Most patients with metastatic cancer eventually develop resistance to systemic therapy. A subset of patients experience oligoprogression, characterized by progression at a limited number of lesions while other disease sites remain controlled by ongoing systemic therapy. This randomized phase 2 trial evaluates whether image-guided 125I seed implantation targeting all oligoprogressive extracranial lesions, combined with standard-of-care systemic therapy, improves progression-free survival compared with standard-of-care systemic therapy alone in patients with metastatic NSCLC or CRC.