Statistical analysis
All statistical analyses were performed using SPSS software (version 21.0; IBM for Windows, Armonk, NY, USA). Categorical variables were analysed using the chi-square test (or Continuity correction chi-square test or Fisher’s exact test). The stepwise multivariate Logistic model using the forward Wald method was used to determine independent impact factors for lymph node metastasis. <0.05 was considered statistically significant.