Evaluation of initial guess values of curve parameter for time-activity curve (TAC) fitting: time-integrated activity coefficient (TIAC) calculation

Purpose: The purpose of this study was to evaluate initial guess values of curve parameter for time-activity curve (TAC) fitting and time-integrated activity coefficient (TIAC) calculations. Methods: This study employed the renal biokinetic data of the patient who underwent radiopharmaceutical therapy using 177Lu-DOTATATE obtained from PMID33443063. The data of activity and time points (i.e., 3.5, 24.8, 102.9, and 152.8 h) were plotted using mono-exponential and bi-exponential fit functions. There were two approaches for determining the initial guess value, i.e., automatic (using the curve stripping method) and manual (initial values were varied from 10 times smaller up to 100 times larger than the result of automatic value). The goodness of fit (GoF) results was evaluated, including by visual inspection of a fitted curve passing through data points and the coefficient of determination (R2). Results: The GoF evaluation showed that both mono-exponential and bi-exponential fit functions meet the quality control criteria with visual inspection, and R2 value closed to 1. The TIAC calculation results using the automatic initial value showed good agreement with relative difference (RD) < 2% compared to the SAAM II results. TIAC calculation results using manual and automatic initial values showed the same value for right kidney and left kidney 1.41 ± 0.04 h and 1.28 ± 0.03 h, respectively, except for initial values of 100 times larger than the automatic value. Conclusion: Determining a good initial value will result in a reliable TIAC value calculation and improve internal dosimetry studies for safety patient management during radiopharmaceutical therapy.