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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 .

Quantitative 166Ho-microspheres SPECT derived from a dual-isotope acquisition with 99mTc-colloid is clinically feasible