Figure 1: Selection of patients and variables in our study. (a) The flow chart of this study with number of cases. The present study was based on data of cancer registry and oral mucosal screening database, which belongs to National Health Insurance Research Database. Data were obtained using the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) and ICD-10 codes for diagnoses. Oral potentially malignant disorders include leukoplakia, erythroplakia, oral submucosal fibrosis, oral verrucous hyperplasia, and oral lichen planus. Oral screening: Taiwanese government has started a free oral mucosal screening since 2010. Malignant transformation: Oral cancer occurring during the follow-up of oral potentially malignant disorders is defined as malignant transformation. (b) The division of Taiwan into four geographic regions. The four geographic regions of Taiwan are based on the most widely used definition from the council for economic planning and development, agency in Executive Yuan, a branch of the Taiwan government. Details of the city/county in each region are given in Table 1.