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

Inclusion Criteria: * Age ≥18 years, ≤80 years, regardless of gender. * ECOG level 0-2. * Expected life span\>3 months. * At least one accessible and measurable lesion should be selected as the target lesion for observation according to RECIST criteria. * 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. * Patients should not be considered eligible for surgical treatment. * Patients with brain metastases that are assessed as clinically stable after treatment through repeated CT and/or MRI scans are eligible. * Patients have complete clinical and pathological information. * Patients should not be borthered by any psychological, family, social or geographical conditions that may hinder compliance with the research protocol. * Patients should be able to understand the informed consent form, voluntarily participate, and sign the informed consent form. * Other indicators accord with the general inclusion criteria for clinical trials. Exclusion Criteria: * Patients with contraindications to radiation therapy and immunotherapy. * Previous occurrence of unacceptable immune related toxic side effects (immune myocarditis, pneumonia, etc.). * Patients who were assessed as hyperprogressive disease (HPD). * Patients who have received pelvic and abdominal radiation therapy within 6 months prior to enrollment. * 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). * Patients with active uncontrolled systemic bacterial, viral, or fungal infections despite optimal treatment. * Significant liver or kidney dysfunction (i.e., laboratory values \>3 times the upper limit of normal). * Active hepatitis B, hepatitis C, HIV, or syphilis. * Brain disorders, symptomatic central nervous system (CNS) or meningeal metastases, or impaired cognitive function. * Hypersensitivity to any drug included in the trial. * Drug and/or alcohol abuse. * Pregnant or breastfeeding women. * Concurrent participation in another therapeutic clinical trial. * Poorly controlled pleural effusion, pericardial effusion, or ascites requiring frequent drainage (recurrence within ≤14 days after intervention). * Major surgery within 30 days. * Use of antibiotics, antifungals, antivirals, antiparasitics, or probiotics within 4 weeks before enrollment.