<qa>  What is the cause of the odynophagia caused by mucositisIn the context of radiotherapy, control of breakthrough pain represents a special challenge. Patients undergoing radiotherapy may experience different situations of pain that may be due to the need to remain immobilized during radiotherapy session, the need to wear an immobilization mask head and neck cancer, the odynophagia caused by mucositis, defecation after the development of proctitis, or sudden pain during the night causing sleep disturbances. 