Calculation of absorbed energy in the gastrointestinal tract.

One goal of this research was to reproduce the photon specific absorbed fraction calculations of Cristy and Eckerman using their gastrointestinal (GI) tract model. A second goal was to calculate photon specific absorbed fraction values for their GI tract model using electron tracking techniques. A final goal was to calculate electron absorbed fraction values for their GI tract model. This paper summarizes the work performed using the currently accepted model of the GI tract provided by Cristy and Eckerman. Their model was coded into the Electron Gamma Shower 4 (EGS4) computational package for calculation of photon specific absorbed fraction values. To benchmark the initial code, the EGS4 program was run so that all secondary particles deposited their energy at the site of the primary photon interaction (i.e., without electron tracking). The results obtained from these preliminary calculations were compared to those provided by Cristy and Eckerman to verify and benchmark the program. Next, specific absorbed fraction values were calculated for twelve discrete photon energies using the electron tracking capabilities of EGS4. These photon specific absorbed fraction values were compared to those calculated without electron tracking. Finally, absorbed fraction values were calculated for twelve discrete electron energies. The electron absorbed fraction values were compared to those calculated without electron tracking. Finally, absorbed fraction values were calculated for twelve discrete electron energies. The electron absorbed fraction values were compared to the ICRP "one-half assumption" for electron energy deposition in the wall of the GI tract.

A revised model for the calculation of absorbed energy in the gastrointestinal tract.

The goal of this research was to develop a more complete gastrointestinal (GI) tract model for use in internal dose assessment. This paper summarizes the development of a revised mathematical model of the GI tract. The current GI tract model assumes the wall can be represented as a single soft tissue layer without regard to the radiosensitivity of the cells. The goal of the GI tract revision was to develop geometric regions that separate the radiosensitive cells from the less radiosensitive cells. Once the model was revised, it was coded into the Electron Gamma Shower 4 (EGS4) computational package for calculation of photon and electron absorbed fraction values. Photon absorbed fraction values were calculated for twelve discrete energies. For the photon absorbed fraction calculations, the EGS4 program was run so that secondary particles created in photon interactions were followed using the electron tracking capabilities of EGS4. The results of the photon absorbed fraction calculations provided better estimates of the energy deposited in the radiosensitive cells when the target organ was the source. In cases where the target organ was not the source, the photon absorbed fraction values did not provide better estimates than those obtained using the current GI tract model. An increase in the number of photon histories should provide better estimates of the photon absorbed fraction for these cases. Electron absorbed fraction values also were calculated for twelve discrete electron energies. The results of these calculations provided the expected pattern of energy deposition and better estimates than those currently available. The annual limit on intake was recalculated for a single radionuclide to demonstrate the affect of these improved absorbed fraction values on internal dose assessment. The radionuclide was selected for two reasons: 1) it was a beta emitting radionuclide; and 2) the annual limit on intake for ingestion was based on the non-stochastic committed dose equivalent limit to the lower large intestine. The calculated annual limit on intake was found to be three times greater than the annual limit on intake provided in ICRP Publication 30. There are many radionuclides that have a section of the GI tract as the limiting organ for ingestion. It is expected that the annual limit on intake value for these radionuclides would increase when the revised GI tract model is employed for internal dose assessment.