Alpha and beta emitters dose to bone and marrow using a trabecular bone model for all ages
To simulate the trabecular bone, two models were created using the Monte Carlo code MCNP, first to represent static trabecular bone and second to represent the dynamic remodeling of trabecular bone. These codes are used to simulate the change in absorbed fraction (AF) and dose caused by the increased target size using the static model with alpha emitters ranging in energy from 0.5 to 10 MeV and four beta emitters (3H, 14C, 153Sm, and 89Sr) from four different radiation sources (bone volume, bone surface, hematopoietic marrow, and fat). The static model is used to simulate different age groups (0, 1, 5, 10 years old, and adult) to compare AFs and doses between age groups, and to calculate marrow toxicity from various radioisotopes assuming uniform distribution of those radionuclides in bone surface or volume. The dynamic model is used to find a potentially better target for bone cancer and, also, to calculate the marrow toxicity from alpha-emitter 223Ra compared to beta-emitters 153Sm and 89Sr, radionuclides which are employed to treat bone metastases or bone pain. With the bone surface as the source, lower energy particles deposited more energy in the 50 µm peripheral marrow target in the younger ages, but as energy increased the particles begin to escape the target area and deposit deeper in the bone. The same can be said about the dose to the entire hematopoietic marrow at low energies. However, at higher energies the fat proportion becomes more important resulting in the dose to increase for newborn and 1 year olds due to their low fat content in marrow. With bone volume as the source, the dose to the 50 µm Endosteum increases dramatically in the newborn relative to the adult (4.5-fold) at low energies and approaches one as the alpha energy increases. The same is true for each age group. The dose to hematopoietic marrow again varies with the fat content in marrow with a higher dose at low energies relative to the adult, but as energy increases more absorption is seen in the fat cells for the older ages. This increase in absorption leads to newborn levels around 2.5 times higher than the adult model. With hematopoietic marrow as the source the lower energy particles deposit the majority of their energy in the hematopoietic marrow for each age group, but as energy increases the proportion of fat begins to absorb some of dose as can be seen comparing newborn to adult, where the ratio is 1.35. By changing the bone cancer target from a 10 µm to a 50 µm thick layer in marrow the dose for each age group decreases by a factor of 2-5 from bone volume or surface to the target for bone seeking alpha- and beta- emitters. However, for marrow seeking alpha- and beta- emitters the dose increased by a factor of 1-1.65 across each age group. The dynamic model simulates the final phase of the bone remodeling process that lasts 135 days in adults that is divided into 7 sub-stages (19.3 days each). The dynamic model shows a much higher AF in the BRC stem cell niche from the alpha-emitting 223Ra than the beta-emitting 153Sm and 89Sr. The dynamic model also shows a much higher absorption in the bone remodeling compartment (BRC) stem cell niche than the 50µm target region from the 223Ra compared to 153Sm and 89Sr ranging from 5.65 to 24.38 compared to 0.67 to 1.75 throughout the entire remodeling process. The absorbed fractions in the static model also show that 223Ra and its daughters have a much lower absorption in marrow than the two beta emitters commonly used to treat bone metastasis. The highest AF that was found in adults from 223Ra and its daughters was 0.121 for 215Po while the AFs for 153Sm and 89Sr are 0.438 and 0.384, respectively. These ratios are nearly constant for each age group. These results show that 223Ra has much lower marrow toxicity than the widely used beta emitters. Also, when comparing the absorption in the 50 µm Endosteum, the AF for 223Ra is 0.120 and for 153Sm and 89Sr are 0.156 and 0.114, respectively, however for younger ages the beta emitters absorption decreases and alpha emitters increases so for a new born the AFs 223Ra is 0.197 and for 153Sm and 89Sr are 0.127 and 0.059, respectively. (Abstract shortened by UMI.)
Nie, Purdue University.
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