Abscopal Effect from the Low-dose Radiotherapy in Polymetastatic Cancer Patients Receiving Stereotactic Radiotherapy

Inclusion Criteria:

* Patients who can provide their written informed consent
* Age ≥19 years
* Patients with histologically confirmed primary solid (irrespective of the status of the primary tumor)
* Patients with ECOG performance status 0-2
* Patients planning stereotactic body radiotherapy (three fractions) for extracranial metastases
* Based on RECIST v1.1, patients with at least one extracranial measurable lesion other than SBRT lesions
* Patients with one or more measurable lesions, which are not suitable for SBRT or palliative radiotherapy and can be considered for LDRT (bone metastasis is not indicated for LDRT)
* Patients with hematologic function suitable for radiotherapy (absolute neutrophil count ≥1,500/mm\^3, hemoglobin ≥9 g/dL, platelet count ≥100,000/mm\^3)
* Patients with a life expectancy of 6 months or more according to the researcher's judgment

Exclusion Criteria:

* Patients participating in other clinical studies that may affect the efficacy/safety of this clinical study
* Patients with brain metastasis
* Patients planning SBRT for all measurable lesions due to oligometastasis
* Patients with a history of radiotherapy for extracranial metastases within 3 months of the enrollment
* Patients unable to cooperate with stereotactic body radiotherapy
* Patients who are pregnant or planning to
* Patients with advanced or multiple malignancies requiring aggressive treatment (excluding skin cancers other than melanoma or intraepithelial cancer)
* Patients who have received systemic steroid therapy or immunosuppressive therapy within 2 weeks of enrollment - Patients with an (e.g. allergic disease, radiation pneumonitis, etc.) (The use of steroids for preventive purposes is allowed (e.g., to prevent vomiting during chemotherapy)) active autoimmune disease requiring systemic treatment within the past 2 years, evidence of clinically severe autoimmune disease, or syndrome requiring systemic steroid or immunosuppressive therapy (Patients who need intermittent use of bronchodilators, inhaled steroids, or topical steroid injections, hypothyroid patients on stable hormone replacement therapy, type 1 diabetes patients, or patients recovering from childhood asthma/atopic dermatitis are permitted)
* Patients with active infection requiring systemic treatment