Dosimetry Calculations on a Tissue Level by Using the MCNP4c2 Monte Carlo Code.

Objective: The aim of this study was to develop a MCNP4c2-code and to further refine the small-scale anatomy intestinal dosimetry model based on a EGS4-code developed by Jnsson et al.1,2Method: The small intestine was modeled as a hexagonal tube system and includes cross-dose contribution from activity in nearby intestine loops. The model includes villi (height, 500 m), radiosensitive crypt cells (height, 150 m), and an overlying mucus layer of thicknesses (5200 m). The developed intestinal model used in either of the two Monte Carlo codes make it possible to calculate S-values and subsequent mean absorbed dose to the radiation-sensitive crypt cells in the small intestinal wall by considering contributions from the self-dose and from the cross-dose from nearby intestinal loops. Results are given for monoenergetic electrons and photons and for full decay schemes of 99mTc, 111In, 131I, 67Ga, 90Y, and 211At. Results: Results show that the cross-dose from nearby intestinal loops is significant, and that the fraction of cumulated activity in the intestinal wall contents is important for accurate absorbed-dose estimation. Conclusion: It is evident from our study that previous Medical Internal Radiation Dose (MIRD) and International Conference on Radiological Protection (ICRP) models tend to overestimate the absorbed dose to the wall. Our work on the gastrointestinal tract model includes several noticeable refinements, as compared to the MIRD- and ICRP model, and the "onion shell" geometry can easily be transferred to similar geometrical dosimetry applications. [ABSTRACT FROM AUTHOR]