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

Positron emission tomography (PET) using (18)F-3'-deoxy-3'-fluorothymidine ([(18)F]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 [(18)F]FLT measurements.

Nine patients with non-small-cell lung cancer (NSCLC) and six with head-and-neck cancer (HNC) underwent [(18)F]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 k(3), 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.

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 k(3) with an ICC of 0.43 and SD of 38%.

Quantitative [(18)F]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.