A Dosimetry Model for the Small Intestine Incorporating Intestinal Wall Activity and Cross-Doses
Current internal radiation dosimetry models for the small intestine, and for most walled organs, lack the ability to account for the activity uptake in the intestinal wall. In existing models the cross-dose from nearby loops of the small intestine is not taken into consideration. The aim of this investigation was to develop a general model for calculating the absorbed dose to the radiation-sensitive cells in the small intestinal mucosa from radionuclides located in the small intestinal wall or contents. Methods: A model was developed for calculation of the self-dose and cross-dose from activity in the intestinal wall or contents. The small intestine was modeled as a cylinder with 2 different wall thicknesses and with an infinite length. Calculations were performed for various mucus thicknesses. S values were calculated using the EGS4 Monte Carlo simulation package with the PRESTA algorithm and the simulation results were integrated over the depth of the radiosensitive cells. The cross-organ dose was calculated by summing the dose contributions from other intestinal segments. Calculations of S values for self-dose and cross-dose were made for monoenergetic electrons, 0.050-10 MeV, and for the radionuclides 99mTc, 111In, 131I, 67Ga, 90Y, and 211At. Results: The self-dose S value from activity located in the small intestinal wall is considerably greater than the S values for self-dose from the contents and the cross-dose from wall and contents except for high electron energies. For all radionuclides investigated and for electrons 0.10-0.20 MeV and 8-10 MeV in energy, the cross-dose from activity in the contents is higher than the self-dose from the contents. The mucus thickness affects the S value when the activity is located in the contents. Conclusion: A dosimetric model for the small intestine was developed that takes into consideration the localization of the radiopharmaceutical in the intestinal wall or in the contents. It also calculates the contribution from self-dose and cross-dose. With this model, more accurate calculations of absorbed dose to radiation-sensitive cells in the intestine are possible.