The expression level of PD-L1 on tumor cells is a pivotal point which might have some influence on prognosis, especially for those who might use PD-1 or PD-L1 antibody for treatment. The aim of this study is to detect the expression of PD-1 on advaced osteosarcoma patient who might use these antibodies for treatment. All those specimen should be taken by pathologist and confirmed with high tumor cellularity.

Inclusion Criteria:

• diagnosis confirmed histologically and reviewed centrally;
• received surgery in Musculoskeletal Tumor Center of Peking University People's Hospital with appropriate sample for immunohistochemical staining;
• progressing upon prior treatment (completed >4 weeks before trial entry) consisted of standard high-grade osteosarcoma chemotherapy agents including doxorubicin, cisplatin, high-dose methotrexate, and ifosfamide; metastatic relapsed and unresectable progressive disease (PD);
• having measurable lesion according to RECIST 1.1;
• Eastern Cooperative Oncology Group performance status 0-1 with a life expectancy >3 months;
• intend to receive anti-PD-1/anti-PD-L1 antibody for therapy.

Exclusion Criteria:

• fail to be followed up regularly.
• sample disqualification.

All those advanced osteosarcoma patients who intend to receive anti-PD-1/anti-PD-L1 therapy.