Determinants of Radiation Dermatitis in Head and Neck Cancer Patients Receiving Preventive Photobiomodulation

The goal of this observational study is to identify factors associated with the occurrence of grade 2 or higher radiation dermatitis in adults with head and neck cancer treated with radiotherapy who receive preventive photobiomodulation (PBM) as part of their standard care. The main question it aims to answer is: which clinical, demographic, tumor-related, treatment-related and functional factors are associated with grade 2 or higher radiation dermatitis (NCI-CTCAE v5.0) despite preventive PBM? Participants will receive their usual radiotherapy and their usual preventive PBM sessions (3 sessions per week throughout radiotherapy). In addition, for the purpose of the study, they will: * perform a 1-minute sit-to-stand test and a handgrip strength measurement at inclusion and at the end of radiotherapy; * complete the EORTC QLQ-H\&N35 quality-of-life questionnaire before radiotherapy, at mid-treatment and at the end of radiotherapy; * have standardized photographs taken of the irradiated skin. Skin toxicity will be graded at each PBM session.

Inclusion Criteria: * Age 18 years or older at the time of inclusion * Diagnosis of head and neck cancer (histological, cytological or imaging confirmation), including the following sites: oral cavity, oropharynx, hypopharynx, larynx, nasopharynx, nasal cavity and paranasal sinuses * Indication for curative-intent external beam radiotherapy (exclusive or adjuvant), with or without concomitant chemotherapy, exposing the cervical skin * Possibility of starting preventive photobiomodulation (PBM) from the start of radiotherapy according to the standardized protocol (3 sessions/week, 3 J/cm2) * Affiliated with or beneficiary of a social security scheme * Free, informed and express written consent obtained after provision of an information sheet Exclusion Criteria: * Inability to receive PBM due to a contraindication, patient refusal, or radiotherapy duration of less than two weeks * Previous radiotherapy to the cervico-facial region * Uncontrolled or active dermatological condition in the area to be irradiated likely to interfere with the assessment of skin toxicity (psoriasis, eczema, cutaneous lupus, etc.) * Ongoing photosensitizing treatment that cannot be interrupted or replaced during radiotherapy * Severe immunosuppression, defined in particular as HIV infection at the AIDS stage, long-term immunosuppressive treatment, or history of organ transplantation * Concurrent participation in another therapeutic trial likely to affect the occurrence or severity of skin toxicity * Person under legal protection (guardianship, curatorship or safeguard of justice) * Pregnant or breastfeeding woman * Patient refusal to participate in the study