Unknown primary metastatic carcinoma is a general term for independent metastatic tumors, which are histologically confirmed as metastatic carcinoma but whose primary anatomic site cannot be determined after examination. It accounts for about 3% \~ 5% of all new cancers in humans, and is the 7th \~ 8th most common malignant tumor and the 4th most common fatal cancer in humans.CUP is typically characterized by aggressive and early metastasis and unpredictable metastasis. The disease is not a single disease, but a heterogeneous collection of tumors composed of different primary tumor types that cannot be recognized Bone is the third most common site of CUP, and bone metastases from cancer of unknown primary (BMCUP) is a type of CUP with poor overall prognosis. The primary site of the tumor was determined. BMCUP is a type of heterogeneous tumor with bone metastases confirmed by biopsy but whose primary anatomic site cannot be determined after detailed history, physical examination and relevant experimental examination.BMCUP accounts for about 10% of CUP, which is common in adults with a median onset age of 65-90 years and slightly more in men than women.The spine is the most common site of BMCUP, followed by the pelvis and long bones.Adenocarcinoma was the main pathologic type of BMCUP, with low and medium differentiated adenocarcinoma (64%), undifferentiated carcinoma (20%), neuroendocrine carcinoma (9%) and squamous cell carcinoma (7%).Patients with BMCUP usually have a poor prognosis, with an average survival of 3 to 12 months.

Currently, empire-based chemotherapy, including platinum and taxanes, is still the main treatment for CUP and BMCUP, but the effect is not ideal and the survival rate is very low (median overall survival, about 6-9 months).In recent years, gene expression profiling is a new diagnostic technique, which makes it possible to predict the origin of tumor tissue based on the expression profiling of specific sites. Several of these molecular profiling methods, including reverse transcription polymerase chain reaction (RT-PCR) analysis and DNA microarray analysis, have been developed to trace the primary foci of CUP and BMCUP. In addition, the results of gene expression profiling can indicate the information of tumor genetic abnormalities and thus potential targeted drugs. However, Hayashietal etal., published in JCO in 2019, concluded that "although the prediction of the primary site seems to have prognostic value,However, cuP-site-specific therapy based on microarray mapping did not result in a significant improvement in 1-year survival compared with empirical platinum-paclitaxel therapy ". At present, the systemic treatment of CUP and BMCUP is still not ideal, and finding new effective and safe drug treatment is the key research content of this kind of disease.

In recent years, with the deepening of the research on the regulation mechanism of the immune system and the emergence of immune checkpoint inhibitors, biological therapy with tumor immunotherapy as the core has developed rapidly, and now has become another major cancer treatment mode after chemotherapy and targeted drugs. When the body encounters cancer cells, antigen presenting cells (APC) capture antigen (Ag) and present it to T cells. After activation of T cells, programmed cell death protein 1 (PD-1) receptor can be expressed in T cells and transmit negative regulatory signals to T cells by binding to PD-L1 ligand on tumor cells and APC surface, thus inhibiting the immune response. Immune checkpoint inhibitors block the binding of the PD-1 receptor to the PD-L1 ligand, thereby enhancing the anti-tumor immune response.Pd-1 and PD-L1 inhibitors play a significant antitumor effect in lung cancer, melanoma, kidney cancer and other malignant tumors, but there are no literature reports on the efficacy and safety of immune checkpoint inhibitors in CUP and BMCUP. According to the search results of our treatment group, ClinicaTtrails. Gov has registered clinical trials under research, and the studies related to the application of immune checkpoint inhibitors in CUP include Tom Baker Cancer Center in Canada, a prospective single-arm study of paprizumab monotherapy in CUP patients with poor prognosis, which is still recruiting patients.

Tislelizumab is a humanized IgG4 monoclonal antibody against PD-1, which is an "immune checkpoint" inhibitor. It can bind to the PD-1 receptor on the cell surface and block the binding of PD-L1 and PD-L2. Thus, the escape state of tumor immunity is reversed and the killing activity of T cells is restored. Compared with other PD-1 antibodies, Tislelizumab can reduce the binding of FcγR on macrophages, thereby reducing antibody-dependent phagocytosis and possibly improving the resistance of PD-1 antibodies. It was approved for market in China on December 26, 2019. For the treatment of patients with recurrent or refractory classic Hodgkin's lymphoma, urothelial carcinoma, non-small cell lung cancer, hepatocellular carcinoma. In terms of safety, the most common grade 3 or higher adverse reactions associated with Tislelizumab were elevated glutamyltransferase, anemia, and glutamic-oxalacetic transaminase.