This single-center randomized controlled trial evaluates the effects of cold (15-20°C) versus room-temperature (30-35°C) water gargling on oral mucositis severity, pain, and comfort in patients with head and neck cancer undergoing radiotherapy. Participants will rinse their mouth four times daily for 30-60 seconds over a 6-week radiotherapy course, with follow-up for two additional weeks. The study hypothesizes that cold-water rinsing can reduce the severity of radiation-induced oral mucositis (RTOM) and pain, improve oral comfort, and minimize treatment interruptions.