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).

Inclusion Criteria:

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 chemotherapy course with high risk of severe mucositis :

   1. high-dose chemotherapy with hematological stem cell transplantation (stratum A) including but not limited to 8 HDC conditioning regimens for solid tumors intensive treatment (Busilvex-Melphalan, VP16-Thiotepa, BAM, BEAM, Thiotepa 900mg/m^2, Carboplatine-Thiotepa (typical teratoid rhabdoid tumor or medulloblastoma), VP16-Carboplatine-Thiotepa, Endoxan-Busilvex and VP16 - Melphalan) and 5 myeloablative conditioning regimens (for example : TBI-VP16, Busilvex-Endoxan-Thiotepa, Busilvex-Endoxan+/-Melphalan and Thiotepa-Busulfan-Fludarabine),
   2. Conventional chemotherapy courses (stratum B) but not limited to COPADM (B lymphoma), CAV/ICE (plexus choroid carcinoma or B lymphoma), Head start induction course (medulloblastoma), N6 course (neuroblastoma), VIDE or VCD (Ewing sarcoma), API/AP (osteosarcoma), Cyclophosphamide and Doxorubicine (nephroblastoma), PLADO/Carboplatine and Doxorubicine (hepatoblastoma), IVADo (rhabdomyosarcoma), induction and consolidation of the Myechild protocol (Cytarabine/Mithoxantrone Fludarabine Aractine +/- Darubicine) (acute myeloid leukemia) and 5FU-cisplatine (undifferentiated carcinoma of nasopharyngeal),
   3. Others courses of chemotherapy (stratum A or B) which may cause mucositis can be considered (please refer to the non-inclusion criterion n° 5 to see the prohibited chemotherapy list). In this case, the coordinating investigator approval is required.

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 protocol specific 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

NB : all patients who respect all the eligibility criteria are recruitable even if the patient haven't a cancer (Patients undergoing a chemotherapy with high risk of severe mucositis for another clinical reason than cancer).

Exclusion Criteria:

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
5. None of this chemotherapy is allowed : Methotrexate, VA or VAD (nephroblastoma), Carboplatine - Etoposide (3 or 5 days), Temozolomide single agent or in combination with Topotecan/Irinotecan or Bevacizumab, Gemzar, Taxotere, Ifosfamide - Etoposide (Ewing sarcoma), OEPA, COPDAC, IgEV (Hodgkin lymphoma)

Multicenter randomized double blind placebo-controlled trial stratified according to the type of chemotherapy (Stratum A and B) and by center. Patients will be randomized in two parallel groups: preventive treatment with LLLT ("Laser" group) or a control group with a placebo intervention ("Placebo" group).