The effect of tumor subregions in the evaluation of electron absorbed fraction and S factor.

Abstract

Absorbed fractions and S factors are essential inputs in MIRD-based dosimetry for radiopharmaceutical therapy (RPT). Their evaluation for six therapeutic β-emitting radionuclides (Y-90, I-131, Lu-177, Sr-89, Sm-153, and Re-186), as well as for monoenergetic electrons over the energy range of interest to RPT (1 keV - 4 MeV) is presented for intermediate-size tumors (∼mm), considering more realistic radionuclide distributions that explicitly account for oxygenated and necrotic tumor subregions. Calculations are performed using both the semi-analytic dose point kernel (DPK) approach and Monte Carlo (MC) simulations with the Geant4 toolkit. The results show that the common assumption of uniform activity distribution in the tumor volume can lead to substantial deviations (27-87%) from more realistic distributions, whereby tumor subregions are considered. The use of the mean β energy instead of the full emission spectrum introduces additional discrepancies, which become even more significant (up to 50-100%) in the examined cases. The comparison between the DPK-based calculations and the Geant4 simulations reveals differences up to 60%, primarily when the electron range is comparable to the characteristic dimensions of the tumor subregions. Although the choice of common cut-off energy parameters in the Geant4 simulations has a limited influence in the case of uniform activity distribution, it may introduce sizeable discrepancies (up to 23%) in the more realistic cases examined. The present findings underscore the importance of incorporating oxygenated and necrotic tumor subregions in the dosimetry of intermediate-size tumors along with appropriate modeling of electron transport.

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