A phantom study to achieve comparable 124I PET recovery and address scanner variability for dosimetry purposes in (re)differentiated thyroid cancer
Maaike Dotinga (Leiden University Medical Center, Netherlands Cancer Institute, TU Delft - Biomechanical Engineering)
Lioe Fee de Geus-Oei (TU Delft - RST/Radiation, Science and Technology, Leiden University Medical Center, University of Twente)
Petra Dibbets-Schneider (Leiden University Medical Center)
Mette K. Stam (Leiden University Medical Center)
Martin Pool (Leiden University Medical Center)
Lenka M. Pereira Arias-Bouda (Leiden University Medical Center)
Ellen Kapiteijn (Leiden University Medical Center)
Dennis Vriens (Radboud University Medical Center)
Floris H.P. van Velden (Leiden University Medical Center)
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Abstract
Background Accurate quantification of iodine uptake is essential for performing pre-treatment dosimetry of 131I therapy after redifferentiation of radioiodine-refractory thyroid cancer. Standardized procedures for 124I PET/CT-based dosimetry are currently lacking. We aim to evaluate the relation between 18F and 124I imaging for two different PET/CT scanners, the effect of bias correction on recovery across scanners and investigate the impact of clinically-encountered background (BG)-to-lesion ratios on recovery correction. Methods Cylindrical and NEMA body phantoms were scanned following vendor-recommended 124I acquisition using clinically representative activity concentrations (5.6–5.7 kBq/mL in cylindrical phantom, 45.1–59.2 kBq/mL in NEMA spheres) and BG-to-sphere ratios (∼1:125 to 1:infinity) using two digital PET/CT scanners (Philips Vereos and GE Healthcare Omni). Additionally, BG-to-sphere ratio ∼1:10 was acquired to compare 124I to the EARL 18F standards 1 (EARL1). Calibration accuracy and recovery coefficients (RCmax, RCmean) were compared between scanners with and without bias correction. Results 124I recovery was ∼20% higher for the Omni compared to the Vereos, showing calibration accuracies of 1.12–1.15 vs. 0.94, and RCmean reaching 0.86 vs. 0.69. After bias correction, RCmean was comparable between scanners ('1%) but below the lower limits of EARL1. A single fit for recovery correction (R2 = 0.97) was obtained for different BG-to-sphere ratios for both scanners as RCmean was comparable (p ' 0.4). Conclusion Vendor-recommended 124I acquisition and reconstruction leads to differences in quantification but can be compensated using a bias correction. Recovery correction is minimally affected by different BG-to-lesion ratios, suggesting that one RC curve is sufficient, simplifying 124I calibration procedures in studies requiring 124I quantification.
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