Baseline and longitudinal variability of normal tissue uptake values of [¹⁸F]-fluorothymidine-PET images

Purpose: [¹⁸F]-fluorothymidine ([¹⁸F]-FLT) is a PET-tracer enabling in-vivo visualization and quantification of tumor cell proliferation. For qualitative and quantitative analysis, adequate knowledge of normal tissue uptake is indispensable. This study aimed to quantitatively investigate baseline tracer uptake of blood pool, lung, liver and bone marrow and their precision, and to assess the longitudinal effect of systemic treatment on biodistribution. Methods: ¹⁸F–FLT-PET(/CT) scans (dynamic or static) of 90 treatment-naïve oncological patients were retrospectively evaluated. Twenty-three patients received double baseline scans, and another 39 patients were also scanned early and late during systemic treatment with a tyrosine kinase inhibitor. Reproducible volume of interest were placed in blood pool, lung, liver, and bone marrow. For semi-quantitative analysis, SUVmean, SUVmax, and SUVpeak with several normalizations were derived. Results: SUVs of basal lung, liver, and bone marrow were not significantly different between averaged dynamic and static images, in contrast with blood pool and apical lung. Highest repeatability was seen for liver and bone marrow, with repeatability coefficients of 18.6% and 20.4% when using SUVpeak. Systemic treatment with TKIs both increased and decreased normal tissue tracer uptake at early and late time points during treatment. Conclusion: Simultaneous evaluation of liver and bone marrow uptake in longitudinal response studies may be used to assess image quality, where changes in uptake outside repeatability limits should trigger investigators to perform additional quality control on individual PET images. Advances in knowledge: For [¹⁸F]-FLT PET images, liver and bone marrow have low intra-patient variability when quantified with SUVpeak, but may be affected by systemic treatment. Implications for patient care: In [¹⁸F]-FLT-PET response monitoring trials, liver and bone marrow uptake may be used for quality control of [¹⁸F]-FLT PET images.