Adaptation of the Geant4 ICRP110 Human Phantom advanced example for nuclear medicine applications to allow overlay of lesions as Geant4 parallel world objects

Introduction: The Geant4 application ‘ICRP110_HumanPhantom’ [1] has recently been implemented as an advanced example in version 10.7 of the Geant4 Monte Carlo Toolkit (https://geant4.web.cern.ch/node/1910). This advanced example implements the adult male and adult female voxelised phantoms published in the ICRP 110 [2] and allows the estimation of radiation doses to single voxels and phantom organs from any radiation field incident on the phantom. In this study, the ICRP110_HumanPhantom was adapted to estimate voxel and organ doses from the internalisation of radionuclides in organs and in lesions that can be positioned within the phantom. The lesions are objects of any shape and/or material composition whose position in the Geant4 simulation can be controlled by the user.

Methods: This implementation of the ICRP110_HumanPhantom, developed in Geant4 version 10.4p01, makes use of the EM Livermore Physics List for modelling particle interactions and of the G4ParticleGun Class to randomly select particles’ initial position, momentum and direction of travel within selected organ(s) or within user defined lesion(s). Lesions are implemented using the Geant4 Parallel World functionality and they can be positioned to overlap the voxelised phantom. The Geant4 code was benchmarked by comparing organ doses from mono-energetic photons and electrons uniformly distributed within organs against OpenDose data [3]. As an example of the code functionality, spherical lesions of 0.5, 1 and 2 cm radius were positioned, in a Geant4 Parallel World, central to the ICRP110 adult male liver, keeping the same material composition as the ICRP110 liver [2]. The isotope ⁹⁰Y was uniformly distributed in the tumours and 50 runs of 10⁶ particles were simulated to evaluate absorbed doses per disintegration to the lesion and to all the ICRP110 organs. Self-dose to the spherical lesions was compared to the values published by Bardies and Chatal [4].

Results: Good agreement between absorbed doses to organs from uniform distribution of mono-energetic electrons and photons in source organs were observed when compared to OpenDose data [3]. Absorbed doses per disintegration to the lesion, liver, spleen, right and left kidneys from spherical lesions are shown in Table 1. Good agreement between self-dose and Bardies and Chatal [4] data was achieved. Conclusion: The Geant4 ICRP110_HumanPhantom advanced example has been adapted to 1. allow users to generate sources within the phantom organs and user defined lesion(s) and 2. record absorbed doses in all the ICRP 110 organs and in the lesion(s). In fact, the use of the Geant4 Parallel World adds the flexibility to input one or more non-spherical lesions within the ICRP110 phantom and to estimate absorbed doses to organs from these lesions. Currently, the code is being imported to the newest version of Geant4 (10.7), this should not have any impact on the validity of the results presented. It is envisaged that this adaptation of the Geant4 ICRP110_HumanPhantom application, of interest for internal dosimetric estimations, will be implemented in future versions of the Geant4 advanced example.

Table 1: Absorbed doses to lesion, spleen, liver and kidneys from spherical tumours