Use of 3′-deoxy-3′-[18F]fluorothymidine PET to monitor early responses to radiation therapy in murine SCCVII tumors

Purpose: 3'-Deoxy-3'-[F-18]fluorothymidine (FLT) is a promising new radiopharmaceutical for imaging cell proliferation. We evaluated whether FLT PET can be used to monitor early responses to radiation treatment. Methods: C3H/HeN mice bearing murine squamous cell carcinomas were randomized to irradiation with 0, 10, or 20 Gy. Twenty-four hours later, the mice were sacrificed for histopathological and biological assessment such as cell cycle analysis, Hoechst staining, and clonogenic cell survival assay. PET scans were performed on other mice after injection of [F-18]FLT or [F-18]fluorodeoxyglucose (FDG) before and after radiation treatment, and tumor growth was assessed over 9 days. Results: Histopathological examination detected no morphological changes 24 h after radiation treatment, but cell cycle analysis showed that irradiated tumors had a decreased fraction of cells in S phase and an increased fraction in G2-M phase, compared with nonirradiated tumors. Irradiated tumors also had a higher incidence of apoptotic features and reduced clonogenic cell survival. Tumor growth was significantly delayed in irradiated mice (p < 0.001) compared with control mice. PET images showed increased tumoral uptake of both FLT and FDG before radiation treatment. Following irradiation, FLT uptake differed significantly (p=0.020) from that in control mice. In contrast, FDG uptake after irradiation did not differ significantly from that in control mice. Conclusion: Our finding that tumor uptake of FLT was reduced at 24 h after radiation treatment suggests that FLT PET may be a promising imaging modality for monitoring the early effects of radiation therapy.