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.