Quantitative (166)Ho-microspheres SPECT derived from a dual-isotope acquisition with (99m)Tc-colloid is clinically feasible

PURPOSE: Accurate dosimetry is essential in radioembolization. To this purpose, an automatic protocol for healthy liver dosimetry based on dual isotope (DI) SPECT imaging, combining holmium-166 ((166)Ho)-microspheres, and technetium-99 m ((99m)Tc)-colloid was developed: (166)Ho-microspheres used as scout and therapeutic particles, and (99m)Tc-colloid to identify the healthy liver. DI SPECT allows for an automatic and accurate estimation of absorbed doses, introducing true personalized dosimetry. However, photon crosstalk between isotopes can compromise image quality. This study investigates the effect of (99m)Tc downscatter on (166)Ho dosimetry, by comparing (166)Ho-SPECT reconstructions of patient scans acquired before ((166)Ho-only) and after additional administration of (99m)Tc-colloid ((166)Ho-DI). METHODS: The (166)Ho-only and (166)Ho-DI scans were performed in short succession by injecting (99m)Tc-colloid on the scanner table. To compensate for (99m)Tc downscatter, its influence was accounted for in the DI image reconstruction using energy window-based scatter correction methods. The qualitative assessment was performed by independent blinded comparison by two nuclear medicine physicians assessing 65 pairs of SPECT/CT. Inter-observer agreement was tested by Cohen’s kappa coefficient. For the quantitative analysis, two volumes of interest within the liver, VOI(TUMOR), and VOI(HEALTHY) were manually delineated on the (166)Ho-only reconstruction and transferred to the co-registered (166)Ho-DI reconstruction. Absorbed dose within the resulting VOIs, and in the lungs (VOI(LUNGS)), was calculated based on the administered therapeutic activity. RESULTS: The qualitative assessment showed no distinct clinical preference for either (166)Ho-only or (166)Ho-DI SPECT (kappa = 0.093). Quantitative analysis indicated that the mean absorbed dose difference between (166)Ho-DI and (166)Ho-only was − 2.00 ± 2.84 Gy (median 27 Gy; p value < 0.00001), − 5.27 ± 8.99 Gy (median 116 Gy; p value = 0.00035), and 0.80 ± 1.08 Gy (median 3 Gy; p value < 0.00001) for VOI(HEALTHY,) VOI(TUMOR,) and VOI(LUNGS), respectively. The corresponding Pearson’s correlation coefficient between (166)Ho-only and (166)Ho-DI for absorbed dose was 0.97, 0.99, and 0.82, respectively. CONCLUSION: The DI protocol enables automatic dosimetry with undiminished image quality and accuracy. CLINICAL TRIALS: The clinical study mentioned is registered with Clinicaltrials.gov (NCT02067988) on 20 February 2014.