A comparison of methods for in vivo activity and absorbed dose quantification with PET/CT following yttrium‐90 radioembolization

Abstract

AbstractBackgroundYttrium‐90 (90Y) positron emission tomography (PET)/computed tomography (CT) imaging is increasingly being used to perform tumor (T) and normal liver (NL) voxel dosimetry after 90Y‐radioembolization (90Y‐RE). Yet, the accuracy of in vivo 90Y‐PET/CT imaging, subject to motion blur and co‐registration inaccuracies, and 90Y‐PET/CT dose quantification, subject to availability of different voxel dosimetry algorithms, are not well understood.PurposeThe purpose of this study was to investigate the accuracy of 90Y‐PET/CT‐based activity estimates following 90Y‐RE and characterize differences between 90Y‐PET/CT‐based voxel dosimetry algorithms.MethodsThirty‐five patients underwent 90Y‐PET/CT imaging after 90Y‐RE with TheraSphere. The net administered 90Y activity (Aadmin) was determined using a dose calibrator and pre‐ and post‐procedure exposure rate measurements. The summation of image‐based activity (Aimage) was extracted from perfused volume (PV) and 3D‐isotropically 2‐cm expanded PV contour (PV+2 cm). Absorbed doses were calculated using voxel S‐value (VSV), local deposition method (LDM), and LDM with known activity (LDMKA) dosimetry algorithms. Linear regression and Bland‐Altman analysis quantified the relationship between Aimage and Aadmin and between mean dose estimates (DLDM, DVSV, DLDM‐KA) for PV, T, and perfused NL volumes.ResultsWhile Aadmin and Aimage in PV were highly correlated (R2 > 0.95), the mean bias ± standard error (SE) and (95% limits of agreement, LOA) was significantly non‐zero with −22.7 ± 4.7% (± 28.4%). In PV+2 cm, the mean bias ± SE (± LOA) decreased to 1.3 ± 3.4% (± 18.0%) consistent with zero mean error. DLDM and DVSV were highly correlated (R2 > 0.99) for all volumes of interest (VOIs) and the mean bias ± SE (± LOA) was 2.2 ± 0.2% (± 1.0%), 0.7 ± 0.4% (± 2.8%), and 3.2 ± 0.5% (± 2.8%) for PV, T, and NL, respectively. DLDM‐KA and DVSV were correlated with R2 = 0.86, 0.80, and 0.86 for PV, T, and NL, respectively. The mean bias ± SE (± LOA) between DLDM‐KA and DVSV was significantly non‐zero with −19.6 ± 5.1% (± 31.0%), −20.8 ± 4.4% (± 29.0%), and −18.1 ± 5.3% (± 31.1%) for PV, T, and NL, respectively.ConclusionsThe summation of Aimage in PV was underestimated relative to Aadmin. Only by accounting for respiratory motion, limited spatial resolution, and PET/CT co‐registration errors through VOI expansion was Aimage, on average, equal to Aadmin. The differences between DLDM and DVSV were not clinically relevant, though DLDM‐KA was approximately 20% greater than DVSV. Given the high quantitative accuracy of dose calibrators and challenges associated with accurate 90Y‐PET/CT quantification, LDMKA is the preferred algorithm for accurate 90Y‐PET/CT‐based dosimetry following 90Y‐RE.