Critères d'inclusion : 1. Age > 4 and ≤ 25 years
2. No mucositis or other mouth lesions at the beginning of chemotherapy that could prevent the laser session and chemotherapy
3. Cooperative patient, able to wear black glasses and to sit with his open mouth during laser session
4. Patients treated in one of the SFCE centers that participate to the study
5. Patients undergoing a chemotherapy with high risk of severe mucositis
a. HDC and HSCT (stratum A) including 8 DC conditioning regimens for solid tumors intensive treatment (Busilvex- Melphalan, VP16-Thiotepa, BAM, BEAM, Thiotepa 900mg/m², Carboplatine-Thiotepa, Endoxan-Busilvex and VP16 - Melphalan) and 4 allograft conditioning regimens (TBI -VP16, Busilvex-Endoxan-Thiotepa and Busilvex-Endoxan+/-Melphalan)
b. Conventional chemotherapy courses (stratum B): COPADM (Burkitt), CAV (neuroblastoma), VIDE (Ewing sarcoma) and 5FU-cisplatine (undifferentiated carcinoma of nasopharyngeal type)
6. Women of childbearing potential must have a negative serum β-HCG pregnancy test prior to the administration of the first laser treatment.
7. French speaking patients
8. Patient and/or parents/legal representatives should understand, sign, and date the written informed consent form prior to any protocolspecific procedures performed. Patient should be able and willing to comply with study visits and procedures as per protocol.
9. Patients must be affiliated to a social security regimen or beneficiary of the same
Critères de non-inclusion : 1. Treatment by opioids on daily basis
2. Orthodontic appliance
3. Pregnant or breastfeeding young ladies or women
4. Patients with cognitive disorder who could not self-evaluate his pain or with a mucositis difficult to evaluate