Pronase Improves Efficacy of Chromoendoscopy Screening on Esophageal Cancer

Inclusion Criteria: * Patients 60 years to 75 years old * Patients less than 60 years old, but with one or multiple high risk factors for esophageal cancer including smoking and drinking addiction,family history of esophageal cancer, personal history of esophageal or head and neck malignancies, previous endoscopy documenting iodine void superficial lesion. Exclusion Criteria: * Allergy to iodine or any other medicine which used in this trial. * Previous endoscopy revealing advanced esophageal cancer or other non-superficial lesions. * Conditions interfering visibility of endoscope including gastrointestinal obstruction and upper gastrointestinal bleeding. * Anatomic variation by surgery. * Pregnancy * Other conditions which investigator consider the patient at high risk for complications.