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Background: Transarterial radioembolization (TARE) is a treatment modality for liver tumors during which radioactive microspheres are injected into the hepatic arterial system. These microspheres distribute throughout the liver as a result of the blood flow until they are trapped in the arterioles because of their size. Holmium-166 (166Ho)-loaded microspheres used for TARE can be visualized and quantified with MRI, as holmium is a paramagnetic metal and locally increases the transverse relaxation rate R*2. The current 166Ho quantification method does not take regional differences in baseline R*2 values (such as between tumors and healthy tissue) into account, which intrinsically results in a systematic error in the estimated absorbed dose distribution. As this estimated absorbed dose distribution can be used to predict response to treatment of tumors and potential toxicity in healthy tissue, a high accuracy of absorbed dose estimation is required. Purpose: To evaluate pre-existing differences in R*2 distributions between tumor tissue and healthy tissue and assess the feasibility and accuracy of voxelwise subtraction-based ΔR*2 calculation for MRI-based dosimetry of holmium-166 transarterial radioembolization (166Ho TARE). Methods: MRI data obtained in six patients who underwent 166Ho TARE of the liver as part of a clinical study was retrospectively evaluated. Pretreatment differences in R*2 distributions between tumor tissue and healthy tissue were characterized. Same-day pre- and post-treatment R*2 maps were aligned using a deformable registration algorithm and subsequently subtracted to generate voxelwise R*2 maps and resultant absorbed dose maps. Image registration accuracy was quantified using the dice similarity coefficient (DSC), relative overlay (RO), and surface dice (=4 mm; SDSC). Voxelwise subtraction-based absorbed dose maps were quantitatively (root-mean-square error, RMSE) and visually compared to the current MRI-based mean subtraction method and routinely used SPECT-based dosimetry. Results: Pretreatment R*2 values were lower in tumors than in healthy liver tissue (mean 36.8 s-1 vs. 55.7 s-1, P = 0.004). Image registration improved the mean DSC of 0.83 (range: 0.70-0.88) to 0.95 (range: 0.92-0.97), mean RO of 0.71 (range 0.53-0.78) to 0.90 (range: 0.86-0.94), and mean SDSC =4 mm of 0.47 (range: 0.28-0.67) to 0.97 (range: 0.96-0.98). Voxelwise subtractionbased absorbed dose maps yielded a higher tumor-absorbed dose (median increase of 9.0%) and lower healthy liver-absorbed dose (median decrease of 13.8%) compared to the mean subtraction method. Voxelwise subtractionbased absorbed dose maps corresponded better to SPECT-based absorbed dose maps, reflected by a lower RMSE in three of six patients. Conclusions: Voxelwise subtraction presents a robust alternative method for MRI-based dosimetry of 166Ho microspheres that accounts for pre-existing R*2 differences, and appears to correspond better with SPECT-based dosimetry compared to the currently implemented mean subtraction method. [ABSTRACT FROM AUTHOR]

Simple Summary: Selective internal radiation therapy (SIRT) is a treatment for patients with liver cancer that involves the injection of radioactive microspheres into the liver artery. For a successful treatment, it is important that tumours are adequately covered with these microspheres; however, there is currently no method to assess this intraoperatively. As holmium microspheres are paramagnetic, MRI can be used to visualize the holmium deposition directly after administration, and possibly to adapt the treatment if necessary. In order to exploit this advantage and provide a personally optimized approach to SIRT, the administration could ideally be performed within a clinical MRI scanner. It is, however, unclear whether all materials (catheters, administration device) used during the procedure are safe for use in the MRI suite. Additionally, we explore the capability of MRI to visualize the microspheres in near real-time during injection, which would be a requirement for successful MRI-guided treatment. We further illustrate our findings with an initial patient case. Selective internal radiation therapy (SIRT) is a treatment modality for liver tumours during which radioactive microspheres are injected into the hepatic arterial tree. Holmium-166 (166Ho) microspheres used for SIRT can be visualized and quantified with MRI, potentially allowing for MRI guidance during SIRT. The purpose of this study was to investigate the MRI compatibility of two angiography catheters and a microcatheter typically used for SIRT, and to explore the detectability of 166Ho microspheres in a flow phantom using near real-time MRI. MR safety tests were performed at a 3 T MRI system according to American Society for Testing of Materials standard test methods. To assess the near real-time detectability of 166Ho microspheres, a flow phantom was placed in the MRI bore and perfused using a peristaltic pump, simulating the flow in the hepatic artery. Dynamic MR imaging was performed using a 2D FLASH sequence during injection of different concentrations of 166Ho microspheres. In the safety assessment, no significant heating (ΔTmax 0.7 °C) was found in any catheter, and no magnetic interaction was found in two out of three of the used catheters. Near real-time MRI visualization of 166Ho microsphere administration was feasible and depended on holmium concentration and vascular flow speed. Finally, we demonstrate preliminary imaging examples on the in vivo catheter visibility and near real-time imaging during 166Ho microsphere administration in an initial patient case treated with SIRT in a clinical 3 T MRI. These results support additional research to establish the feasibility and safety of this procedure in vivo and enable the further development of a personalized MRI-guided approach to SIRT. [ABSTRACT FROM AUTHOR]