RADIATION ABSORBED DOSES TO THE WALLS OF HOLLOW ORGANS

Many radiopharmacueticals are excreted from the body through the gastr ointestinal (GI) tract. The doses to the walls of the organs involved often are very significant. As significant fractions of the administer ed activity pass through them, these organs may receive the highest do ses in the body for many radiopharmaceuticals. The absorbed dose to th ese walled organs, from activity in their contents, is typically calcu lated as 50% of the average absorbed dose to the contents, for nonpene trating emissions. The internal surface of the GI tract, and to a cert ain extent the urinary bladder, is lined with a variable thickness of mucus. In addition, the radiosensitive cell populations (crypt or stem cells) are located at some depth into the mucosa. These two factors s uggest that the surface dose, often used to characterize the clinicall y relevant absorbed doses for walled organs, may represent an overesti mate in some cases. Methods: In this study, the radiation transport co de MCNP was used to simulate the deposition of energy from nonpenetrat ing emissions of several radionuclides of interest: Y-90, Tc-99m, I-12 3 and I-131. Absorbed doses as a function of distance from the wall-co ntents interface were calculated for three geometric shapes representi ng different organs along the routes of excretion. Results: The absorb ed dose from nonpenetrating emissions to the sensitive cell population s was consistently lower than estimated by the standard model assumpti on. The simulated absorbed doses to radiosensitive cells in the GI tra ct for Tc-99m and I-123 are tenfold lower; those for I-131 are fivefol d lower and those for Y-90 are 20% lower. Conclusion: This study demon strates that the normally reported dose to the walls of hollow organs probably should be modified to account for the attenuation of these no npenetrating emissions in the linings of the walls. This study also de monstrates that Monte Carlo codes continue to be useful in the evaluat ion of the dose to sensitive cells in walled organs.