Calculation of radiation dose at a bone-to-marrow interface using Monte Carlo modeling techniques (EGS4)

Radiation dose rate profiles at a bone-to-marrow interface were calculated by simulating a uniform radiation source at the center of the endosteal layer in a long bone. Isotopes (153Sm, 186Re, and 166Ho) were assumed to assimilate as surface agents and the dose profiles were calculated on a microscopic scale using the Electron-Gamma Shower (EGS4) computer program. We validated our computational model against published dose factors (delta) for uniform volume distributed sources replicating them to an accuracy of better than 95%. The calculated dose distributions illustrate the relative contribution of atomic electrons, beta, and photon fractions. The backscatter contribution to marrow dose increased from 3% to 4% at the source to 6% to 8% at a marrow depth of 100 microns. Backscattered dose fraction was not significantly different among the three isotopes. The dose contribution from the three isotopes was remarkably similar at ranges between 25 and 125 microns.