The purpose of this study is to determine if positron emission tomography (PET) imaging with an imaging agent called 18F-fluorodeoxythymidine([18F]-FLT) will allow investigators to measure how well tumor(s) respond to treatment without taking a tissue sample (biopsy). Additionally, the investigators want to determine if it is possible to predict how well tumor(s) might respond to treatment with [18F]-FLT PET imaging.