Following the first dose of gemcitabine investigators would like to draw special blood and spinal fluid samples to help learn how much of the drug is in the blood and spinal fluid. These studies are called pharmacokinetics. A total of 10 samples will be collected. The blood samples may be collected from an intravenous catheter or a central venous catheter. The spinal fluid samples may be collected either via Ommaya reservoir or lumbar reservoir.