Comparison of 8th and 7th editions of TNM staging in terms of mortality, persistent disease, and response to treatment in patients with differentiated thyroid cancer.

Objective: To compare the eighth and seventh editions of TNM staging (TNM-8 and TNM-7) on disease-related mortality, persistent disease, and response to treatment in patients with differentiated thyroid cancer (DTC).

Methods and materials: We studied 400 patients (79% female) with DTC with a mean age of 40.93±14.11 years. TNM staging was recorded according to the 7th and 8th editions and patients were followed for at least 1 year and response to therapy was recorded according to ATA response categorization.

Results: The mean follow up time was 42.5±15.24 months. Overall, 108 patients (27%) were down-staged using the TNM-8, mainly due to the changes in the age cut-off (14.5%), N (9.25%), and T categorization (3.25%). All patients in stage III and 82.8% in stage IV were down-staged. The mean Tg levels were significantly higher in stages III and IV in TNM-8 compared to TNM-7. Four disease-related death were recorded during follow up, all in stage IV according to TNM-7, while one was in stage II according to TNM-8. One year after treatment, persistent disease was detected in 12% and 77% of patients in stage III according to the 7th and 8th editions, respectively (P= 0.04). Similarly, biochemical incomplete response one year after treatment was seen in 7.3% and 87% in stage III disease using 7th and 8th editions (P = 0.006) that fell to 2.4% and 22% in the last visit respectively (P = 0.04).

Conclusion: Persistent disease and incomplete response to therapy were more common in stages III and IV in TNM-8 compared to TNM-7. The eighth edition was a better predictor of persistent disease in stages III and IV disease.