The aim of this pilot study is therefore to retrospectively measure the volume and percentage of necrosis on diagnostic MRI in T1 sequence and in parallel to study the expression of immunohistochemical markers of hypoxia (HIF-1α, CAIX , HIF-2α, pS6, phosphomTor, CD163 and CD68) on diagnostic biopsies of high-grade osteosarcomas from 2007 to 2018 in the Strasbourg center, focusing on the pediatric population. The investigators will systematically carry out a correlation analysis between these different parameters and with the clinical data of these same patients (response to chemotherapy, presence of metastases or not and overall and recurrence-free survival). This will eventually make it possible to highlight new prognostic markers at diagnosis.

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

Subject ≤ 20 years having been treated for osteosarcoma in the OS2006 protocol or treated according to.