Researchers in the cancer nanomedicine community debate whether use of tiny structures, called nanoparticles, can best deliver drug therapy to tumors passively — allowing the nanoparticles to diffuse into tumors and become held in place, or actively — adding a targeted anti-cancer molecule to bind to specific cancer cell receptors and, in theory, keep the nanoparticle in the tumor longer. Now, new research on human and mouse tumors in mice suggests the question is even more complicated.