The Role of Recoiled Nuclei Effective Rbe In Equivalent Dose Estimation for Alpha Dart (224Ra) Dosimetry
Abstract
Purpose
This study aims to evaluate the effective RBE of recoiled nuclei and their contribution to equivalent dose estimation in Alpha DaRT, whose radiobiological impact remains poorly characterized.
Methods
According to recent studies [1,2], the RBE of heavy ions relative to X-rays (for colony formation with SF = 10%) reaches ~1.06 at LET ≃ 2000 keV/µm. At high LET values, the RBE of heavy ions is known to saturate due to the “overkill effect”. In this study, we present a quantitative estimation of the effective RBE of recoiled nuclei (RBEEff) in Alpha DaRT, primarily originating from 212Pb progeny, by integrating their intracellular biodistribution probabilities with energy deposition characteristics. Using parameters derived from a previous study using γ-ray spectroscopy measurements with an HPGE detector [3], we modeled a maximum value case for the RBE scenario assuming 72% of intracellular activity of 212Pb(PDC)2 localizes within the cell nucleus (Pnucleus ≃ 0.72), and ~35% of the nuclear dose fraction is bound to DNA (PDNA ≃ 0.35).
Results
The effective RBE of recoiled nuclei was defined as the product of the nuclear dose fraction (RBEEff ), the nuclear dose fraction is bound to nuclear DNA (PDNA) and the intrinsic RBE. Results show that RBEEff varies between 0.005 and 0.268 and reaches a maximum of 0.268 at distances beyond 3.35 mm from the source.
Conclusion
Although recoiled nuclei contribute to the total physical dose, their effective RBE remains significantly lower than that of alpha particles. More importantly, as Alpha DaRT does not involve the use of a PDC ligand to facilitate nuclear uptake, the recoil energy is likely to be entirely dissipated outside the nuclei region, resulting in an almost negligible radiobiological effect. Therefore, recoiled nuclei do not appear to play a significant role in the estimation of the radiobiological equivalent dose for Alpha DaRT.