(166)Holmium–(99)(m)Technetium dual-isotope imaging: scatter compensation and automatic healthy-liver segmentation for (166)Holmium radioembolization dosimetry

BACKGROUND: Partition modeling allows personalized activity calculation for holmium-166 ((166)Ho) radioembolization. However, it requires the definition of tumor and non-tumorous liver, by segmentation and registration of a separately acquired CT, which is time-consuming and prone to error. A protocol including (166)Ho-scout, for treatment simulation, and technetium-99m ((99m)Tc) stannous phytate for healthy-liver delineation was proposed. This study assessed the accuracy of automatic healthy-liver segmentation using (99m)Tc images derived from a phantom experiment. In addition, together with data from a patient study, the effect of different (99m)Tc activities on the (166)Ho-scout images was investigated. To reproduce a typical scout procedure, the liver compartment, including two tumors, of an anthropomorphic phantom was filled with 250 MBq of (166)Ho-chloride, with a tumor to non-tumorous liver activity concentration ratio of 10. Eight SPECT/CT scans were acquired, with varying levels of (99m)Tc added to the non-tumorous liver compartment (ranging from 25 to 126 MBq). For comparison, forty-two scans were performed in presence of only (99m)Tc from 8 to 240 MBq. (99m)Tc image quality was assessed by cold-sphere (tumor) contrast recovery coefficients. Automatic healthy-liver segmentation, obtained by thresholding (99m)Tc images, was evaluated by recovered volume and Sørensen–Dice index. The impact of (99m)Tc on (166)Ho images and the role of the downscatter correction were evaluated on phantom scans and twenty-six patients’ scans by considering the reconstructed (166)Ho count density in the healthy-liver. RESULTS: All (99m)Tc image reconstructions were found to be independent of the (166)Ho activity present during the acquisition. In addition, cold-sphere contrast recovery coefficients were independent of (99m)Tc activity. The segmented healthy-liver volume was recovered fully, independent of (99m)Tc activity as well. The reconstructed (166)Ho count density was not influenced by (99m)Tc activity, as long as an adequate downscatter correction was applied. CONCLUSION: The (99m)Tc image reconstructions of the phantom scans all performed equally well for the purpose of automatic healthy-liver segmentation, for activities down to 8 MBq. Furthermore, (99m)Tc could be injected up to at least 126 MBq without compromising (166)Ho image quality. Clinical trials The clinical study mentioned is registered with Clinicaltrials.gov (NCT02067988) on February 20, 2014.