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90 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 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 In selective internal radiation therapy (SIRT), 90 Y-microsphere radioembolization is a valuable therapeutic option in patients presenting with unresectable hepatocellular carcinoma (HCC) not eligible for other therapeutic options (1-3).SIRT with 90 Y-charged microspheres relies on differential vascularization between tumor and nontumor liver parenchyma, resulting in favorable, potentially tumoricidal, deposition of microsphere activity in tumors while minimizing absorbed dose to the functional parenchyma, thus preventing toxicity. Two microsphere types are clinically available: resin spheres (SIR spheres; SirTex Medical Ltd.) and glass spheres (TheraSphere; Nordion Inc.). Despite being of similar size (;30 mm), these two types of sphere differ in specific activity, density (ffi 4 · 10 5 glass spheres/GBq; ffi 2 · 10 7 resin spheres/GBq), and injection solution (NaCl for glass spheres; water for resin spheres), leading to potential differences in embolic effect and local variations in the radiation dose deposited in tissues.Predictive dosimetry has included hepatic CT angiography for catheter positioning and partition modeling based on 99m Tcmacroaggregate albumin (MAA) SPECT/CT acquisition (4,5).The manufacturer-recommended activity for resin spheres is based on a semiempiric formula including body surface area (6,7) and tumor burden. This approach can be refined using a 3-compartment partition model (4) including the lungs, liver TVs, and liver NTVs derived from a pretreatment 99m Tc-MAA SPECT/CT scan. The prescribed glass sphere activity is based on a 2-compartment model (lungs and targeted liver regions) aiming to deliver a dose of 80-150 Gy to the targeted liver volume.90 Y time-of-flight (TOF) PET/CT dosimetry (8) provides a valuable tool to verify 99m Tc-MAA SPECT/CT-based predictive dosimetry.

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