Specific Absorbed Fractions and Radionuclide S Values for Human Spleen Microstructure for Radiopharmaceutical Dosimetry
Abstract
Purpose
Following radiopharmaceutical administration, absorbed doses can be estimated by coupling radionuclide S values with cumulated activity. For alpha-emitting radiopharmaceuticals, S values should be established at a microscale level to account for heterogeneous dose distributions within organs. To enable the accurate dose estimates to spleen, which contribute to hemotoxicity, this study aims to establish a full set of specific absorbed fractions (SAFs) and radionuclide S values using seven microscale mesh-type human spleen models constructed from reticulin-stained histology slides.
Methods
Seven polygonal-mesh models of human splenic tissue—including white pulp (germinal, mantle, marginal zones), red pulp, trabeculae, and blood vessels—were converted to tetrahedral meshes for Monte Carlo calculations. PHITS version 3.35 was used to compute SAFs for photons, electrons, and alpha particles across all source-target combinations. Marginal zones, red pulp, and blood were considered key sources due to the presence of somatostatin receptor types used in peptide-receptor radionuclide therapy with known uptake of common radioligands, while white pulp was an important target for B-cell selective uptake contributing to lymphocyte toxicity. Radionuclide S values were subsequently calculated using SAFs and radionuclide decay data.
Results
A comprehensive set of SAFs were established for 27 monoenergetic photons and electrons ranging from 1 keV to 10 MeV and 23 monoenergetic alpha particles from 1 MeV to 12 MeV. The S values were generated for 1,252 radionuclides, including all 333 radionuclides relevant to nuclear medicine. The results revealed that the alpha SAFs and S values were non-zero in cross irradiation cases, underscoring the significance of tissue-level dosimetry for short-range particles for accurate dose estimates.
Conclusion
The established SAF and S value datasets provide a robust foundation for microscale dosimetry in the human spleen, enabling more accurate prediction of hemotoxicity and supporting optimization of administered activity in radiopharmaceutical therapy.