Abstract: Background and purpose: The effect of minimal extrathyroid invasion (MEI) on mortality in differentiated thyroid cancer (DTC) patients was eliminated from the 8^th TNM staging system. This study aimed to analyze the correlation between MEI and recurrence risk in DTC patients. Methods: We retrospectively analyzed 942 DTC patients without distant metastasis who were treated in Peking Union Medical College Hospital with a median follow-up of 24 months. Patients were divided into two groups: disease recurrence/persistence patients as structural incomplete response group (SIR, n=55), and non-SIR as NSIR group (n=887) according to their response to therapy. Chi-square test and rank-sum test were used to evaluate the statistical differences in basic clinicopathologic features between two groups. Multivariate analysis was used to quantify the influence factors for SIR. Correlation analysis was conducted between MEI and recurrence. We compared the clinical-pathologic features and responses between low-risk group (G1, n=39) and minimal extrathyroid invasion group (G2, no other risk factors, n=65). Result: There were statistical differences in tumor size (P=0.018), lymph node stage (P=0.008) and macroscopic extrathyroid invasion (P=0.008) between SIR group and NSIR group, and no significant difference in MEI (P=0.444) between the groups. Tumor size (P=0.007) and macroscopic extrathyroid invasion (P=0.036) were two independent influence factors for SIR in multivariate analysis. It showed no correlation between MEI and SIR (r=-0.026, P=0.425). G2 showed a high rate of female (P=0.018) and age at diagnosis (P=0.033) compared with G1. There was no significant difference in tumor size (P=0.517), tumor multifocality (P=1.000) and dose of ¹³¹I (P=1.000 ), as well as the recurrence between G1 and G2 (1.5% vs 2.6%, P=0.244). Conclusion: MEI should not be an independent risk factor for recurrence in DTC patients.

Key words: Differentiated thyroid cancer, ¹³¹I therapy, Minimal extrathyroid invasion, Recurrence, Response