The main end point for efficacy is the reduction of severe mucositis (grade 3 or more). The secondary efficacy endpoints are the reduction of mucositis, whatever the grade, the reduction of time with mucositis, the time to onset of mucositis symptoms, the reduction of pain intensity and evaluation of quality of life.The secondary endpoints for safety are severe adverse events and potential toxicity. Patients will be allocated at inclusion with a 1/1 ratio either to laser-on or laser-off groups. In the laser group, patients will undergo laser therapy performed by nurses, which will consist of irradiation of the whole oral cavity during 2 minutes with 250mW power. In the laser-off group, laser therapy will be carried out with equipment off during the same time, and still performed by nurses. Protective eye shields will be used to avoid detrimental effects on eyes and to keep the subjects blind.