Radiotherapy (RT) and chemotherapy-induced oral side effects, as oral mucositis, and xerostomia are extremely challenging for professionals in the supportive care for head and neck squamous cell carcinoma (HNSCC) patients. There is a substantial evidence about the benefits of photobiomodulation in the management of these oral side effects, mainly in the decrease of oral mucositis (OM) rates and OM severity, healing of OM ulcers and pain relief by intraoral low-level lasers. However, new parameters as extraoral applications by defocused high-power lasers have been investigated to improve the OM outcomes, the patients' complaints, and the experience in the clinical assistance by professionals. Thus, 132 H&N cancer patients will be, globally and simultaneously, randomized according to clinical stage of head and neck cancer, type of treatment (isolated RT or RT and QT), type of RT (RTC3D or IMRT) and center involved (A.C. Camargo Cancer Center or State of São Paulo Cancer Institute or Porto Alegre Clinics Hospital). The randomized patients can be allocated into two groups: Group I: intraoral photobiomodulation with low level laser therapy (660nm, 100 mW, 10 J/cm2, 3 s/point) and Group II: extraoral photobiomodulation with high level laser therapy (810 + 980 nm, 1000 mW, 6.11 J/cm2, 30 s/point). PBMT will be performed daily, 5 times per week during the radiotherapy or until wound healing of OM lesions by a trained professional. Oral mucositis, pain, xerostomia, dysphagia, dysgeusia, oral functions, labial hydration and radiodermatitis will be evaluated daily. Salivary flow and trismus will be assessed weekly, and two questionaries (OHIP-14 and Vanderbilt) will be assessed on the first day of RT, on the middle of RT regimen, on the final of RT regimen and three months later RT sessions.