PET Correlates with Thymidine Kinase 1 Expression: Static and Kinetic Analysis of 18 F-FLT PET Studies in Lung Tumors

corrected maximum-pixel standardized uptake value from 60- to 90-min summed dynamic data) was significantly correlated with the overall (r 5 0.57, P 5 0.006) and maximal (r 5 0.69, P , 0.001) immunohistochemical expressions of Ki-67 and TK1 (overall expression: r 5 0.65, P 5 0.001; maximal expression: r 5 0.68, P , 0.001) but not with TK1 enzymatic activity (r 5 0.34, P 5 0.146). TK1 activity was significantly correlated with TK1 protein expression only when immunohistochemistry was scored for maximal expression (r 5 0.52, P 5 0.029). Dynamic analysis of 18 F-FLT PET revealed correlations between the flux constant (KFLT) and both overall (r 5 0.53, P 5 0.014) and maximal (r 5 0.50, P 5 0.020) TK1 protein expression. KFLT was also associated with both overall (r 5 0.59, P 5 0.005) and maximal (r 5 0.63, P 5 0.002) Ki-67 expression. We observed no significant correlations between TK1 enzyme activity and KFLT. In addition, no significant relationships were found between TK1 expression, TK1 activity, or Ki-67 expression and any of the compartmental rate constants. Conclusion: The absence of observable correlations of the imaging parameters with TK1 activity suggests that 18 F-FLT uptake and retention within cells may be complicated by a variety of still undetermined factors in addition to TK1 enzymatic activity.