18F-FLT PET/CT imaging may hold potential as a noninvasive method to assess both tumor response in chemotherapy and immune activity during cancer immunotherapy. It may serve as a valuable tool for the early detection of treatment failure and adverse effects, leading to more personalized and adaptive treatment strategies in prostate cancer. This chapter reviews two pilot studies which demonstrated limited success. Our own study for the early response evaluation of chemotherapy in prostate cancer showed that FLT PET/CT may work in the assessment of response in lymph nodes—it will probably not work in the evaluation of prostate bed, but it will definitely not work in the evaluation of skeletal or liver disease. In immunotherapy, FLT-PET could provide an early indicator of treatment efficacy by measuring tumor and immune cell proliferation, which could guide clinical decisions more effectively than traditional imaging or PSA measurements.