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

Purpose: The aim of this proof-of-concept study is to propose a simplified personalized kidney dosimetry procedure in Lu-177 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. Methods: We conducted a phantom study involving external count-rate measurements in an abdominal phantom setup filled with activity concentrations of (99)mTc, reproducing patient-relevant organ effective half-lives occurring in Lu-177 PRRT. GATE Monte Carlo (MC) simulations of the experiment, using (99)mTc and Lu-177 as sources, were performed. Furthermore, we tested this method via MC on a clinical case of Lu-177-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. Results: 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 (99)mTc and Lu-177, 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. Conclusions: 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.