Specific absorbed fractions and radionuclide S-values for tumors of varying size and composition

Abstract

Accurate estimates of tumor absorbed dose are essential for the evaluation of treatment efficacy in radiopharmaceutical cancer therapy. Although tumor dosimetry via the MIRD schema has been previously investigated, prior studies have been limited to the consideration of soft-tissue tumors. In the present study, specific absorbed fractions (SAFs) for monoenergetic photons, electrons, and alpha particles in tumors of varying compositions were computed using Monte Carlo simulations in MCNPX after which self-irradiation S-values for 22 radionuclides (along with 14 additional alpha-emitter progeny) were generated for tumors of both varying size and tissue composition. The tumors were modeled as spheres with radii ranging from 0.10 cm to 6.0 cm and with compositions varying from 100% soft tissue (ST) to 100% mineral bone (MB). The energies of the photons and electrons were varied on a logarithm energy grid from 10 keV to 10 MeV. The energies of alpha particles were varied along a linear energy grid from 0.5 MeV to 12 MeV. In all cases, a homogenous activity distribution was assumed throughout the tumor volume. Furthermore, to assess the effect of tumor shape, several ellipsoidal tumors of different compositions were modeled and absorbed fractions were computed for monoenergetic electrons and photons. S-values were then generated using detailed decay data from the 2008 MIRD Monograph on Radionuclide Data and Decay Schemes. Our study results demonstrate that a soft-tissue model yields relative errors of 25% and 71% in the absorbed fraction assigned to uniform sources of 1.5 MeV electrons and 100 keV photons, respectively, localized within a 1 cm diameter tumor of MB. The data further show that absorbed fractions for moderate ellipsoids can be well approximated by a spherical shape of equal mass within a relative error of < 8%. S-values for 22 radionuclides (and their daughter progeny) were computed with results demonstrating how relative errors in SAFs could propagate to relative errors in tumor dose estimates as high as 86%. A comprehensive data set of radionuclide S-values by tumor size and tissue composition is provided for application of the MIRD schema for tumor dosimetry in radiopharmaceutical therapy.

An International Study of Factors Affecting Variability of Dosimetry Calculations, Part 1: Design and Early Results of the SNMMI Dosimetry Challenge

“…The IDAC software (18), which follows International Commission on Radiological Protection publication 133 (19), accounted for 19% of submissions. Two submissions used OpenDose (20) in combination with factors published by Olguin et al (21) for the tumors. Two submissions reported using local energy deposition instead of S factors.…”

MIRD Pamphlet No. 28, Part 1: MIRDcalc—A Software Tool for Medical Internal Radiation Dosimetry

“…MIRDcalc allows selection of sphere volume, volume uncertainty, tissue composition (bone/soft-tissue mixture), radionuclide, TIAC, and TIAC uncertainty. These dose calculations are based on S values provided by Olguin et al (35). MIRDcalc implementation utilizes S-value interpolation or extrapolation of published data points for user-specified volumes.…”

Electron Absorbed Fractions and S Factors for Intermediate Size Target Volumes: Comparison of Analytic Calculations and Monte Carlo Simulations

“…In that study, the absorbed fractions in spheres for monoenergetic electrons were determined for comparison with the recommended values of Stabin and Konijnenberg [37]. These studies [37,38] are included in the more recent work of Olguin et al [39], which presented the absorbed fraction and the S factor for targets of different sizes and compositions. Specific absorbed fractions were calculated by the MCNPX code for monoenergetic photons, electrons, and α-particles in spherical and ellipsoidal tumor volumes, while S factors for 22 radionuclides were also provided.…”