Author: Bryan Bednarz, Larry A. DeWerd, Sean Jollota, Ahtesham Ullah Khan, Ohyun Kwon, Jeff Radtke 👨🔬
Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison 🌍
Purpose: The accurate quantification of absorbed dose is essential for alpha-emitting radionuclides used in radiopharmaceutical therapy (RPT). This study presents the first direct comparison of physical measurements of absorbed dose to air for 225Ac for a Monte Carlo (MC) platform, TOPAS, and a research RPT activity to absorbed dose calculation platform, the Radiopharmaceutical Assessment Platform for Internal Dosimetry (RAPID), which is a Geant4-based tool.
Methods: Absorbed dose to air was measured using an extrapolation chamber with an in-house plated 225Ac source. Spatial distribution of the deposited source was evaluated using digital autoradiography with an ionizing-radiation quantum imaging detector (iQID), while activity was determined through gamma spectrometry. Self-attenuation was quantified with an alpha spectrometer, and Monte Carlo correction factors (kself, kback, and kvox ) were applied to facilitate comparisons between the calculations and the experimental results. RAPID simulations utilized iQID-derived images as the activity distribution input for dose calculations.
Results: Across an air gap range of 0.3 to 1.0 mm, absorbed dose to air measurements and TOPAS simulations agreed within 3%, with experimental values being consistently higher. RAPID simulations showed a maximum of 5% discrepancy compared to physical measurements. Trends in absorbed dose and percent difference as a function of air gap highlighted the importance of accounting for geometric effects and progeny behavior in both experimental and computational models.
Conclusion: This work validates both TOPAS and RAPID platforms for 225Ac dosimetry, demonstrating their reliability for modeling alpha-emitting radionuclides. The study establishes a robust methodology for correlating physical measurements with computational dose calculations, providing a foundation for future work involving complex decay chains and improving traceability in RPT dosimetry workflows.