**5. Conclusions**

This study showed the potential impact of the microsphere concentration in the vial on the microsphere concentration in the blood during radioembolization. A relationship between the concentration of microspheres in the vial and the peak of the microsphere concentration in the blood was observed. Further research on the influence of injection characteristics (i.e., injection velocity and concentration of microspheres in the vial) is needed to confirm the relationship observed in this study, in addition to studying the effects of the microsphere concentration in the blood on the microsphere distribution in tumors. The results of this and future studies could be of use for planning optimal patient-specific radioembolization procedures.

**Author Contributions:** Conceptualization, J.A. and R.A.; methodology, J.A. and R.A.; formal analysis, U.L.; investigation, U.L. and J.A.; writing—original draft preparation, U.L. and J.A.; writing—review and editing, M.R.-F., B.S. and R.A.; visualization, U.L.; supervision, J.A. and R.A.; project administration, M.R.-F. and J.A.; funding acquisition, M.R.-F., J.A. and R.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Diputación Foral de Gipuzkoa (the Provincial Council of Gipuzkoa), gran<sup>t</sup> number 2021-CIEN-000076-04-01, and by the PI18/00692 project, integrated in the 2013–2016 National R&D Plan and co-financed by the ISCIII General Division for Research Evaluation and Promotion and the European Regional Development Fund. The APC was funded by the PI18/00692 project, integrated in the 2013–2016 National R&D Plan and co-financed by the ISCIII General Division for Research Evaluation and Promotion and the European Regional Development Fund.

**Data Availability Statement:** Data and codes are available on request from the authors.

**Acknowledgments:** U.L. gratefully acknowledges the financial support of Eusko Jaurlaritzako Hezkuntza Saila (Basque Government Department of Education) through the Non-Doctor Research Personnel Predoctoral Training Program.

**Conflicts of Interest:** J.A. and R.A. have received speaker honoraria from Sirtex Medical and research fees from Terumo and Sirtex Medical. M.R.-F. has received consultation fees and speaker honoraria from Sirtex Medical. B.S. has received consulting fees from BTG, Sirtex Medical and Terumo; speaker fees from Sirtex Medical and Terumo; and institutional research funding from Sirtex Medical. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
