A prospective clinical study of the influence of oral protein intake on [18F]FET-PET uptake and test–retest repeatability in glioma

Abstract

Background: O-(2-[18F]fluoroethyl)-l-tyrosine positron emission tomography ([18F]FET PET) scanning is used in routine clinical management and evaluation of gliomas with a recommended 4 h prior fasting. Knowledge of test–retest variation of [18F]FET PET imaging uptake metrics and the impact of accidental protein intake can be critical for interpretation. The aim of this study was to investigate the repeatability of [18F]FET-PET metrics and to assess the impact of protein-intake prior to [18F]FET PET scanning of gliomas. Results: Test–retest variability in the non-protein group was good with absolute (and relative) upper and lower limits of agreement of + 0.15 and − 0.13 (+ 9.7% and − 9.0%) for mean tumour-to-background ratio (TBRmean), + 0.43 and − 0.28 (+ 19.6% and − 11.8%) for maximal tumour-to-background ratio (TBRmax), and + 2.14 cm3 and − 1.53 ml (+ 219.8% and − 57.3%) for biological tumour volume (BTV). Variation was lower for uptake ratios than for BTV. Protein intake was associated with a 27% increase in the total sum of plasma concentration of the l-type amino acid transporter 1 (LAT1) relevant amino acids and with decreased standardized uptake value (SUV) in both healthy appearing background brain tissue (mean SUV − 25%) and in tumour (maximal SUV − 14%). Oral intake of 24 g of protein 1 h prior to injection of tracer tended to increase variability, but the effects on derived tumour metrics TBRmean and TBRmax were only borderline significant, and changes generally within the variability observed in the group with no protein intake. Conclusion: The test–retest repeatability was found to be good, and better for TBRmax and TBRmean than BTV, with the methodological limitation that tumour growth may have influenced results. Oral intake of 24 g of protein one hour before a [18F]FET PET scan decreases uptake of [18F]FET in both tumour and in healthy appearing brain, with no clinically significant difference on the most commonly used tumour metrics.