Pemetrexed(PEM), a standard therapy for non-squamous non-small cell lung cancer (NSCLC), inhibits the de novo thymidine pathway, resulting in a transient burst of metabolism through the salvage pathway, an effect detected as a flare of activity by 18F-thymidine (FLT)-PET. FLT is a reliable biomarker of proliferation, and post-therapeutic changes in tumor FLT avidity predicts therapeutic response in a range of malignancies. However, FLT as a measure of therapeutic response in NSCLC is not well studied. Our overarching hypothesis is that FLT-PET will allow early assessment of therapeutic response in NSCLC with the following specific aims:Specific Aim 1: To determine the optimal timing to measure FLT flare, an imaging biomarker of successful PEM-induced TS inhibition in a NSCLC preclinical model. Specific Aim 2: To conduct a proof-of-concept clinical study of FLT flare and FLT-measured changes in tumor proliferation at 2 weeks of therapy as predictors of NSCLC response to PEM. During the research period, we have determined that the optimal time to observe FLT flare in vivo is at 2 hrs following therapy start. This data has been translated to an open clinical trial of FLT-PET as a biomarker of therapy response in NSCLC proposed in Specific Aim #2.