Low Level Laser Therapy has been used for a few years in some pediatric hospital centers for chemo- and radiotherapy-induced oral mucositis care. It may promote ulceration's healing, limits mucositis severity and associated pain. As its use is recommended in the treatment of radio induced mucositis in adults, the level of evidence in pediatric studies does not allow a precise treatment protocol. The investigators present here the protocol of a feasibility study in the haemato-oncology department of the Children University Hospital of Toulouse, with the view to a future efficacy study.

This study aims to evaluate the feasibility of using low level laser therapy (LLLT) in the haemato-oncology department of the Children University Hospital of Toulouse. The feasibility will be considered as acceptable if 60% of patients hospitalized for oral mucositis had successfully benefit from the procedure. The secondary objectives are the evaluation of efficacy of LLLT on pain and mucositis grade, the evaluation of tolerance of this treatment and the medical time needed for this supportive care. The athermic phototherapy laser diode will be used by scanning the entire oral mucosa area. It will be applied uniformly every two days as long as grade of mucositis is 2 or more, with a fluence of 4J/cm², under two wavelengths (635 nm and 815 nm).

Inclusion Criteria:

• Hospitalization in the oncology ward of the university hospital Toulouse France
• Onco-haematological paediatric patients treated by radio- and /or chemo-therapy, affected by an oral mucositis of WHO grade 2 to 4,
• lack of any physical or psychological disease which could interfere with the realization of the study.

Exclusion Criteria:

• Onco-haematological paediatric patients treated by radio- and /or chemo-therapy, affected by an oral mucositis of WHO's grade under 2
• Child suffering from epilepsy
• Patient wearing a cardiac pacemaker
• Patient not able to open their mouth at least 20 mm
• Patient refusing the wearing of safety goggles.