Test-Retest Repeatability of [¹⁸F]FDG Uptake by Solid Tumors on PET/MRI: A Prospective, Multicenter Study Using the...

Prior oncologic PET/MRI repeatability studies have been limited by single-center designs, small cohorts, or select patient populations. Our primary aim was to estimate the test-retest repeatability of uptake metrics on [¹⁸F]FDG PET/MRI studies across a variety of cancer types. A secondary aim was to assess the impact of intersession μ-map variability on repeatability. Methods: In this prospective study involving 2 institutions with different PET/MRI scanners, subjects completing standard-of-care [¹⁸F]FDG PET/CT or MRI examinations were recruited to undergo 2 research [¹⁸F]FDG PET/MRIs (i.e., test and retest) within a 1-9 d period per Quantitative Imaging Biomarker Alliance specifications. Enrollment required at least 1 measurable lesion (long axis ≥ 2 cm, known or expected SUV_max ≥ 4.0) and no active oncologic therapy. SUV_max and SUV_peak were extracted from measurable lesions (1-5 per subject). At site 1, attenuation-coefficient maps (i.e., μ-maps) were inspected for substantial differences across sessions. Additionally, at site 1, the retest μ-map was coregistered to the test μ-map, and the PET attenuation-correction process was repeated for the test session based on the retest μ-map. Intersession repeatability was assessed via Bland-Altman plots, and within-subject coefficients of variation (wCVs) were calculated. Results: At site 1, 10 subjects with 11 measurable lesions were analyzed, and the wCVs for SUV_max and SUV_peak were 12.3% and 9.6%, respectively. At site 2, 11 subjects with 30 measurable lesions were analyzed, and the wCVs for SUV_max and SUV_peak were 9.5% and 8.8%, respectively. At site 1, repeating attenuation correction of the test PET data with the retest μ-map slightly improved repeatability, reducing the wCV to 10.1% for SUV_max and to 9.1% for SUV_peak For 8 of 11 lesions at site 1, the μ-map substitution altered the test-retest change by less than 5%, with no appreciable intersession μ-map differences or artifacts. Conclusion: The test-retest repeatability of SUV_max among hypermetabolic lesions on [¹⁸F]FDG PET/MRI, using data from 2 scanner models, was similar to the Quantitative Imaging Biomarker Alliance's wCV benchmark of 10%-12% for [¹⁸F]FDG PET/CT. Intersession μ-map variations had minimal negative impact on repeatability.