FIELD : medicine.SUBSTANCE : invention relates to the field of medicine, and more specifically to the field of oncology, traumatology and orthopedics, and can be used in the surgical treatment of patients with osteosarcoma. Method of determining the border of resection in the treatment of osteosarcoma, in which computed tomography of the area of interest is performed in the preoperative period, 3D modeling thereof is carried out, the results of the study are used intraoperatively, performing resection using the navigation system. Computed tomography is performed with contrast enhancement in the dynamics before and after chemotherapy to the patient with the construction of 3D models of the zone of interest; results of computed tomography in the DICOM format are loaded into the Autoplan program, the original and changed tumor sizes are correlated, the final 3D model of the zone of interest is built with the determination of the optimal resection limits, which are performed under the control of the navigation system.EFFECT : invention provides the choice of the optimal border of resection in the treatment of osteosarcoma.1 cl, 1 ex