The superior predictive value of 166Ho-scout compared with 99mTc-macroaggregated albumin prior to 166Ho-microspheres radioembolization in patients with liver metastases

Purpose As an alternative to technetium-99m-macroaggregated albumin (Tc-99m-MAA), a scout dose of holmium-166 (Ho-166) microspheres can be used prior to Ho-166-radioembolization. The use of identical particles for pre-treatment and treatment procedures may improve the predictive value of pre-treatment analysis of distribution. The aim of this study was to analyze the agreement between Ho-166-scout and Ho-166-therapeutic dose in comparison with the agreement between Tc-99m-MAA and Ho-166-therapeutic dose. Methods Two separate scout dose procedures were performed (Tc-99m-MAA and Ho-166-scout) before treatment in 53 patients. First, qualitative assessment was performed by two blinded nuclear medicine physicians who visually rated the agreement between the Tc-99m-MAA, Ho-166-scout, and Ho-166-therapeutic dose SPECT-scans (i.e., all performed in the same patient) on a 5-point scale. Second, agreement was measured quantitatively by delineating lesions and normal liver on FDG-PET/CT. These volumes of interest (VOIs) were co-registered to the SPECT/CT images. The predicted absorbed doses (based on Tc-99m-MAA and Ho-166-scout) were compared with the actual absorbed dose on post-treatment SPECT. Results A total of 23 procedures (71 lesions, 22 patients) were included for analysis. In the qualitative analysis, Ho-166-scout was superior with a median score of 4 vs. 2.5 for Tc-99m-MAA (p < 0.001). The quantitative analysis showed significantly narrower 95%-limits of agreement for Ho-166-scout in comparison with Tc-99m-MAA when evaluating lesion absorbed dose (- 90.3 and 105.3 Gy vs. - 164.1 and 197.0 Gy, respectively). Evaluation of normal liver absorbed dose did not show difference in agreement between both scout doses and Ho-166-therapeutic dose (- 2.9 and 5.5 Gy vs - 3.6 and 4.1 Gy for Tc-99m-MAA and Ho-166-scout, respectively). Conclusions In this study, Ho-166-scout was shown to have a superior predictive value for intrahepatic distribution in comparison with Tc-99m-MAA.