years. No significant difference in C-statistic was found between the best alternate and current NTCTCS systems (p=0.200). The best five alternate follicular cancer systems had age cut-points of 50-55 years, with the highest scoring model using 50 years. All five best alternate staging systems performed better compared with the current system (p=0.003-0.035). There was no significant difference in discrimination between the best alternate system (cut-point age 50 years) and the best system of cut-point age 45 years (p=0.197). Conclusions: No alternate papillary cancer systems assessed were significantly better than the current system. New alternate staging systems for follicular cancer appear to be better than the current NTCTCS system, although they require external validation.