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

Prior treatment

• Patients with heart defects and / or cardiac arrhythmias requiring permanent treatment with antiarrhythmic drugs.
• Prior treatment with regorafenib.
• Taking medications that cannot be used while under regorafenib treatment.
• Persistent toxicity related to previous therapy, preventing drug incorporation.
• Diagnosis of other neoplastic disease prior to inclusion in the study.

Other illness

• Failure to meet any of the inclusion criteria.
• Known hypersensitivity to the drug or any of its ingredients.
• Patients with uncontrolled hypertension.
• Patients with diseases related to the coagulation disorders.

Lab values

• Other acute or chronic medical conditions, behaviors, or abnormal laboratory values that may increase the risk of participating in this clinical trial or taking the study medication, or may affect the interpretation of the study results, or, in the investigator's opinion, may cause that the patient should not be enrolled in the study.

Other

• Pregnancy and breastfeeding.

• Age> 9 years and ≤ 21 years at the time of inclusion for the study.
• Ewing's sarcoma or osteosarcoma confirmed by histopathological examination and the tests performed so far.
• Treatment failure identified no earlier than 30 days prior to study treatment initiation (at least one subsection must be met for the patient to meet this criterion): a. progression on treatment of I or another line or b. relapse.
• Giving written, informed consent to participate in the study prior to the commencement of the procedures included in the study protocol, including treatment with regorafenib in accordance with the current legal regulations.
• Life expectancy of at least 12 weeks from signing the informed consent.
• Possibility of swallowing the tablet.
• Consent to use effective contraception throughout the period of regorafenib treatment and at least 2 years after its discontinuation in patients in puberty.