Could a tiny dose of radiation reboot cancer immunotherapy?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Age≥18 years old; * ECOG score 0-1; * Histologically or cytologically confirmed esophageal squamous cell carcinoma that is locally advanced (unresectable) or metastatic (AJCC/TNM 8th edition). * Progression during or after one prior systemic first-line regimen that contained both a platinum-based chemotherapy and a PD-1/PD-L1 inhibitor (progression must be documented radiologically or clinically). Patients who received neoadjuvant/adjuvant therapy containing a PD-1/PD-L1 inhibitor are considered first-line failures if progression/recurrence occurs during or within 6 months after completion of that therapy. * At least one measurable lesion per RECIST 1.1 within 4 weeks before enrollment. NOTE: a previously irradiated lesion cannot serve as a target lesion unless clear progression after radiotherapy is documented. * Life expectancy ≥ 3 months. * Adequate organ function within 1 week before enrollment: * Hematologic: Hb ≥ 80 g/L; WBC ≥ 3.0 × 10⁹/L or ANC ≥ 1.5 × 10⁹/L; platelets ≥ 100 × 10⁹/L. * Hepatic: total bilirubin ≤ 1.5 × ULN (direct bilirubin ≤ ULN if total \> 1.5 × ULN); ALT/AST ≤ 2.5 × ULN. * Renal: serum creatinine \< 1.5 × ULN or creatinine clearance ≥ 50 mL/min; BUN ≤ 200 mg/L; albumin ≥ 30 g/L. * Ability to understand and provide written informed consent. Exclusion Criteria: * Active autoimmune disease (e.g., inflammatory bowel disease, rheumatoid arthritis, systemic lupus erythematosus, vasculitis). * Symptomatic interstitial lung disease or active infectious/non-infectious pneumonitis. * Tumor invasion into adjacent organs (aorta or trachea) with high risk of bleeding or fistula; prior esophageal stent placement. * Other malignancies within the past 2 years (except adequately treated basal-cell carcinoma, cervical carcinoma in situ, etc.). * Active infection, heart failure, myocardial infarction within 6 months, unstable angina, or uncontrolled arrhythmia. * Any condition that, in the investigator's opinion, could interfere with study results or increase patient risk. * Mixed small-cell histology. * Pregnant or breastfeeding women. * Congenital or acquired immunodeficiency, HIV infection, prior organ or allogeneic stem-cell transplantation. * Active HBV, HCV, or tuberculosis infection. * Prior tumor vaccine or any live vaccine within 4 weeks (inactivated influenza vaccine is allowed). * Concurrent use of other immunosuppressive agents, chemotherapy, investigational drugs, or chronic corticosteroids. * Psychiatric illness, substance abuse, or social issues that could compromise compliance. * Prior intolerance, hypersensitivity, or contraindication to PD-1/PD-L1 inhibitors or chemotherapy components.