Inclusion Criteria:  Subject ≤ 20 years at diagnosis Subject having been treated for osteosarcoma in the OS2006 protocol or treated according to Supported at Strasbourg University Hospitals between 05/01/2007 and 10/31/2018 Availability of its diagnostic tumor material stored in paraffin having an MRI diagnostic evaluation comprising the injected T1 sequences necessary for the analysis of the volume / necrotic proportion.  Exclusion Criteria:  Opposition of the patient (or his legal representatives if he is a minor) to participate in the study Diagnosis other than osteosarcoma Biological tissue from a patient who does not meet all the inclusion criteria Patient under legal protection Patient under guardianship or guardianship