The Efficacy of N-acetylcysteine on Alleviating Symptom Caused by Lugol Chromoendoscopy

Inclusion Criteria: * Patients at risk of esophageal squamous cell cancer including those over 50 years old, or over 45 years old with family history of esophageal squamous cell cancer; * Patients with suspected esophageal lesions by screening endoscopy; * Patients requiring surveillance after endoscopic resection or radiotherapy for esophageal squamous cell cancer one year later。 Exclusion Criteria: * Patients allergic to iodine or with hyperthyroidism; * Patients with advanced esophageal cancer, esophageal varices, esophageal ulcer or other conditions inadvisable for Lugol chromoendoscopy; * Patients with severe gastroesophageal reflux disease or reflux symptoms which may interfered with the outcome measures of current study; * Patients with postoperative esophageal stenosis affecting endoscopic observation; * Patients with serious comorbidities that cannot tolerate un-sedated gastroscopy; * Patients who could not cooperate with the observation including patients with mental disorders, severe neurosis or dysgnosia; * Patients undergoing sedation or general anesthesia during endoscopy. * Patients who do not agree to sign informed consent forms or follow the trial requirement.