Dual-optimization hypothesis: integrating radiation dosimetry and mechanical reinforcement in radioactive seed implantation for lumbar metastases

Objective To explain the rapid pain relief observed hours after radioactive seed implantation for lumbar metastases (which precedes radiobiological effects) and propose a novel therapeutic framework that integrates two core functions of titanium-encased radioactive seeds: delivering therapeutic radiation and providing immediate mechanical reinforcement to compromised vertebrae. Methods A nonlinear finite element analysis (FEA) was conducted on an L4-L5 vertebral metastasis model to quantify the biomechanical effects of seed implantation. The analysis focused on changes in cortical bone stress peaks and load redistribution patterns in fracture-prone zones, while correlating seed activity levels with implantation density, spatial distribution, dosimetric coverage, and biomechanical reinforcement effects. Conclusion Finite element simulations in a patient-specific L4–L5 model indicate titanium-encased seed implantation reduces cortical stress peaks (16.2% in this model) and redistributes loads from fracture-prone regions. These mechanical changes align with immediate stabilization, potentially aiding early pain relief—though causality cannot be established, as pain is multifactorial and our model only addresses mechanical aspects. We thus propose a dual-optimization framework integrating TPS-based dosimetry with biomechanical objectives to inform both short-term stabilization potential and long-term radiobiological control. Within the scope of the present L4–L5 case, this integrated TPS–biomechanics framework provides a hypothesis-driven approach to optimize implantation planning, while extension of quantitative findings to other spinal levels requires dedicated modeling and validation.