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

Abstract

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. Retrospective analysis of 142 patients age >18 withdifferentiated thyroid cancerwho 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 +velesions (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. 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. I-PET is a simple readily acquired imaging biomarker that potentially enhances the dynamic risk stratification and guide treatment in thyroid cancer.