PET-CT post therapy dosimetry in radioembolization with resin 90Y microspheres: Comparison with pre-treatment SPECT-CT 99mTc-MAA results

Resin microspheres radioembolization is an effective treatment for liver tumors when the surgical option is not feasible. Doses delivered to tumor and normal liver can be assess in the pre-therapy phase by means of a Tc-99m-MAA SPECT-CT simulation and after the treatment with Y-90 PET-CT acquisition. The optimal therapeutic Y-90 activity is determined on Tc-99m-MAA SPECT-CT dose results in order to avoid healthy parenchyma toxicity and to effectively irradiate the tumor. The assumption of identical radiopharmaceutical distribution between simulation and verification is still under debate and literature data showed controversial results. In this study 10 HCC patient's dosimetry performed on Tc-99m SPECT-CT and Y-90 PET-CT were compared. Patients were selected when a good agreement between the pre and post-therapy distribution was observed in order to investigate the intrinsic dosimetric variations between the two imaging modalities. Mean doses (MIRD and Voxel approaches) showed a good correlation (Pearson's coefficient r > 0.90) both for tumor and normal liver. Dose Volume Histogram curves were compared with a good agreement particularly for normal liver (D50). Goal doses were achieved for 90% of patients. Bland-Altman analysis indicates lower variations for healthy parenchyma than for tumor (1.96 SD equal to 9.1 Gy and 68 Gy respectively) confirming the robustness of the dose-toxicity approach. PET-CT dosimetry well correlates with SPECT-CT doses (under assumption of same catheter position and Y-90 activity). Better agreement was showed for 7/10 and 8/10 patients for T and NL respectively, confirming dosimetry as effective tool to optimize and individualize the treatment.