| Summary: | <b>Introduction:</b> Neutron-activated samarium-153-oxide-loaded polystyrene ([<sup>153</sup>Sm]Sm<sub>2</sub>O<sub>3</sub>-PS) microspheres has been developed in previous study as a potential theranostic agent for hepatic radioembolization. In this study, the therapeutic efficacy and diagnostic imaging capabilities of the formulation was assessed using liver cancer Sprague-Dawley (SD) rat model. <b>Methods:</b> Twelve male SD rats (150–200 g) that implanted with N1-S1 hepatoma cell line orthotopically were divided into two groups (study versus control) to monitor the tumour growth along 60 days of treatment. The study group received an intra-tumoural injection of approximately 37 MBq of [<sup>153</sup>Sm]Sm<sub>2</sub>O<sub>3</sub>-PS microspheres, while control group received an intra-tumoural injection of 0.1 mL of saline solution. A clinical single photon emission computed tomography/computed tomography (SPECT/CT) system was used to scan the rats at Day 5 post-injection to investigate the diagnostic imaging capabilities of the microspheres. All rats were monitored for change in tumour volume using a portable ultrasound system throughout the study period. Histopathological examination (HPE) was performed after the rats were euthanized at Day 60. <b>Results:</b> At Day 60, no tumour was observed on the ultrasound images of all rats in the study group. In contrast, the tumour volumes in the control group were 24-fold larger compared to baseline. Statistically significant difference was observed in tumour volumes between the study and control groups (<i>p</i> < 0.05). The SPECT/CT images clearly displayed the location of [<sup>153</sup>Sm]Sm<sub>2</sub>O<sub>3</sub>-PS in the liver tumour of all rats at Day 5 post-injection. Additionally, the [<sup>153</sup>Sm]Sm<sub>2</sub>O<sub>3</sub>-PS microspheres was visible on the CT images and this has added to the benefits of <sup>153</sup>Sm as a CT contrast agent. The HPE results showed that the [<sup>153</sup>Sm]Sm<sub>2</sub>O<sub>3</sub>-PS microspheres remained concentrated at the injection site with no tumour cells observed in the study group. <b>Conclusions:</b> Neutron-activated [<sup>153</sup>Sm]Sm<sub>2</sub>O<sub>3</sub>-PS microspheres demonstrated excellent therapeutic and diagnostic imaging capabilities for theranostic treatment of liver cancer in a SD rat model. Further studies with different animal and tumour models are planned to validate this finding.
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