Evaluation of Long Axial Field-of-View (LAFOV) PET/CT for Post-Treatment Dosimetry in Yttrium-90 Radioembolization of Liver Tumors: A Comparative Study with Conventional SPECT Imaging

Purpose Long axial field-of-view (LAFOV) PET/CT scanners enable high sensitivity and wide anatomical coverage. Therefore, they seem ideal to perform post-selective internal radiation therapy (SIRT) ⁹⁰ Y scans, which are needed, to confirm that the dose is delivered to the tumors and that healthy organs are spared. However, it is unclear to what extent the use of LAFOV PET is feasible and which dosimetry approaches results in accurate measurements. Methods In this retrospective analysis, a total number of 32 patients was included (median age 71, IQR 64-76), which had HCC, CCC or liver metastases. All patients underwent SIRT, and the post-therapy scan was acquired on a SPECT/CT and a LAFOV Biograph Quadra PET/CT with a 20-minute acquisition time. Post-treatment dosimetry, regarding the tumor, whole-liver and lung (LMD) absorbed dose was done using an organ-wise (Simplicit90Y) and a voxel-wise approach (HERMIA Dosimetry) which used a semi-Monte Carlo algorithm. The lung shunt fraction (LSF) was also measured using the voxel-wise approach and compared to the planned. Results The _{planning, post-treatment SPECT and PET (SPECT pre , SPECT post , PET post ) median tumor doses based on the organ-wise dosimetry were 276.0 Gy (200.0-330.0 Gy), 232.0 Gy (158.5-303.5 Gy) and 267.5 Gy (182.5-370.8 Gy). In contrast, the median voxel-wise PET post dose was significantly smaller than the planned SPECT pre (152.5 Gy (94.8-223.8 Gy); p<0.00001). Moreover, the median tumor absorbed dose at 90% (D90) of the tumor volume was significantly higher in SPECT post compared with PET post (123.5 Gy; 81.5 -180.0 vs. 30.5 Gy; 11.3-106.3; p < 0.00001). The PET post measured LSF was significantly lower compared to the planned SPECT pre (0.89%; 0.4-1.3% vs. 2.3%; 1.5-3.6%; p<0.0001). Similarly, the measured PET post median LMD was considerably lower to the planned SPECT pre (1.2 Gy; 0.6-2.3 vs. 2.5 Gy; 1.4-4.7; p<0.0001) Conclusion LAFOV PET enabled the direct measurement of post therapy lung dose and tumor doses that correlated well with the planned treatment doses. However, current voxel-wise-based tumor dosimetry seems to be inaccurate for LAFOV PET. In addition, dose volume histogram-based metrics also significantly underestimate the delivered dose. Therefore, improved dosimetry tools are needed for reliable voxel-wise ⁹⁰ Y dosimetry to leverage the sensitivity and spatial resolution of LAFOV PET scanners.}