Impact of gadolinium concentration and cell oxygen levels on radiobiological characteristics of gadolinium neutron capture therapy technique in brain tumor treatment.

Neutron capture therapy (NCT) with various concentrations of gadolinium (¹⁵⁷Gd) is one of the treatment modalities for glioblastoma (GBM) tumors. Current study aims to evaluate how variations of ¹⁵⁷Gd concentration and cell oxygen levels can affect the relative biological effectiveness (RBE) of gadolinium neutron capture therapy (GdNCT) technique through a hybrid Monte Carlo (MC) simulation approach. At first, Snyder phantom including a spherical tumor was simulated by Geant4 MC code and relevant energy electron spectra to different ¹⁵⁷Gd concentrations including 100, 250, 500, and 1000 ppm were calculated following the neutron irradiation of simulated phantom. Scored energy electron spectra were then imported to Monte Carlo damage simulation (MCDS) code to estimate RBE values (both RBE_SSB and RBE_DSB) at different gadolinium concentrations and oxygen levels from 10 to 100%. The results indicate that variations of ¹⁵⁷Gd can affect the energy spectrum of released secondary electrons including Auger electrons. Variation of gadolinium concentration from 100 to 1000 ppm in tumor region can change RBE_SSB and RBE_DSB values by about 0.1% and 0.5%, respectively. Besides, maximum variations of 4.3% and 2% were calculated for RBE_DSB and RBE_SSB when cell oxygen level changed from 10 to 100%. From the results, variations of considered gadolinium and oxygen concentrations during GdNCT can influence RBE values. Nevertheless, due to the not remarkable changes in the intensity of Auger electrons, a slight difference in RBE values would be expected at various ¹⁵⁷Gd concentrations, although considerable RBE changes were calculated relevant to the oxygen alternations inside tumor tissue.