To evaluate the efficacy and safety of regorafenib in patients with refractory primary bone tumors.

Description of the study: The main goal of this study it to evaluate the efficacy and safety of regorafenib in patients with refractory primary bone tumors. The secondary goal of the study it to determine the optimal dosing regimen of the test substance in patients from 2 years to 21 years through monitored pharmacokinetic parameters and pharmacodynamic effects and to determine the safety of regorafenib treatment in patients with primary bone tumors refractory to conventional therapy, aged 2 to 21 years. The secondary aim of the study is the determination of pharmacokinetic parameters Cmaxs, Cmins, Css, time to steady-state concentration and evaluation of clinical response to regorafenib treatment in terms of pharmacokinetics / serum drug concentration as well as the assessment of adverse effects of regorafenib in terms of pharmacokinetics / serum drug concentration. The assessment of the molecular profile in patients with primary bone tumors refractory to conventional therapy, aged 2 to 21 years and the sssessment of the molecular profile as a prognostic factor in comparison with other recognized factors is also the part of the study.