Student’s t test, chi-squared test, and Mann–Whitney U test were used to determine the significance of associations between smoking status and extracted clinicopathological parameters. Recurrence was defined as the development of OED at the same biopsy site after total resection, including excisional biopsy. Malignancy-free survival was defined as the period from the date of diagnosis to malignant transformation [39]. The 5-year cumulative malignant transformation rate was estimated using the Nelson–Aalen method [40]. Patients without an event were censored at last follow-up.

The independent significance of prognostic factors was determined using the Cox proportional hazards model. The Schoenfeld residual was used to test whether the proportional hazards assumptions were true for any data set. Cox–Snell residuals were used to evaluate the overall fitness of the model [41, 42]. Variables with univariate associations showing P<0.1 were included as confounding factors in the multivariate analysis.

All statistical analyses were conducted using STATA version 15 (StataCorp, College Station, Texas, USA). P<0.05 was considered statistically significant.

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