Thanks to the intrinsic qualities of the proton beam, proton therapy will reduce adverse effects of irradiation. The Proteus®One is the latest generation of proton therapy equipment, enabling the Centre Antoine Lacassagne to expand its range of treatments by carrying out new proton therapy treatments. It has an innovative compact isocentric rotating head (Gantry) that allows the radiation beam to be directed at different angles around the patient. In some cases, two beams are used to treat tumours, and by convention, both beams are delivered during the same session. However, it is necessary to position the patient before each beam, which is time-consuming because 2 beams have to be positioned very precisely each day. The aim of this study is therefore to assess the toxicity of proton therapy delivered by a single daily beam compared with proton therapy delivered by two daily beams, which is the conventional technique.

Inclusion Criteria:

• Chordoma, chondrosarcoma of the skull base and spine, Ewing's sarcoma, and osteosarcoma meeting the criteria for treatment by proton therapy
• Tumour requiring 2 beams
• MRI less than one month old
• PS 0-2.
• Patient who has read the patient information note and signed the consent form.
• Patient with healthcare insurance cover.
• Age over 18 years.
• For women of childbearing age, negative urine pregnancy test and effective contraception in place for the duration of treatment and for six months following the end of treatment.

Exclusion Criteria:

• Persons deprived of their liberty or under guardianship.
• Unable to undergo the medical follow-up of the clinical investigation for geographical, social or psychological reasons.
• Patient eligible for symptom reduction surgery Vulnerable populations and participants defined in Articles 64 to 68 of Regulation (EU) 2017/745 of the European Parliament and of the Council of 5 April 2017.