177Lu-based radioligand therapy: A retrospective multicenter analysis to calculate the effective half-life and follow-up dose for the public

Abstract The study investigated routinely used radiopharmaceuticals containing lutetium-177 (177Lu), providing reference values for their effective half-life (T1/2,eff). So far, no guidelines regarding discharging this patients exist, are essential to ensure radiation protection of the public. This study contributes to the development of binding discharge criteria regarding 177Lu therapies. This retrospective multicenter study comprised eight nuclear medicine departments. Two commercially available products and two in-house preparations were considered. Radiation measurement methods, in terms of measuring device, setup, and time points were reported, as well as dose rate measurements for T1/2,eff calculation. The study includes 210 dose rate measurement sets with a mean administered activity of (7149 ± 522) MBq. When only measurements up to 48 h p.i. were taken into account the T1/2, eff were shorter (Pluvicto 1.30 d, Lutathera 1.40 d, ihPSMA 1.45 d, ihRRT 1.97 d) compared to 168 h (1.60 d, 1.89 d, 2.10 d, and 2.85 d, respectively). Differences were statistically significant for all compounds except Lutathera. The 90th percentile of the T1/2,eff observed in late measurements (Pluvicto 2.0 d, Lutathera 3.0 d, ihPSMA 3.5 d, ihRRT 4.0 d) can be considered conservative values for discharge calculations. The T1/2,eff of 177Lu is considerably shorter than its T1/2,phy, owing to the pharmacokinetics of the entire molecule. This highlights the need of considering biological clearance mechanisms into calculations of discharge times for patients treated with 177Lu. If direct measurements are not feasible in clinical routine, reference values support inpatient planning while ensuring radiation protection.