This 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 parameters of each worker can be acquired for an exact geometric representation of his or her anatomy, which is particularly important for low-energy gamma ray emitting sources such as thorium, uranium, plutonium and other actinides. The software discussed here enables automatic creation of an MCNP input data file based on computed tomography (CT) scanning data. The utility was first tested for low- and medium-energy actinide emitters on Livermore phantoms, the mannequins generally used for lung counting, in order to compare the results of simulation and measurement. From these results, the utility's ability to study uncertainties in in vivo calibration were investigated. Calculations and comparison with the experimental data are presented and discussed in this paper.