Medicine (1). Although the use of PET has exploded within the last few years, this has primarily resulted from the application of 18F-FDG in oncology research and clinical practice. The study of Buck et al. further explores the use of 18F-FLT (Fig. 1) and begins a necessary comparison with 18F-FDG. 18F-FDG has found wide applicability in clinical oncology, since it is simple to use and can be distributed commercially. 18F-FDG is useful in the diagnosis, staging, and restaging of a wide variety of tumor types and in the evaluation of their treatment. Although 18F-FDG is the only PET tracer now routinely used in clinical oncology, many other tracers have been produced and tested by investigators. If PET is going to continue to expand, other tracers that complement the information provided by 18F-FDG will need to be found.

For oncologic use, the measurement of tumor growth and DNA synthesis are attractive targets for imaging. Many investigators have thus been led to test a variety of DNA precursors for tumor imaging. Based on the work originally done in the laboratory using 3H-and 14C-labeled thymidine, synthesis of 11C-thymidine was developed for use with PET (2, 3). Imaging with 11C-thymidine is of great interest, since it is the native pyrimidine base used in