The paper reports on a new utility for development of computational phantoms for Monte Carlo calculations and data analysis for in vivo measurements of radionuclides deposited in tissues. The individual dimensions of each worker can be acquired for a rather precise geometric representation of his(her) anatomy, which is particularly important for low energy emitting sources such as thorium, uranium, plutonium and other actinides. The software discussed here enables automatic creation of an MCNP^TM input data file based on CT or MRI images. The utility was firstly tested for low and medium energy actinide emitters on anthropomorphic phantoms, the mannequins generally used for in vivo counting, in order to compare the results of simulation and measurement. From these results, the demonstration of the utility's abilities for the study of geometry uncertainties, such as different anthropomorphic phantoms or different source geometries, on in vivo calibration was investigated. Calculations and comparison with the experimental data are presented and discussed in this paper.