Quantitative Comparison of Yettirium-90 (Y90) PET/CT and Y90 SPECT/CT Voxel-Based Dosimetry post Resin Y90 Microsphere Radioembolization.

PURPOSE: To quantitatively compare Y90 PET/CT and Y90 SPECT/CT voxel-based dosimetry after RESIN Y90 TARE in hepatocellular carcinoma (HCC). This was a secondary aim of a previously published prospective single-center trial evaluating low-dose scout RESIN Y90 for treatment planning (NCT04172714). MATERIALS AND METHODS: Thirty patients with 33 treatment-naïve HCC lesions underwent Y90 PET/CT and SPECT/CT after scout and therapeutic RESIN Y90. Voxel-based dosimetry (local deposition method) yielded mean, minimum, and the mean dose to the highest-dosed 30% (V30) and 70% (V70) of the volume, for tumor dose (TD) and non-tumoral liver dose (NTLD). Identical PET/CT-derived contours were applied to the SPECT/CT, and agreement was assessed by Bland-Altman analysis and intraclass correlation (ICC). RESULTS: TD-mean (494 vs. 366 Gy), TD-V30 (594 vs. 408 Gy), and TD-V70 (405 vs. 326 Gy) were higher on PET/CT than SPECT/CT, while NTLD-mean (103 vs. 182 Gy) and NTLD-V70 (77 vs. 141 Gy) were lower on PET/CT (all p < 0.001). Correlations were very strong for TD-mean, TD-min, TD-V30, TD-V70, NTLD-mean, and NTLD-V30 (r > 0.9). Relative to PET/CT, SPECT/CT underestimated TD-mean by 127 Gy (95% limits of agreement -435 to 181 Gy) and overestimated NTLD-mean by 79 Gy; ICC was low for several non-tumoral endpoints (NTLD-mean 0.57, 95% CI -0.09 to 0.86) despite strong correlation. CONCLUSIONS: After RESIN Y90 TARE for HCC, PET/CT calculated higher TD and lower NTLD than SPECT/CT, correlating strongly but with a potentially clinically relevant systematic bias and wide limits of agreement. The modalities should not be considered interchangeable without modality-specific thresholds or further validation.