Pharmacological ascorbate (AscH(-)) induces cytotoxicity and oxidative stress selectively in pancreatic cancer cells compared with normal cells. Positron emission tomography (PET) with the thymidine analog 3'-deoxy-3'-((18)F) fluorothymidine (FLT) enables noninvasive imaging and quantification of the proliferation fraction of tumors. We hypothesized that the rate of tumor proliferation determined by FLT-PET imaging, would be inversely proportional to tumor susceptibility to pharmacological AscH(-)-based treatments. Indeed, there was decreased FLT uptake in human pancreatic cancer cells treated with AscH(-) in vitro, and this effect was abrogated by co-treatment with catalase. In separate experiments, cells were treated with AscH(-), ionizing radiation or a combination of both. These studies demonstrated that combined AscH(-) and radiation treatment resulted in a significant decrease in FLT uptake that directly correlated with decreased clonogenic survival. MicroPET (18)F-FLT scans of mice with pre-established tumors demonstrated that AscH(-) treatment induced radiosensitization compared to radiation treatment alone. These data support testing of pharmacological ascorbate as a radiosensitizer in pancreatic cancer as well as the use of FLT-PET to monitor response to therapy.