Absorbed fractions in discretized spheres of many sizes

In internal dosimetry, radiation is emitted by internal sources distributed in the volume of an organ or region of the human body. Doses estimates are determined through the simulation of the radiation transport using mathematical models for the energy deposition in matter. Several methods are used to calculate the absorbed fractions for photons and perhaps the Monte Carlo method is the most useful technique. With the recent advances in nuclear medicine and medical images, such as, CT (Computerized Tomography) and SPECT (Single Photon Emission Computerized Tomography) have made possible new challenges in internal dosimetry. Images taken from CT, for instance, give detailed anatomic information for organs but in a discretized form which can introduce errors caused by some geometrical errors. This present work shows the Monte Carlo simulation of radiation transport, using the MCNP-4B code, for the calculation of absorbed fractions in discretized spheres of various sizes. The results are compared with the MIRD publication (MIRD pamphlet 03 and 08). It has been shown that, in general, the errors introduced by the discretization in geometrical modeling have small influences in the results, but increase as the geometric errors increase. Comparisons with results obtained in normal spheres show maximum discrepancies of 4% and 16% respectively for two spheres discretization levels studied. (author)