To assess the efficacy of preventive Low-Level Laser Therapy (LLLT) for reducing the incidence of WHO's grade 3-4 oral mucositis in children and young adults receiving chemotherapy regimens associated with a high rate of mucositis: conventional chemotherapy or High Dose Chemotherapy (HDC) conditioning regimens with Hematological Stem Cell Transplantation (HSCT).
Age > 4 and ≤ 25 years
No mucositis or other mouth lesions at the beginning of chemotherapy that could prevent the laser session and chemotherapy
Cooperative patient, able to wear black glasses and to sit with his open mouth during laser session
Patients treated in one of the SFCE centers that participate to the study
Treatment by opioids on daily basis
Orthodontic appliance
Pregnant or breastfeeding young ladies or women
Patients with cognitive disorder who could not self-evaluate his pain or with a mucositis difficult to evaluate