Simplified patient-specific renal dosimetry in ¹⁷⁷Lu therapy: a proof of concept.

The aim of this proof-of-concept study is to propose a simplified personalized kidney dosimetry procedure in <sup>177</sup> Lu peptide receptor radionuclide therapy (PRRT) for neuroendocrine tumors and metastatic prostate cancer. It relies on a single quantitative SPECT/CT acquisition and multiple radiometric measurements executed with a collimated external probe, properly directed on kidneys. We conducted a phantom study involving external count-rate measurements in an abdominal phantom setup filled with activity concentrations of <sup>99m</sup> Tc, reproducing patient-relevant organ effective half-lives occurring in <sup>177</sup> Lu PRRT. GATE Monte Carlo (MC) simulations of the experiment, using <sup>99m</sup> Tc and <sup>177</sup> Lu as sources, were performed. Furthermore, we tested this method via MC on a clinical case of <sup>177</sup> Lu-DOTATATE PRRT with SPECT/CT images at three time points (2, 20 and 70 hrs), comparing a simplified kidney dosimetry, employing a single SPECT/CT and probe measurements at three time points, with the complete MC dosimetry. The experimentally estimated kidney half-life with background subtraction applied was compatible within 3% with the expected value. The MC simulations of the phantom study, both with <sup>99m</sup> Tc and <sup>177</sup> Lu, confirmed a similar level of accuracy. Concerning the clinical case, the simplified dosimetric method led to a kidney dose estimation compatible with the complete MC dosimetry within 6%, 12% and 2%, using respectively the SPECT/CT at 2, 20 and 70 hrs. The proposed simplified procedure provided a satisfactory accuracy and would reduce the imaging required to derive the kidney absorbed dose to a unique quantitative SPECT/CT, with consequent benefits in terms of clinic workflows and patient comfort