Lesion-Allocated Therapy for Multiple Metastases and Tumor-Zoned Treatment for Bulky Tumors: A Phase II Study of Iodine-125 Seed Implantation Combined With Cryoablation in High-Burden Metastatic Disease

This open-label, randomized Phase II trial evaluates whether a strategy-based, multimodal local treatment approach can improve local control and disease outcomes compared with single-modality local therapy in patients with high-burden metastatic disease. The trial targets a population in which tumor burden is driven by both multiple metastatic deposits and at least one bulky lesion, where uniform local treatment is often limited by anatomic heterogeneity, safety constraints, and incomplete coverage of high-risk tumor regions.

The study is based on a complementary rationale at both the inter-lesion and intra-lesion levels. For multiple metastases, different lesions may be optimally treated with different modalities: cryoablation offers rapid cytoreduction and prompt symptom relief in appropriately accessible lesions with favorable safety margins, while iodine-125 (125I) seed implantation provides sustained low-dose-rate irradiation that can better sterilize residual disease in lesions where ablative margins are difficult to achieve or where durable peripheral control is prioritized. For bulky tumors, a tumor-zoned concept is applied to address intratumoral heterogeneity and margin risk: the core region is treated to achieve efficient debulking (typically with cryoablation), while the peripheral/high-risk margin zones are treated to enhance durable control (typically with 125I seeds) under protocol-defined organ-at-risk constraints. The trial is designed to determine whether this structured "allocation plus zoning" strategy translates into superior local disease control and meaningful tumor-burden reduction without unacceptable toxicity.

Participants will be randomized in parallel to one of two arms: (1) strategy-based combined local therapy, consisting of protocol-defined lesion allocation (each prespecified metastatic target lesion assigned to 125I seed implantation or cryoablation based on predefined anatomic/technical suitability criteria) plus tumor-zoned treatment for at least one prespecified bulky index lesion; or (2) single-modality local therapy, in which all prespecified target lesions are treated using a single local modality (either 125I seed implantation alone or cryoablation alone, as specified by the protocol and/or center capability) with standard supportive care. Treatment assignment is open label. To reduce assessment bias, imaging-based efficacy endpoints will be evaluated using a standardized blinded imaging review process by independent assessors according to protocol-defined criteria, with prespecified rules for target lesion selection, timing windows, and adjudication of progression events.

The primary endpoint is local control rate (LCR) of prespecified treated target lesions at a fixed time point and local progression-free survival (LPFS) of the bulky index lesion (as defined in the protocol). Key secondary endpoints include progression-free survival (PFS), overall response rate (ORR) (patient-level by RECIST), time to systemic progression (including appearance of new lesions), overall survival (OS), tumor-burden reduction metrics (e.g., change in sum of diameters and/or total measurable tumor volume), technical success rates for each procedure, need for re-intervention on treated target lesions.

Safety will be assessed throughout the study and includes procedure-related and radiation-related adverse events graded by CTCAE v5.0, with focused monitoring for clinically relevant risks such as bleeding, infection, fistula/organ injury, nerve or vascular injury, cryo-related complications, seed migration, radiation injury to adjacent organs, and any grade ≥3 treatment-emergent adverse events. Prespecified dose planning and procedural quality assurance measures will be implemented for 125I seed implantation, and standardized cryoablation parameters and post-procedure imaging checks will be used to ensure consistent delivery and safety oversight. Exploratory analyses will evaluate dosimetry-outcome relationships (e.g., coverage and dose-volume parameters for seed implants), zonal treatment response patterns within bulky lesions, imaging biomarkers and radiomics features associated with benefit, and the dynamics of peripheral blood biomarkers as potential correlates of local control and systemic progression.