Purpose: Dosimetry is of major value in the preclinical phase of radiopharmaceutical development and its clinical use may be considered an inherent part of therapy to establish the maximum tolerated dose and dose-response relationship. The OpenGATE collaboration is constantly improving the GATE Monte Carlo simulation platform for dosimetry calculations. Our team, as part of the collaboration, has proposed a method implementing S-factor calculation using GATE. Methods: The usual strategy of internal emitter dose estimation using the MIRD formalism establishes that dose can be determined by the product of S-factor times cumulated activity in the source organ.Our procedure is based on the definition of sources and targets volumes from RT-struct DICOM files obtained from patient CT scans. In GATE 8.0 (estimated release by spring 2017) it will be possible to manage DICOM files; we modified in addition the code to have the possibility to manage RT-struct files.Contoured organs from CT scan can be converted into voxelized volumes inside GATE, then simulations can be easily performed choosing a source volume and specifying target volumes. This approach has been validated in different preclinical dosimetry studies. Results: We are now able to perform S-factor calculations in preclinical models with a computing time of around 6 hours on a single CPU (Intel Xeon 3.00GHz) for an initial number of particles (gamma E=140keV) of 10⁷ and with 3 defined materials (soft tissue, lung and bone). Conclusion: We look ahead to create a useful and convenient tool for personalized internal dosimetry in the purpose of speed up S-factor calculation in preclinical and clinical trials. We consider adding S-factor calculation tool to GATE after the next release (by end 2017). Funding Support, Disclosures, and Conflict of Interest: This work was supported by grants from ANR (Agence Nationale pour la Recherche), Programme Recherche translationnelle en sante (DS0411) 2015 under contract ANR-15-CE18-003 and from ITMO "Technologies pour la sante" - AVIESAN