The effect of detailing the trabecular structure of bone phantoms on the assessment of the bone marrow dose from 89,90Sr

Abstract

Today there exist two main approaches to developing computational phantoms for bone dosimetry. The first approach is based on a detailed description of the microarchitecture of the spongiosa filling the phantoms. This microarchitecture includes trabeculae and bone marrow separately, i.e., the source tissue and the detector tissue are separated. The second approach involves generating a homogeneous bone where the target and source tissues are combined. In both cases the simulation results are conversion factors that allow converting the specific activity of incorporated radionuclides into the absorbed dose in the bone marrow. For dosimetry of the Techa River population exposed due to incorporated 89,90Sr, the skeletal phantoms were created for people of different sex and age, starting with a newborn. These phantoms included a detailed description of the trabecular bone microstructure, i.e., they belong to the first approach. Also, phantoms of the skeleton of the fetus and pregnant woman at various gestation stages have been developed, which involves modeling the bone as a homogeneous medium. These phantoms are designed for dosimetry of external and internal exposure, including 89,90Sr dosimetry. The usage of two fundamentally different approaches to bone dosimetry for the pre- and postnatal period raises the issue of compatibility of these approaches and possibility of their combining within a single dosimetric system. Objective: to evaluate the effect of detailing the trabecular structure of bone phantoms on the evaluation of conversion factors of bone marrow exposure due to 89,90Sr. Computational phantoms of eight regions of a newborn’s skeleton filled in with trabecular bone were generated. For each bone region two phantoms were generated: one phantom with a detailed description of the spongiosa microstructure and one phantom with spongiosa modeled as a homogeneous media. For all phantoms, the radiation transport from 89,90Sr incorporated in the source tissue was simulated using the MCNP 6.2 code, and the values of conversion factors were calculated. As a result, 16 conversion factors were obtained for all phantoms. On the average the conversion factors obtained for phantoms with homogeneous spongiosa exceed those for phantoms with a detailed description of the spongiosa microstructure by 2.4 times. Such significant difference between the results makes it possible to conclude that the detailing description of trabecular structure of bone phantoms has a significant impact on the assessment of the bone marrow dose due to incorporated 89,90Sr.