Beyond the MAA-Y90 Paradigm: The Evolution of Radioembolization Dosimetry Approaches and Scout Particles

Abstract

Radioembolization is a well-established treatment for primary and metastatic liver cancer. There is increasing interest in personalized treatment planning supported by dosimetry, as it provides an opportunity to optimize dose delivery to tumor and minimize nontarget deposition, which demonstrably increases the efficacy and safety of this therapy. However, the optimal dosimetry procedure in the radioembolization setting is still evolving; existing data are limited as few trials have prospectively tailored dose based on personalized planning and predominantly semi-empirical methods are used for dose calculation. Since the pretreatment or "scout" procedure forms the basis of dosimetry calculations, an accurate and reliable technique is essential. ^99m Tc-MAA SPECT constitutes the current accepted standard for pretreatment imaging; however, inconsistent patterns in published data raise the question whether this is the optimal agent. Alternative particles are now being introduced to the market, and early indications suggest use of an identical scout and treatment particle may be superior to the current standard. This review will undertake an evaluation of the increasingly refined dosimetric methods driving radioembolization practices, and a horizon scanning exercise identifying alternative scout particle solutions. Together these constitute a compelling vision for future treatment planning methods that prioritize individualized care.

Keywords: Y90; novel techniques; positron emission tomography; radioembolization; single-photon emission computed tomography; technetium-MAA.

Fig. 1

BSA dosimetry calculation method. The equation for activity calculation is equivalent to that presented in Giammarile et al.

Fig. 2

MIRD single-compartment dosimetry calculation method. The factors of 50 (J/GBq) and 15.9 (J/GBq) account for the energy deposited in the region over time per activity injected for ⁹⁰ Y and ¹⁶⁶ Ho, respectively. The equations for absorbed dose have been adapted from those presented by Gulec et al and are the formalism used in Simplicit ⁹⁰ Y (Mirada medical Ltd, Oxford, UK).

Fig. 3

MIRD multi-compartment dosimetry calculation method. The factors of 50 (J/GBq) and 15.9 (J/GBq) account for the energy deposited in the region over time per activity injected for ⁹⁰ Y and ¹⁶⁶ Ho, respectively. The equations for absorbed dose have been adapted from those presented by Gulec et al and are the formalism used in Simplicit ⁹⁰ Y (Mirada medical Ltd). (Printed with permission from Hermann A, Dieudonné A, Ronot M, et al. Relationship of tumor radiation–absorbed dose to survival and response in hepatocellular carcinoma treated with transarterial Radioembolization with ⁹⁰ Y in the SARAH Study. Radiology 2020;296:673–684).

Fig. 4

MIRD voxel-wise dosimetry calculation method. The factors of 50 (J/GBq) and 15.9 (J/GBq) account for the energy deposited in the region over time per activity injected for ⁹⁰ Y and ¹⁶⁶ Ho, respectively. The equations for absorbed dose have been adapted from those presented by Gulec et al and are the formalism used in Simplicit ⁹⁰ Y (Mirada medical Ltd). This formalism assumes application of local deposition model.

Fig. 5

Kaplan–Meier curves for overall survival for participants with optimal agreement. This research was initially published in Radiology by Hermann et al. (Copyright by CC BY 4.0.)

Fig. 6

MAA particles ( a ) and TheraSphere (Boston Scientific, Marlborough, MA) microspheres ( b ) as viewed under a microscope. It is evident that the morphology differences between the two particles are significant. (Images provided by Boston Scientific.)

Fig. 7

( a ) A patient with diffuse uptake, and multiple small lesions. Registration of the baseline diagnostic dataset and the ^99m Tc MAA is difficult and therefore the alignment is suboptimal. ( b ) A patient with a large, hypervascular lesion. There is less random activity distribution and it is inherently easier to register the datasets; therefore, the alignment is optimal.

Fig. 8

¹⁶⁶ Ho microspheres as viewed under a scanning electron microscope. This research was initially published in JECCR by Lin and Alessio. (Copyright by CC BY 2.0.)