Radiation depth dosimetry models of 5-spheres with volumes between 0.4 to 250 ml

Objectives This study is designed to evaluate 21 radionuclides with therapeutic potentials (+ Tc99m as reference) using Monte Carlo simulations of photons and beta emissions inside spheres to simulate most situations of clinical relevance.

Methods Depth dose up to 15 cm around 0.4cc, 2cc, 10cc, 50cc, and 250cc spheres were estimated for Rb-82, Tc-99m, F-18, Ga-68, In-111, Ga-67, Tl-201, Re-188, Sm-153, Ho-166, Re-186, I-131, Y-90, P-32, Sr-89, I-125, I-124, Cu-62, Cu-64, Cu-61, Ga-66 and Lu-177. Absorbed fractions were derived from Monte Carlo simulation of the radiation transport in models of spherical sources and surrounding tissue defined in the Cristy Eckerman phantom. Concentric spherical target shells were modeled starting outwards from the outside surface of the spheres. For photon simulations, sufficient numbers of histories were run such that relative errors were < 5%. For betatron simulations sufficient numbers of histories were run such that relative errors were <10% for all shells. Non-linear regressions were performed using SigmaPlot 11 to predict normalized S-values (cGy/(mCi/ml)/Hr) and depth fractions.

Results A stable predictable relationship is noted between normalized S-values and volumes of spheres. Stable and predictable relationships are also found between depth dose fractions versus distances from spheres and volumes of spheres.

Conclusions Results from this study allow prediction of radiation dosimetry inside and outside of radioactive spherical sources. The requsite parameters are the radioactivity, the volume of the source and distance from the sphere.

Research Support US DOD BCRP DAMD17-03-1-0455