Cellular Dosimetry Using the Geant4 Monte Carlo Toolkit

TO THE EDITOR: Cai et al. (1) reported Monte Carlo calculations to estimate cellular doses using ¹¹¹In in different cell configurations. Therefore, the authors used the Monte Carlo N-particle (MCNP) code and compared their results with values provided by Goddu et al. (2,3) for a single-cell model. Their results were within 66.2%–153.4% of the results of Goddu et al.

We performed similar simulations for a single-cell model using the Geant4 tool kit (4). Using the low-energy extensions, Geant4 is able to simulate electron–photon interactions down to 250 eV. The Geant4 source code is freely available and can be downloaded from the official Geant4 collaboration Web site (www.geant4.org). According to cellular experiments in our laboratory, we considered ^99mTc, ¹²³I, and ¹¹¹In. Emission spectra were taken from Howell (5); unlike Cai et al., we used electron and photon emissions. Decay sites were assumed to be homogeneously distributed inside the nucleus or cytoplasm or on the cell surface; dose deposition was considered only in the nucleus for different cell and nucleus radii. For each run, 10⁷ particles were simulated to give at least 10,000 hits in the nucleus.

Our results corresponded well to the S values given by the authors in their Table 1. For a single cell, S values S(N←N) are 0.3%–3% higher, S(N←Cy) deviates from −1.3% to 10.1%, and S(N←CS) deviates from −5.1% to 7.4% (N, Cy, and CS are nucleus, cytoplasm, and cell surface, respectively).

Furthermore, Cai et al. calculated S values for cell clusters and cell monolayers. These values are important when cells in those configurations were irradiated. Thus far, we have examined clonogenic cell survival, whereby cells can be considered as single cells. Hence, we do not calculate S values for clusters or monolayers. Nevertheless, the compliance between the results of Cai et al. (1), Goddu et al. (2), and our simulations show the feasibility of using Monte Carlo methods to assess absorbed doses in cellular dimensions. In contrast to MCNP, Geant4 is free and is available on the Web.

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