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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]

Background: Quantitative imaging is a crucial step for dosimetry in radionuclide therapies. Traditionally, SPECT/CT imaging is quantified based on scanner-specific conversion factors or self-calibration, but recently absolute quantification methods have been introduced in commercial SPECT reconstruction software (Broad Quantification, Siemens Healthineers). In this phantom study we investigate the accuracy of three quantification methods for holmium-166 SPECT/CT imaging, and provide recommendations for clinical dosimetry., Methods: One cylindrical phantom, filled with a homogeneous holmium-166-chloride activity concentration solution, was imaged at one time point to determine a scanner-specific conversion factor, and to characterize the spatial dependency of the activity concentration recovery. One Jaszczak phantom with six fillable spheres, 10:1 sphere-to-background ratio, was imaged over a large range of holmium-166 activities (61-3130 MBq). The images were reconstructed with either an ordered subset expectation maximization (OSEM, Flash3D-reconstruction; scanner-specific quantification or self-calibration quantification) or an ordered subset conjugate gradient (OSCG, xSPECT-reconstruction; Broad Quantification) algorithm. These three quantification methods were compared for the data of the Jaszczak phantom and evaluated based on whole phantom recovered activity, activity concentration recovery coefficients (ACRC), and recovery curves., Results: The activity recovery in the Jaszczak phantom was 28-115% for the scanner-specific, and 57-97% for the Broad Quantification quantification methods, respectively. The self-calibration-based activity recovery is inherently always 100%. The ACRC for the largest sphere (Ø60 mm, ~ 113 mL) ranged over (depending on the activity level) 0.22-0.89, 0.76-0.86, 0.39-0.72 for scanner-specific, self-calibration and Broad Quantification, respectively., Conclusion: Of the three investigated quantification methods, the self-calibration technique produces quantitative SPECT images with the highest accuracy in the investigated holmium-166 activity range., (© 2024. The Author(s).)