• Written informed consent signed before any trial-related procedures are carried out.

• Histologically confirmed osteosarcoma, with a diagnosis of pulmonary metastases without the existence of local recurrence (previous re-resection of local recurrence with wide margin is allowed).

• Resectable pulmonary nodule(s), defined as nodule(s) that are removable by wedge resection/ segmentectomy/lobectomy without necessitating a total pneumonectomy (e.g., nodules immediately adjacent to the main stem bronchus or main pulmonary vessels), and no evidences of malignant pleural effusion.

• Participants have received at least one standardized systemic treatment regimen at the time of enrollment, and have not received gemcitabine in the past.

• Patient has adequate pulmonary function eligible for one-staged or two-staged thoracic surgery.

• Aged no less than 10 years old and no more than 65 years old;

• For patients ≥16 years old, ECOG score is between 0 and 2 (for patients with amputations, if they can basically take care of themselves and can move freely for more than 50% of their waking hours with the assistance of stretchers, walkers, wheelchairs, etc.) still included);

• For patients under 16 years old, Lansky score is at least 70 or above (for patients with amputations who are unable to participate in active recreational activities due to amputation, if they can participate in most active recreational activities with the assistance of walkers, wheelchairs, etc., they are still eligible included).

• The expected survival time is greater than 24 weeks;

⁃ The majority of the recurrent lesions with an established radiological diagnosis could receive SBRT;

⁃ Major organ functions meet basic safety standards within 7-14 days before treatment.

⁃ Women of childbearing age should agree that they must use contraceptive measures (such as intrauterine devices, birth control pills or condoms) during the study and within 6 months after the end of the study; if in doubt, serum or urine tests within 7 days before study enrollment The pregnancy test is negative and the patient must be non-lactating; the male should agree that contraceptive measures must be used during the study period and within 6 months after the end of the study period;

⁃ If there are recurrent lesions previously treated by surgery, radiofrequency ablation or radiotherapy:

∙ If the image of the metastatic lesion is stable, enrollment is allowed and SBRT is not required for that lesion;

‣ If the metastatic lesion has image progression, if it was previously treated with surgery and SBRT can be performed, enrollment is allowed; if it was previously treated with radiofrequency ablation or radiotherapy, if repeat SBRT can be considered, enrollment is still allowed.