Quantitative evaluation of ⁹⁰Y-PET/CT and ⁹⁰Y-SPECT/CT-based dosimetry following Yttrium-90 radioembolization

Background: Following yttrium-90 radioembolization (⁹⁰Y-RE), ⁹⁰Y-PET/CT and ⁹⁰Y-SPECT/CT imaging provide the means to calculate the voxelized absorbed dose distribution. Given the widespread use of the two imaging modalities and lack of well-established standardized dosimetry protocols for ⁹⁰Y-RE, there is a clinical need to systematically investigate and evaluate differences in the performance of voxel-based dosimetry between ⁹⁰Y-PET/CT and ⁹⁰Y-SPECT/CT.

Purpose: To quantitatively analyze and compare ⁹⁰Y-PET/CT and ⁹⁰Y-SPECT/CT-based dosimetry following ⁹⁰Y-RE.

Methods: ⁹⁰Y-PET/CT and ⁹⁰Y-SPECT/CT imaging was acquired for 35 patients following ⁹⁰Y-RE with TheraSphere for the treatment of unresectable hepatocellular carcinoma. Dosimetry was performed using the local deposition method with known activity and the mean dose (D_mean) was calculated for perfused liver volumes (PV), tumors (T), and perfused normal livers (NL). Additionally, the absorbed dose to x% of the volume (D_x, x $\in$ [5%, 10%, …, 90%, 95%]) and the volume receiving y Gy (V_y, y $\in$ [10 Gy, 20 Gy, …, 190 Gy, 200 Gy]) were calculated for T and NL, respectively. Dose metrics were compared using linear regression, Bland-Altman analysis, and statistical testing.

Results: Both ⁹⁰Y-SPECT/CT and ⁹⁰Y-PET/CT-based tumor D_mean were strongly correlated (R² ≥ 0.90) with D_x, excluding metrics on the extrema. Intra-modality comparisons of various D_x and V_y metrics yielded statistically significant differences (ANOVA, p < 0.001) for both⁹⁰Y-PET/CT and ⁹⁰Y-SPECT/CT. Based on statistical testing, only D_x metrics separated by greater than 20%-30% coverage, and only V_y metrics separated by greater than 40-70 Gy, reported significant differences. For PV, there was a strong correlation (R² ≥ 0.99) between D_mean derived separately from ⁹⁰Y-PET/CT and ⁹⁰Y-SPECT/CT imaging. The strength of the correlation was slightly reduced for T and NL with R² = 0.91 and R² = 0.95, respectively. For PV, the mean bias ± standard error (SE) and 95% limits of agreement (LOA) between D_mean from the two modalities was effectively zero with -0.8 ± 0.4% (± 2.5%). For T and NL, the mean bias ± SE (± LOA) was -14.5 ± 3.7% (± 24%) and 9.4 ± 4.7% (± 27%), respectively.

Conclusion: The strong correlation between D_mean and D_x suggests information from multiple dose metrics (e.g., D₇₀ and D_mean) is largely redundant when establishing dose-response relationships in ⁹⁰Y-RE. D_mean is highly correlated between ⁹⁰Y-PET/CT and ⁹⁰Y-SPECT/CT-based dosimetry, for all liver VOIs. Relative to ⁹⁰Y-SPECT/CT, ⁹⁰Y-PET/CT, on average, yielded higher D_mean to tumors (14%) and lower D_mean to perfused normal livers (9%). Absorbed dose differences for perfused liver volumes between ⁹⁰Y-SPECT/CT and ⁹⁰Y-PET/CT were negligible.