Astatine-211 ( \({}^{211}At\) ) has emerged as a promising alpha-emitting radionuclide for targeted alpha therapy (TAT) due to its favorable radiobiological properties. However, precise dosimetry remains a critical challenge, as the complex decay characteristics of \({}^{211}At\) introduce uncertainties in energy deposition modeling and dose estimation. Accurate assessment of absorbed dose at the voxel level is essential for optimizing treatment efficacy while minimizing off-target toxicity. To address this challenge, a voxel-based \({}^{211}At\) dosimetry framework has been developed, integrating computed tomography and single-photon emission computed tomography imaging with an optimized equivalent source term (EST) model. The EST model is derived from a detailed analysis of the \({}^{211}At\) decay chain, enabling a refined representation of energy deposition and dose distribution. Monte Carlo simulations are employed to compute high-resolution, patient-specific dose distributions, ensuring accurate quantification of dose absorption at the microscopic level. The incorporation of multimodal imaging enhances both anatomical and functional fidelity, facilitating precise localization of radionuclide uptake and dose mapping. To evaluate the reliability and feasibility of this approach, previously reported preclinical small-animal experimental data have been analyzed, demonstrating consistency between the proposed dosimetric framework and empirical findings. The results underscore the importance of EST refinement in \({}^{211}At\) dosimetry and highlight the advantages of Monte Carlo-based methodologies for TAT applications. By providing a comprehensive dosimetric assessment framework, this study establishes a foundation for the future development of clinically applicable \({}^{211}At\) treatment planning systems, bridging the gap between preclinical research and personalized radionuclide therapy.
