Radiotherapy is an important treatment for many malignant tumors. However, radioresistance remains a major cause of treatment failure, often leading to tumor recurrence or progression. As a key factor affecting prognosis, radioresistance currently lacks reliable methods for identifying high-risk individuals before treatment. Liquid biopsy, as a non-invasive and dynamic monitoring tool, offers unique advantages for real-time assessment. Previous studies from our group have shown that peripheral blood levels of CHI3L1 are significantly elevated in patients with radioresistant tumors and may serve as a predictive biomarker for radiotherapy sensitivity. This study aims to conduct a multicenter, bidirectional cohort study to systematically evaluate the predictive value of peripheral blood CHI3L1 as a pan-cancer biomarker for tumor response to radiotherapy.