Intestinal Low Dose Radiotherapy Combined With Immunotherapy in Immune-resistant Metastatic Solid Tumors

Inclusion Criteria: 1. Age ≥18 years, ≤80 years, regardless of gender. 2. ECOG level 0-2. 3. Expected life span\>3 months. 4. At least one accessible and measurable lesion should be selected as the target lesion for observation according to RECIST criteria. 5. Patients with metastatic solid tumors (of any histology) without standard therapy options, who have previously received immunotherapy, immunotherapy combined with chemotherapy, or immunotherapy combined with anti-angiogenesis treatment and have shown disease progression. 6. The patient is considered ineligible for surgical treatment. 7. Patients with brain metastases assessed as clinically stable after treatment through repeated CT and/or MRI scans are eligible. 8. Patients have complete clinical and pathological information. 9. Any psychological, family, social or geographical conditions may hinder compliance with the research protocol. 10. Patients are able to understand the informed consent form, voluntarily participate, and sign the informed consent form. 11. Other indicators accord with the general inclusion criteria for clinical trials. Exclusion Criteria: 1. Patients with contraindications to radiation therapy and immunotherapy. 2. Previous occurrence of unacceptable immune related toxic side effects (immune myocarditis, pneumonia, etc.). 3. Patients who were assessed as hyperprogressive disease (HPD). 4. Patients who have received pelvic and abdominal radiation therapy within 6 months prior to enrollment. 5. The adverse reactions from prior treatment have not yet recovered to a CTCAE5.0 rating of ≤ 1 (excluding toxicity that has been determined to be risk-free, such as fatigue or hair loss). 6. Accompanied by severe infections. 7. Serious liver disease (such as cirrhosis), kidney disease, respiratory disease, or chronic system diseases such as uncontrollable diabetes and hypertension; Patients who cannot tolerate radiation therapy. 8. Clinical symptoms of brain metastases or meningeal metastasis. 9. The patients with known allergies or allergies to the test drug ingredients. 10. Substance/alcohol abuse. 11. Patients who are pregnant or planning to. 12. Patients participating in other clinical studies that may affect the efficacy/safety of this clinical study. 13. Patients who have undergone major surgical procedures within 30 days. 14. Patients who have received antibiotics, antifungal drugs, antiviral, antiparasitic drugs, or probiotics within 4 weeks.