Dosimetrical studies on a tissue level using the MCNP4c2 Monte Carlo Simulations

Abstract: The purpose of this thesis has been to learn the MCNP-code, its theory and limitations, and how to use it in the field of small-scale dosimetry. In order to evaluate the applicability of the code for future dosimetry studies it was applied on a novel small-intestinal dosimetry model. For some radiopharmaceuticals a significant amount of activity is excreted through the gastrointestinal tract. It has also been shown that the activity may concentrate in the intestinal wall. The absorbed dose to the wall might therefore be significant and need to be considered in risk estimations relating to the radiopharmaceuticals. Since, the small intestinal wall is impossible to distinguish from the intestinal contents with modern nuclear medicine imaging; the general dosimetric models assume that all activity is located in the contents. This work is based on a model previously published to simulate S values to the radiosensitive crypt cells and villi in the small intestinal wall using Monte Carlo technique. The activity is assumed to be located in both intestinal contents and wall. The MC-program used in this work was the MCNP4c2- and MCNPX-code. The model calculates the contribution of nearby intestinal loops by assuming that the small intestine is a cylinder and that the intestinal loops build a hexagonal pattern. The work includes simulations with monoenergetic photons and electrons and the radionuclides 99mTc, 111In, 131I, 67Ga, 90Y and 211At. The results verify that for radionuclides used in nuclear medicine the cross-dose from nearby loops are essential. It is also confirmed that the fraction of cumulated activity in the intestinal wall to the contents is of great importance in order to estimate the absorbed dose. It was stated that the MIRD-, MIRDOSE3- and ICRP-model tends to overestimate the absorbed dose to the intestinal wall.