Estimation of absorbed fraction for Ac-225 targeted alpha therapy using cortex and medulla geometry

Kidneys are sensitive to radiation and can develop radiation-induced conditions. Administered radiopharmaceuticals accumulates differently in the kidney, with varying accumulation rate in the cortex and medulla. This study aims to estimate the S-value for different kidney regions (Cortex and medulla) while considering different distribution cases in 225Ac targeted alpha therapy. A comprehensive kidney model was implemented in Gate 9.2 software to generate a 3D dose map. 225Ac (0.3 μCi) and its two daughters 213Bi, 221Fr, and lastly 177Lu with activity of 200 μCi were investigated. Emission spectrum of each source was obtained to confirm the source definition and the Radioactive decay. Dose maps were generated by uniformly distributing the activity once in the cortex or medulla, throughout cortex & medulla, throughout the entire kidney. Finally, S-values were calculated for Cortex, Medulla and whole kidney. The emission spectrums indicated that the sources were well defined. 3D dose map Results showed that the sources were posited within the targeted region and the highest levels of deposited energy were observed in regions where the sources were attached. The Medulla consistently exhibited a higher absorbed fraction (S-value) than the Cortex for all radionuclides tested, with S-value ratios of 2.8:1, 2.7:1, 2.8:1, and 2.6:1 for 225Ac, 213Bi, 221Fr, and 177Lu, respectively. Among the radionuclides studied, Fr-221 had the highest S-value of 3.71 in the whole kidney, while 225Ac, 213Bi, and 177Lu had S-values of 3.41, 0.253, and 0.083, respectively. We concluded that the absorbed fraction varies across different regions of the kidney, and that different radionuclides exhibit varying absorbed fractions. furthermore, francium-221 should be given high priority and consideration, as it had the highest absorbed fraction in the kidney. Overall, our study emphasizes that regional variations in absorbed fraction for different radionuclides should be considered.