High Quality Imaging and Dosimetry of Yttrium-90 (⁹⁰Y) Radioembolization Using a SiPM-Based PET/CT scanner

Objectives: Yttrium-90 radioembolization (⁹⁰Y RE) is a treatment modality for primary and secondary hepatic malignancies. Personalized dosimetry is crucial and aims for maximum treatment effect while keeping toxicity acceptably low. Dose calculation methods are based on empirical evidence with regard to efficacy and safety, but do not take the radiation absorbed dose in the tumor(s) or the normal liver tissue into account. Here we evaluated the image quality of ⁹⁰Y post RE scans using a SiPM-based PET/CT scanner (Discovery MI) compared to standard of care pre-therapy ^99mTc MAA SPECT/CT. We assessed the accuracy for both distribution and dosimetry and compared the absorbed dose in the tumor(s) and normal liver tissue in the pre- and post-therapy imaging.

Methods: SurePlan (MIM) software (version 6.7.10, MIM Software, Cleveland, OH) was used to analyze the ^99mTc MAA SPECT/CT and 90Y PET/CT imaging. Tumor volume and normal liver tissue were contoured on the diagnostic CT images and then registered to the ^99mTc MAA SPECT/CT and the post-therapy ⁹⁰Y PET/CT images respectively. Image quality of the lesions detected with digital PET/CT was compared to ^99mTc-MAA SPECT/CT using the 5-point Likert scale. All PET images were acquired for 30 minutes and reconstructed in 10, 15 and 20 minute datasets. Two nuclear medicine physicians reviewed all scans independently.

Results: We enrolled eight patients (age range 55 - 88 years, mean age 67 years; 5 males and 3 females) treated for primary and secondary liver malignancies. The median administered activity was 2.42 (0.2-4.4) GBq. Mean dose of tumor computed from ^99mTc MAA SPECT/CT was 82.4 Gy versus 83.1 Gy from post-therapy ⁹⁰Y PET/CT. There was no significant average difference between Tumor-SPECT/CT and Tumor-⁹⁰Y PET/CT doses (t = 0.78, P=0.938). On average, Tumor-SPECT/CT scores were 0.7 Gy higher than Tumor-⁹⁰Y PET/CT doses (95% CI [-18, 20]). For normal liver tissue a mean dose of 21.3 Gy was found in the ^99mTc MAA SPECT/CT images and 17.8 Gy for ⁹⁰Y PET/CT (T= -1.114, P=0.302). On average, normal liver tissue doses in the ^99mTc MAA SPECT/CT were 3.5Gy lower than ⁹⁰Y PET/CT doses (95% CI [-11, 4]). The tumor uptake of pre- and post-therapy imaging was well correlated in all patients. For ⁹⁰Y PET/CT imaging, the best image quality was found to be at 20 min scan time (Likert-Scale 4.5 ± 0.5) vs. 15 min Likert-Scale 4 ± 0.6 and 10 min Likert-Scale 2 ± 0.6.

Conclusions: Our preliminary data show excellent image quality from digital ⁹⁰Y PET/CT at a reduced scan time of 20 minutes, and SurePlan (MIM) software computed no significant divergent tumor or normal liver tissue doses for ^99mTc MAA SPECT/CT and ⁹⁰Y PET/CT. However, more patients have to be evaluated to confirm these findings. The faster imaging time may allow for inclusion of ⁹⁰Y PET/CT in routine clinical workflows.

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