(Clinical features) a The prevalence about 0.5 per cent Females account for at least 65 of patients Patients usually over 40 years old (40-49 Untreated disease can persist for 10 or more years Department of Oral Pathology, Shandong UniversityDepartment of Oral Pathology, Shandong University 6 【Clinical features】  The prevalence about 0.5 per cent  Females account for at least 65% of patients  Patients usually over 40 years old (40-49)  Untreated disease can persist for 10 or more years