Patients with more than five and up to ten metastatic lesions often have limited tolerance for surgery, radiotherapy, or thermal ablation because of cumulative treatment burden or expected toxicity. In this setting, systemic therapy alone frequently remains the primary treatment option.

This prospective, open-label, randomized phase 2 study evaluates whether image-guided iodine-125 (125I) seed implantation, when added to standard-of-care systemic therapy, can improve disease control compared with standard systemic therapy alone in patients with multiple metastatic lesions. Clinical outcomes including progression-free survival, overall survival, safety, and quality of life will be prospectively assessed.

This is a prospective, open-label, randomized phase 2 interventional study designed to evaluate the efficacy and safety of image-guided iodine-125 (125I) seed implantation in combination with standard-of-care systemic therapy compared with standard systemic therapy alone in patients with multiple metastatic lesions. Patients with more than five and up to ten extracranial metastatic lesions often have limited tolerance for surgery, radiotherapy, or thermal ablation due to cumulative treatment burden or expected toxicity, and in this setting, systemic therapy alone frequently remains the only treatment option. This study therefore explores the potential role of image-guided radioactive seed implantation as an additional local treatment strategy for this patient population.

After eligibility confirmation and baseline assessments, participants will be randomly assigned in a 1:1 ratio to receive either standard-of-care systemic therapy alone or image-guided 125I seed implantation plus standard-of-care systemic therapy. Randomization will be performed using a predefined allocation scheme, and the study is conducted in an open-label manner due to the interventional nature of the procedure.

The primary endpoint of the study is progression-free survival (PFS), defined as the time from randomization to disease progression or death from any cause, whichever occurs first. Secondary endpoints, aligned with prior randomized studies of metastasis-directed therapy, include overall survival, time to initiation of a new systemic therapy, treatment-related adverse events, and patient-reported quality of life. Exploratory objectives include descriptive analyses of patterns of disease progression.

Participants assigned to the experimental arm will undergo image-guided 125I seed implantation targeting multiple metastatic lesions, performed under CT guidance (with PET/CT fusion planning when clinically indicated) according to institutional standards. Systemic anticancer therapy will be administered in both study arms at the discretion of the treating physician in accordance with current clinical guidelines. All participants will undergo scheduled clinical and imaging follow-up, with disease progression assessed using RECIST version 1.1 and/or PERCIST criteria.

Inclusion Criteria:

• Age ≥ 18 years.
• Histologically or cytologically confirmed malignant solid tumor with metastatic disease.
• Presence of more than five and up to ten (6-10) extracranial metastatic lesions, identified on CT or PET/CT imaging and assessable by RECIST version 1.1 (and/or PERCIST when PET imaging is used).
• At least one metastatic lesion considered suitable for image-guided iodine-125 (125I) seed implantation according to institutional assessment.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0-2.
• Adequate organ function to undergo interventional procedures and systemic therapy, as determined by institutional standards.
• Ability to understand and willingness to provide written informed consent.

Exclusion Criteria:

• Diffuse or unstable central nervous system involvement, including leptomeningeal disease or uncontrolled/symptomatic brain metastases requiring immediate local intervention.
• Medical conditions that preclude safe interventional procedures, including uncontrolled infection, severe cardiopulmonary dysfunction, or other serious systemic illness, as judged by the investigator.
• Contraindications to percutaneous implantation, such as uncorrectable coagulation disorders, high bleeding risk, lack of a safe needle path, or unacceptable risk to critical organs.
• Pregnancy or breastfeeding.
• Inability to provide informed consent or comply with study procedures, due to severe psychiatric illness, cognitive impairment, or other limiting conditions.
• Any other condition that, in the investigator's judgment, would make the patient unsuitable for study participation or compromise patient safety or study integrity.

De-identified individual participant data (IPD) will be made available to qualified researchers upon reasonable request after completion of the study and publication of the primary results. Data to be shared may include demographic information, treatment assignment, key efficacy outcomes, adverse events, and imaging-derived parameters. A data-sharing agreement will be required to ensure appropriate use of the dataset.

Individual participant data (IPD) will be available beginning 6 months after publication of the primary study results and will remain available for 5 years following publication, or until the main study database is closed, whichever occurs first.

Qualified researchers affiliated with academic institutions or recognized research organizations may request access to de-identified individual participant data (IPD), including imaging-derived parameters, dosimetric results, and clinical outcome data.Requests must include a brief research proposal describing the scientific rationale, objectives, and planned analyses. Approval will be granted by the study's principal investigator and institutional ethics committee. Data will be shared through secure institutional data transfer systems after execution of a formal data sharing agreement that ensures patient confidentiality and compliance with applicable privacy regulations.

Due to the interventional nature of iodine-125 (125I) seed implantation, participants and treating clinicians are not blinded to treatment allocation. However, radiologists and nuclear medicine physicians who assess imaging outcomes are masked to treatment assignment, and imaging data are reviewed using anonymized datasets to minimize assessment bias.