4-Step Renal Dosimetry Dependent on Cortex Geometry Applied to 90Y Peptide Receptor Radiotherapy: Evaluation Using a Fillable Kidney Phantom Imaged by 90Y PET

Accurate dosimetry in Y-90 peptide receptor radionuclide therapy (PRRT) helps to optimize the injected activity, to prevent kidney or red marrow toxicity, while giving the highest absorbed dose to tumors. The aim of this study was to evaluate whether direct Y-90 bismuth germanate or lutetium yttrium orthosilicate time-of-flight PET was accurate enough to provide dosimetry estimates suitable to Y-90 PRRT. Method: To overcome the statistical uncertainty arising from the low Y-90 positron counting rate, the computation of the cortex mean-absorbed dose was divided into 4 steps: delineation of the cortex volume of interest (VOI) on the CT scan, determination of the recovery coefficient from the cortex VOI using the point-spread function of the whole imaging process, determination of the mean cortex-absorbed dose per unit cumulated activity in the cortex (S-cortex <- cortex value) from the cortex VOI using a Y-90 voxel S value kernel, and determination of the number of decays in the cortex VOI from the PET reconstruction. Our 4-step method was evaluated using an anthropomorphic abdominal phantom containing a fillable kidney phantom based on the MIRD kidney model. Vertebrae with an attenuation similar to that of bone were also modeled. Two tumors were modeled by 7-mL hollow acrylic spheres and the spleen by a plastic bag. Activities corresponded to typical tissue uptake in a first Y-90-DOTATOC cycle of 4.4 GBq, considered as free of significant renal toxicity. Eight successive 45-min scans were acquired on both systems. Results: Both PET systems were successful in determining absorbed dose to modeled tumors but failed to provide accurate red marrow dosimetry. Renal cortex dosimetry was reproducible for both PET systems, with an accuracy of 3% for the bismuth germanate system but only 18% for the lutetium yttrium orthosilicate time-of-flight system, which was hindered by the natural radioactivity of the crystal, especially in the most attenuated area of the kidney. Conclusion: This study supports the use of direct Y-90 PET of the first PRRT cycle to assess the kidney-absorbed dose and optimize the injected activity of the following cycles.