Dose-point kernels for Auger electron emitting radionuclides used in targeted radionuclide therapy

Objectives Dose point kernels (DPKs) provide information on the spatial pattern of energy deposition around the site of decay of the radionuclide. This information could inform on the choice of radionuclide used with a specific cancer cell targeting strategy. In this work, DPKs are presented for some Auger electron emitting isotopes used for targeted radiotherapy, namely ¹²⁵I, ¹²³I, ¹¹⁹Sb, ¹¹¹In, and ⁶⁷Ga.

Methods The DPKs of the aforementioned radionuclides were calculated with the Monte Carlo code PENELOPE. To this end, point sources were placed in an infinite medium and the absorbed dose produced by the emitted electrons (Auger, Coster-Kronig and internal-conversion electrons) and photons (gammas and characteristic x-rays) was scored in spherical shells. Simulations were done in tissue equivalent material as well as liquid water. The generated DPKs were then employed to calculate C←C, C←CS, N←N and N←CS S-values for spherical cells of various sizes. The S-values so obtained are compared to those tabulated by the Medical Internal Radiation Dose (MIRD).

Results Summary of S-values (Gy/Bqs) for R_C = 10 µm and R_N = 5 µm in liquid water (Table 1).

Conclusions Cellular S-values can be used to estimate the absorbed dose at the cellular level from intracellularly localized radionuclides. The contribution of photons is much smaller than that of electrons and can be neglected. S-values are comparable with the MIRD data when the source and target regions strongly overlap, e.g. for the C←C and N←N configurations. But discrepancies become visible when the source is far from the target, in the N←CS case. This could be ascribed to MIRD S-values being based on approximate DPKs evaluated from the collision stopping power (i.e. assuming straight trajectories of the electrons and neglecting energy straggling).

Research Support This research was supported by Cancer Research UK (CRUK), the Medical Research Council (MRC), and the Nuffield Oxford Trust

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Table 1. Comparison of S-values from MC calculated DPKs and MIRD data.

^†MIRD cellular S values. Goddu et al. 1997. ^‡Not included in MIRD monograph.