Article Dose constraints Dose objectives Dose-Response Relationship, Radiation Head Head and Neck Neoplasms Head and neck Humans IMRT Incidence Medline Neck Organs at Risk Organs at risk Radiation Injuries Radiotherapy Dosage Radiotherapy, Intensity-Modulated adverse event bone necrosis brain stem injury cancer patient cancer radiotherapy dosimetry dysphagia esophagitis evidence based practice head head and neck cancer head and neck tumor hearing impairment