A new approach for calculating internal dose estimates was developed through the use of a more realistic computational model of the human body. The study demonstrates the capability of building a patient-specific phantom with voxel-based data for the simulation of radiation transport and energy deposition using Monte Carlo methods such as the MCNP-4B code. MCNP-4B was used to calculate absorbed fractions for photons in a voxel-based phantom, and values were compared to reference values from traditional phantoms used for many years. Results obtained in general agreed well with previous values, but considerable differences were found in some cases due to two major causes; differences in the organ masses between the phantoms and the occurrence of organ overlap in the voxel-based phantom (which is not well modeled in the mathematical phantoms). These new techniques offer promise of developing a new generation of more realistic phantoms for internal, as well as external, dose assessment. The principal area of implementation in internal dose assessment should be the development of patient-specific dose estimates in nuclear medicine therapy, such as radioimmunotherapy (RIT). However, as new voxel-based phantoms for different individuals can be developed, they may also be used with the techniques developed here to derive new absorbed fractions and replace the traditional values usedfor other applications in internal and external dose assessment, which have been based on mathematical constructs that are not always very representative of real human organs.