@article {Mei265245, author = {Riccardo Mei and Lukas Kessler and Kim M. Pabst and Manuel Weber and Christian Schimdkonz and Christoph Rischpler and Helle Damgaard Zacho and Thomas Hope and Sarah M. Schwarzenb{\"o}ck and Martin Allen-Auerbach and Louise Emmett and Justin Ferdinandus and Marcus Unterrainer and Benedikt M. Schaarschmidt and Lale Umutlu and Andrea Farolfi and Paolo Castellucci and Cristina Nanni and Silvi Telo and Stefano Fanti and Ken Herrmann and Wolfgang P. Fendler}, title = {68Ga-FAPI PET/CT Interobserver Agreement on Tumor Assessment: An International Multicenter Prospective Study}, elocation-id = {265245}, year = {2023}, doi = {10.2967/jnumed.122.265245}, publisher = {Society of Nuclear Medicine}, abstract = {68Ga-fibroblast activation protein inhibitors (FAPIs) are promising radiotracers for cancer imaging, with emerging data in the recent years. Nonetheless, the interobserver agreement on 68Ga-FAPI PET/CT study interpretations in cancer patients remains poorly understood. Methods: 68Ga-FAPI PET/CT was performed on 50 patients with various tumor entities (sarcoma [n = 10], colorectal cancer [n = 10], pancreatic adenocarcinoma [n = 10], genitourinary cancer [n = 10], and other types of cancer [n = 10]). Fifteen masked observers reviewed and interpreted the images using a standardized approach for local, local nodal, and metastatic involvement. Observers were grouped by experience as having a low (\<30 prior 68Ga-FAPI PET/CT studies; n = 5), intermediate (30{\textendash}300 studies; n = 5), or high level of experience (\>300 studies; n = 5). Two independent readers with a high level of experience and unmasked to clinical information, histopathology, tumor markers, and follow-up imaging (CT/MRI or PET/CT) served as the standard of reference (SOR). Observer groups were compared by overall agreement (percentage of patients matching SOR) and Fleiss κ with mean and corresponding 95\% CI. We defined acceptable agreement as a κ value of at least 0.6 (substantial or higher) and acceptable accuracy as at least 80\%. Results: Highly experienced observers agreed substantially on all categories (primary tumor: κ = 0.71; 95\% CI, 0.71{\textendash}0.71; local nodal involvement: κ = 0.62; 95\% CI, 0.61{\textendash}0.62; distant metastasis: κ = 0.75; 95\% CI, 0.75{\textendash}0.75), whereas observers with intermediate experience showed substantial agreement on primary tumor (κ = 0.73; 95\% CI, 0.73{\textendash}0.73) and distant metastasis (κ = 0.65; 95\% CI, 0.65{\textendash}0.65) but moderate agreement on local nodal stages (κ = 0.55; 95\% CI, 0.55{\textendash}0.55). Observers with low experience had moderate agreement on all categories (primary tumor: κ = 0.57; 95\% CI, 0.57{\textendash}0.58; local nodal involvement: κ = 0.51; 95\% CI, 0.51{\textendash}0.52; distant metastasis: κ = 0.54; 95\% CI, 0.53{\textendash}0.54). Compared with SOR, the accuracy for readers with high, intermediate, and low experience was 85\%, 83\%, and 78\%, respectively. In summary, only highly experienced readers showed substantial agreement and a diagnostic accuracy of at least 80\% in all categories. Conclusion: The interpretation of 68Ga-FAPI PET/CT for cancer imaging had substantial reproducibility and accuracy among highly experienced observers only, especially for local nodal and metastatic assessments. Therefore, for accurate interpretation of different tumor entities and pitfalls, we recommend training or experience with at least 300 representative scans for future clinical readers.}, issn = {0161-5505}, URL = {https://jnm.snmjournals.org/content/early/2023/05/25/jnumed.122.265245}, eprint = {https://jnm.snmjournals.org/content/early/2023/05/25/jnumed.122.265245.full.pdf}, journal = {Journal of Nuclear Medicine} }