Partition Model–Based 99mTc-MAA SPECT/CT Predictive Dosimetry Compared with ⁹⁰Y TOF PET/CT Posttreatment Dosimetry in Radioembolization of Hepatocellular Carcinoma: A Quantitative Agreement Comparison

Y-microsphere selective internal radiation therapy (SIRT) is a valuable treatment in unresectable hepatocellular carcinoma (HCC). Partition-model predictive dosimetry relies on differential tumor-to-nontumor perfusion evaluated on pretreatment 99m Tcmacroaggregated albumin (MAA) SPECT/CT. The aim of this study was to evaluate agreement between the predictive dosimetry of 99m Tc-MAA SPECT/CT and posttreatment dosimetry based on 90 Y time-of-flight (TOF) PET/CT. Methods: We compared the 99m Tc-MAA SPECT/CT results for 27 treatment sessions (25 HCC patients, 41 tumors) with 90 Y SIRT (7 glass spheres, 20 resin spheres) and the posttreatment 90 Y TOF PET/CT results. Three-dimensional voxelized dose maps were computed from the 99m Tc-MAA SPECT/CT and 90 Y TOF PET/CT data. Mean absorbed dose (D mean ) was evaluated to compute the predicted-to-actual dose ratio (DR mean 5 D MAA mean =D 90Y mean ) in tumor volumes (TVs) and nontumor volumes (NTVs) for glass and resin spheres. The Lin concordance (r c ) was used to measure accuracy (C b ) and precision (r). Results: Administered activity ranged from 0.8 to 1.9 GBq for glass spheres and from 0.6 to 3.4 GBq for resin spheres, and the respective TVs ranged from 2 to 125 mL and from 6 to 1,828 mL. The mean dose D 90Y mean was 240 Gy for glass and 122 Gy for resin in TVs and 72 Gy for glass and 47 Gy for resin in NTVs. DR TV mean was 1.46 0.58 (0.65-2.53) for glass and 1.16 0.41 (0.54-2.54) for resin, and the respective values for DR NTV mean were 0.88 0.15 (0.56-1.00) and 0.86 0.2 (0.58-1.35). DR variability was substantially lower in NTVs than in TVs. The Lin concordance between D MAA mean and D 90Y mean (resin) was significantly better for tumors larger than 150 mL than for tumors 150 mL or smaller (r c 5 0.93 and C b 5 0.95 vs. r c 5 0.57 and C b 5 0.93; P , 0.05). Conclusion: In 90 Y radioembolization of HCC, predictive dosimetry based on 99m Tc-MAA SPECT/CT provided good estimates of absorbed doses calculated from posttreatment 90 Y TOF PET/CT for tumor and nontumor tissues. The low variability of DR NTV demonstrates that pretreatment dosimetry is particularly suitable for minimizing radiation-induced hepatotoxicity.