• dosimetry
• hepatocellular carcinoma (HCC)
• holmium-166 microspheres
• liver
• liver metastasis
• selective internal radiation therapy
• transarterial radioembolization
• tumor-absorbed dose

Purpose Transarterial radioembolization (TARE) with Holmium-166 poly(L-lactic acid)-loaded microspheres (Ho-PLLA-MS) is a promising technique for the treatment of both primary and secondary liver cancers. Thanks to his paramagnetic properties, it is possible to depict the deposition of holmium particles through MRI imaging. The purpose of our study is to compare the radiation absorbed dose both in liver and treated tumor calculated through the software for 166Ho (Q-suite™ 2.1) of MR images acquired before and after treatment, with the standard SPECT dose calculation.
Materials and Methods We retrospectively analyzed the data of patients who were treated with 166Ho-radioembolization in our Institution between January 2018 and September 2022. Only patients with both pre- and post-treatment MRI were included. Axial MERGE (Multiple Echo Recombined Gradient Echo) sequences were acquired. Post-treatment dosimetry was assessed by SPECT-CT performed before patients’ discharge. MR images were used to calculate liver- and tumor-absorbed dose using Q-suite™ 2.1 software. The paired Student’s t-test (two tailed) and the Pearson’s correlation were used to compare the means of liver- and tumor-absorbed dose by SPECT and MRI; the correlation coefficient (r) was indicated. Analyses were performed using SPSS statistical software, version 28. A P value of less than 0.05 was considered statistically significant.
Results Eleven patients were included (9 male and 2 females; mean age 71 years) with hepatocellular carcinoma (HCC; n=6) intrahepatic cholangiocarcinoma (ICC, n=2), and metastatic lesions (n=3). Mean absorbed dose in liver calculated with SPECT and MR by using the Q-Suite™ software were 31.09 Gy and 27.45 Gy respectively, with a good agreement between the two methods (r=0.893, p<0.001). The mean absorbed dose in tumors calculated with SPECT and MR by using Q-Suite™ software were 163.72 Gy and 112.72 Gy respectively, lower at MRI in comparison to SPECT (r=0.922, p<0.001), particularly when higher doses were delivered.
Conclusions Our small series shows promising results for the use of MRI in the evaluation of the distribution of holmium particles and dose deposition in tissues, nevertheless larger studies are needed to overcome limits of MRI quantification.