Reproducibility of quantitative ¹⁸F-3′-deoxy-3′- fluorothymidine measurements using positron emission tomography

Purpose: Positron emission tomography (PET) using ¹⁸F-3′- deoxy-3′-fluorothymidine ([¹⁸F]FLT) allows noninvasive monitoring of tumour proliferation. For serial imaging in individual patients, good reproducibility is essential. The purpose of the present study was to evaluate the reproducibility of quantitative [¹⁸F]FLT measurements. Methods: Nine patients with non-small-cell lung cancer (NSCLC) and six with head-and-neck cancer (HNC) underwent [¹⁸F]FLT PET twice within 7 days prior to therapy. The maximum pixel value (SUV_max) and a threshold defined volume (SUV_41%) were defined for all delineated lesions. The plasma to tumour transfer constant (K_i) was estimated using both Patlak graphical analysis and nonlinear regression (NLR). NLR was also used to estimate k₃, which, at least in theory, selectively reflects thymidine kinase 1 activity. The level of agreement between test and retest values was assessed using the intraclass correlation coefficient (ICC) and Bland-Altman analysis. Results: All primary tumours and >90% of clinically suspected locoregional metastases could be delineated. In total, 24 lesions were defined. NLR-derived K_i, Patlak-derived K_i, SUV _41% and SUV_max showed excellent reproducibility with ICCs of 0.92, 0.95, 0.98 and 0.93, and SDs of 16%, 12%, 7% and 11%, respectively. Reproducibility was poor for k₃ with an ICC of 0.43 and SD of 38%. Conclusion: Quantitative [¹⁸F]FLT measurements are reproducible in both NSCLC and HNC patients. When monitoring response in individual patients, changes of more than 15% in SUV_41%, 20-25% in SUV_max and Patlak-derived K_i, and 32% in NLR3k-derived K_i are likely to represent treatment effects.