Organ-Specific Differences in Immunotherapy Response Among Metastatic Cancer Patients

Inclusion Criteria: * Age ≥18 years. * Pathologically confirmed solid tumor with a documented primary tumor site. * At least one metastatic site in the brain, liver, lung, or bone confirmed by imaging or pathology (single or multiple lesions allowed). * Received first-line or second-line immune checkpoint inhibitor (ICB) therapy, including PD-1, PD-L1, or CTLA-4 inhibitors, either alone or in combination. * Completed at least 3 cycles of ICB therapy. * At least one evaluable imaging follow-up after initiation of immunotherapy. * Available clinical and follow-up data, including treatment initiation date, response assessment, progression status, and survival status. * Meets institutional ethics requirements for retrospective studies. Exclusion Criteria: * Unclear or undocumented metastatic site, or lack of imaging/pathologic evidence supporting metastasis. * ICB treatment regimen cannot be clearly determined (e.g., unclear combination therapy components). * Missing more than 20% of key clinical variables or incomplete follow-up data (e.g., missing PFS or OS information). * Received fewer than 3 cycles of ICB therapy. * Major treatment interruption or poor treatment adherence. * Concurrent active malignancy that may interfere with outcome assessment. * Severe immune-related disease (e.g., systemic lupus erythematosus) or organ transplantation requiring long-term immunosuppression. * Participation in another interventional clinical trial that may affect treatment evaluation. * Data errors or logical inconsistencies that cannot be resolved after review.