Illustrating the sensitivity of [18F]-FLT PET to de novo pathway utilization, PET imaging of HCT-116 (Fig. 5C) and HCT-116p21-/- (Fig. 5D) xenografts closely reflected the relative tumor cell TK1 levels inherent to each model, where HCT-116 xenografts (8.56 ± 1.17 %ID/g) exhibited approximately 1/3 greater uptake than analogous HCT-116p21-/- xenografts (6.91 ± 1.07 %ID/g; (p  =  0.005) (Fig. 5E). Here, TK1 is linked to neoplasm.