I‐PET score: Combining whole body iodine and 18F‐FDG PET/CT imaging to predict progression in structurally or biochemically incomplete thyroid cancer

Abstract Objective We propose a new scoring system (I‐PET) combining whole body scan (WBS) and FDG findings to identify patients who have or are likely to become refractory to radioactive iodine. Design Retrospective analysis of 142 patients age >18 with differentiated thyroid cancer who had a F‐18 labelled fluoro‐2‐deoxyglucose ( 18 F‐FDG) positron emission tomography (PET) and WBS within a 6‐month period between 2010 and 2020. Pairs of 18 F‐FDG PET and WBS were reviewed by three independent nuclear medicine physicians and an I‐PET score was assigned: I‐PET [0] : Iodine −ve/FDG −ve, I‐PET [1] : Iodine +ve/FDG −ve, I‐PET [2] : Iodine +ve/FDG +ve and I‐PET [3] : Iodine −ve/FDG +ve. Patients with FDG +ve lesions (I‐PET [2] and I‐PET [3]) were further classified into groups A and B if SUVmax was ≤5 or >5, respectively. Follow‐up data were obtained by chart review. Progression was defined as structural progression as per RECIST 1.1 or further surgical intervention; or biochemical progression as unstimulated thyroglobulin increasing >20% from baseline. Results Of 142 patients included in the study 121 patients had follow‐up data available for review. At baseline, 49 patients were classified as I‐PET [0], 10 as I‐PET [1], 16 as I‐PET [2] and 46 as I‐PET [3]. Progression was seen in 11/49 (22%) of I‐PET [0], 4/10 (40%) of I‐PET [1], 10/16 (63%) of I‐PET [2] and 34/46 (74%) of I‐PET [3] ( p < 0.001). I‐PET [2B] and I‐PET [3B] had a progression rate of 88% (7/8) and 78% (25/32), respectively. I‐PET [3B] were 9.6 times more likely to commence multikinase inhibitor therapy ( p = 0.001) and had 8 times greater mortality ( p = 0.003) than patients in other I‐PET groups combined. Conclusion I‐PET is a simple readily acquired imaging biomarker that potentially enhances the dynamic risk stratification and guide treatment in thyroid cancer.