Framework to calculate 225Ac, 177Lu, and 161Tb radiation dose and biological effect in metastatic castration-resistant prostate cancer treatment

Abstract

Background: In the treatment of metastatic castration-resistant prostate cancer (mCRPC), the standard radionuclide 177Lu (β⁻ emitter) is being challenged by alternatives, particularly the α-emitter 225Ac and Auger electron emitter 161Tb, due to their superior radiobiological properties. These include higher linear energy transfer (LET) and shorter ranges, which enhance localized cell killing while minimizing off-target effects. While these radionuclides induce DNA damage both in source-cells and neighboring cells (crossfire effect), their distinct radiation profiles provide critical metrics to compare their therapeutic efficacy. By quantifying these differences, especially in micrometastatic settings, the optimal radionuclide for specific clinical scenarios could be selected. Purpose: This Study aims to develop a comprehensive pipeline based on Monte Carlo (MC) simulations to compare the therapeutic efficacy of 225Ac, 177Lu, and 161Tb in the treatment of mCRPC based on clinically administered activity (7.4 GBq for 177Lu, 161Tb, and 7 MBq for 225Ac) in a multi-cell model. Besides evaluating the absorbed dose at the cellular level, the Biological Effect Cell Kernel (BECK) method is proposed to compare the radiobiological effect of radionuclides, accounting for the crossfire effect using 3D convolution. Methods: A 2 µm resolution cell model was constructed with a 20 µm cell, an 8 µm nucleus diameter, and a 26 µm center-to-center distance. This configuration resulted in a cellular fraction of 0.24 mL/g, in agreement with that estimated from six prostate cancer patients using CT Perfusion. The prostate time-integrated activity (TIA) in the model was estimated from a patient based on a dynamic 300 MBq 18F-DCFPyL PET scan after scaling to account for the higher administered activity in therapy. The TOPAS-nBio MC tool was used to calculate the absorbed dose and the DNA breaks in the cell model. To account for the crossfire effect, we created the BECK, an isotropic 3D kernel, demonstrating the DNA breaks in the source-containing cell, and those induced in its neighboring cells. The BECK was convolved with the 3D TIA maps of the cell model to obtain the DNA break maps. Results: The cellular absorbed dose was higher than the macro-scale dose based on SPECT-derived TIA, by 31.3%, 15.7%, and 39.8% for 225Ac, 177Lu, and 161Tb, respectively. At the single-cell level, 225Ac induced markedly higher DNA breaks per source – 48 double-strand breaks (DSBs), and 32 complex DSBs, compared to 177Lu – 0.022 and 0.017, and 161Tb – 0.083 and 0.073, respectively. Crossfire effects were dominant for 255Ac and 177Lu at ∼75% and less pronounced for 161Tb at ∼41%. The maximum ranges at which 99.99% of the total DNA breaks were observed were approximately 85 µm for 225Ac, 150 µm for 177Lu, and 110 µm for 161Tb. Conclusions: DPKs and BECKs of 22⁵Ac, ¹⁷⁷Lu, and ¹⁶¹Tb were developed to quantify cellular-level dose distributions and biological efficacy, revealing micrometer-scale heterogeneity accentuated by short-range emissions (α, CEs, AEs). Results demonstrate ¹⁶¹Tb's optimal performance for micrometastases—surpassing ¹⁷⁷Lu's efficacy with lower toxicity than 22⁵Ac—while relative biological effectiveness predicts activity requirements: 22⁵Ac << ¹⁶¹Tb < ¹⁷⁷Lu.