The Effects of Intra-Individual Variability Setting on the Accuracy of Time-Integrated Activity Calculations
Fira Dwi Ananda (a), Assyifa Rahman Hakim (a), Rien Ritawidya (b), Deni Hardiansyah (a*)

a) Medical Physics and Biophysics, Physics Department, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Depok, Indonesia
*denihardiansyah[at]ui.ac.id
b) Research Center for Radioisotope Technology, Radiopharmaceuticals, and Biodosimetry, Nuclear Power Research Organization, National Research and Innovation Agency, South Tangerang, Indonesia

Abstract

Background: Accurate estimation of kidney time-integrated activity (TIA) is essential for treatment planning in peptide receptor radionuclide therapy (PRRT). In the context of pharmacokinetic modeling, the intraindividual variability (IAV) setting-associated with measurement uncertainty-can influence TIA estimates. This study investigates the impact of varying IAV settings on TIA calculation.
Methods: Kidney biokinetic data following [177Lu]Lu-DOTATATE administration were obtained from 10 patients with neuroendocrine tumors using serial SPECT/CT imaging. A Nonlinear Mixed-Effects (NLME) model was used for TIA estimation. In Method 1, reference TIA (rTIA) was calculated by estimating both interindividual variability and IAV. In Method 2, IAV was fixed at half (hTIA) and twice (tTIA) the value obtained in Method 1. The influence of altered IAV on TIA accuracy was evaluated by comparing hTIA and tTIA against rTIA using relative deviation (RD), root-mean-square error (RMSE), and mean absolute percentage error (MAPE).
Results: Fixing IAV at half the reference value resulted in RMSE and MAPE of 4% and 3%, respectively. Furthermore, doubling the IAV led to an RMSE of 13% and MAPE of 10%.
Conclusion: Modifying the IAV setting had a measurable impact on TIA accuracy when doubled, but a negligible effect when halved in our population biokinetic data.

Keywords: PRRT, NLME, Intraindividual variability (IAV)

Topic: Medical Physics and Biophysics