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) positron emission tomography/computed tomography (PET/CT) scanners enable high sensitivity and wide anatomical coverage. Therefore, they seem ideal to perform post-selective internal radiation therapy (SIRT) ⁹⁰Y 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 14), which had hepatocellular carcinoma, cholangiocarcinoma, or liver metastases. All patients underwent SIRT, and the post-therapy scan was acquired on a single photon emission computed tomography/computed tomography (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 ⁹⁰Y dosimetry to leverage the sensitivity and spatial resolution of LAFOV PET scanners.