Optimization of patient geometry based on CT data in GEANT4 for medical application

The GEANT4 Monte Carlo code provides many powerful functions for conducting particle transport simulations with great reliability and flexibility. GEANT4 has been extending the application fields for not only the high energy physics but also medical physics. In radiation therapy, patient data are usually given in DICOM format and have to be imported into a simulation as geometry. For this purpose, a DICOM interface has been developed for retrieving CT data from DICOM files and converting it into suitable format for making geometry. The GEANT4 defines a patient geometry as a series of voxels. The "parameterized volumes" in GEANT4 is capable to deal with such voxelized geometry. Alternatively, more efficient functionality, "nested parameterization volumes" has been introduced since GEANT4 version 8.1. The performances of these functionalities have been studied in terms of computational efficiency about a patient geometry of a series of voxels with GEANT4 version 8.2 and 9.0. The functionality of the DICOM interface and the result of the performance study have been reported in this paper. The dramatic improvement of memory consumption has been confirmed by adopting the "nested parameterization volumes" instead of normal "parameterization volumes".