75 - A Clinical Comparison Between Nuclear Medicine and CT-based Post-treatment Dosimetry Following 90Y-radioembolization with Multi-modality Imageable Eye90 Microspheres

Purpose: This report presents interim dosimetry analysis from a prospective, multi-center study (Route90, NCT05953337) comparing the mean absorbed dose (D_mean) derived from nuclear medicine (NM) imaging and CT imaging of patients undergoing ⁹⁰Y-radioembolization (⁹⁰Y-RE) with imageable Eye90 microspheres®.

Materials and Methods: A total of 27 patients were treated for unresectable HCC using ⁹⁰Y-RE with Eye90 microspheres. Immediately post-treatment, patients were imaged with CT and NM (SPECT (n=9), PET (n=18)) to measure the absorbed dose to the perfused volume (PV) and tumor (T). In NM-based dosimetry, D_mean was calculated using voxel S-values for PET and the local deposition method for SPECT {1}. For CT, D_mean was calculated through calibration phantom analysis and convolution with a ⁹⁰Y dose voxel kernel {2}. D_mean to PV and T was compared between NM- and CT-based dosimetry using linear regression and Bland-Altman analysis. Correlation coefficients (R²), the percent difference ± standard error (bias), and 95% limits of agreement (precision) between D_mean are reported. NM- and CT-based D_mean were also independently compared to a single compartment MIRD-based D_mean, considered a ground truth.

Results: When comparing D_mean from CT- and NM-based dosimetry to a MIRD-based D_mean, CT demonstrates significantly improved agreement. There is a very strong correlation between CT- and MIRD-based D_mean to PV (R²=0.96, p< 0.001), while in NM, the correlation was far weaker (R²=0.56, p< 0.001). The MIRD-based D_mean was generally greater than both CT- or NM-based D_mean, particularly NM. For CT, the bias and precision were 44 ± 13% and 64%, respectively. For NM, the bias and precision were 70 ± 16% and 78%. When comparing D_mean directly between CT and NM, there was a moderate correlation between for PV (R²=0.57, p< 0.001) and a weak correlation for T (R²=0.36, p< 0.001). D_mean was generally greater in CT-based dosimetry relative to NM. For PV, the bias and precision were 31 ± 19% and 94%, respectively. For T, the bias and precision were 10 ± 21% and 104%.

Conclusion: For PV, CT-based dosimetry revealed that D_mean maintained a significantly stronger correlation with the MIRD-based D_mean, relative to NM. CT-based dosimetry also had a reduced bias and improved precision relative to NM-based dosimetry. Future research will leverage the high spatial resolution of CT to more accurately characterize tumor dose heterogeneity.