It depends how large the nanoparticles are, I suppose. Down to a certain size and they can just be filtered out by the kidneys. I have some minor experience working with iron oxide nanoparticles to be used as MRI contrast agents and I believe there was just a window of time during which the MRI could be done and then after a few days the particles would be cleared from the bloodstream. To be honest, I didn't really think too much about it and was kind of shooting off the cuff. Obviously a lot depends on the size and type of the nanoparticles, how they are injected, and how they targeted the tumors. Being too lazy to RTFA, I don't really know what they did in this case.