An In-House 3D Voxel Dosimetric Tool to Compare Predictive and Post- Treatment Dosimetry in 90Y Radioembolization: A Proof of Concept.

Aim: The aim of this study was to implement an in-house dosimetric tool to assess tumour- absorbed doses in pre and post-dosimetry for ⁹⁰Y radioembolization with resin spheres.

Materials and methods: To perform dosimetric calculations we set up a dosimetric procedure and developed homemade software to calculate tumour absorbed dose and dose volume histograms (DVHs). The method is based on a simplified voxel dosimetry for an estimated 3D absorbed dose and it can be applied to both ^99mTc-MAA SPECT/CT and ⁹⁰Y PET/CT acquisitions for pre and post-dosimetry. We tested the software performance in a retrospective study using the data of 22 patients with hepatocellular carcinoma who underwent radioembolization with ⁹⁰Y resin spheres in the period 2016-2021. The software calculates tumour doses (mean, minimum and maximum doses) from voxel counts and dose-volume histograms (DVH_spect, DVH_pet) for both ^99mTc-MAA SPECT/CT and ⁹⁰Y PET/CT imaging. DVH_spect and DVH_pet data were analyzed and compared with the aim to assess an agreement between them. Concordance between dosimetric data were evaluated with the Wilcoxon Signed Ranked test, descriptive statistical analysis and Pearson correlation coefficient.

Results: The mean administrated activity was 1313 MBq (range 444 MBq - 2200 MBq). Tumour volumes ranged from 75 mL to 1012 mL. The mean absorbed dose for tumour volume was 161 ± 66 Gy (Dm_spect) and 173 ± 79 Gy (Dm_pet). From Wilcoxon Signed Rank Test the differences between the dosimetric data extrapolated from DVH_spect and DVH_pet results were not significant with α = 0.05 (two-sided test). A good linear correlation was found between ^99mTc-MAA and ⁹⁰Y dosimetric data (Pearson correlation coefficient 0.887 p < 0.001). Generally, DVHs calculated on ^99mTc-MAA SPECT/CT and ⁹⁰Y PET/CT gave comparable results, some discrepancies were observed particularly with those patients where SPECT and PET imaging presented a visual mismatching.

Conclusion: A simplified 3D dosimetry methodology was implemented and tested retrospectively on patient data treated with ⁹⁰Y resin spheres. Even if the clinical feasibility of our approach has to be further validated on an extended patient cohort, the preliminary results of our study highlight the potential of the implemented dosimetric tool for tumour dose assessment.