A hybrid composite made up of superparamagnetic iron oxide nanoshells encapsulating the anticancer drug doxorubicin and bound together by poly(vinyl alcohol) was developed. Transcatheter arterial delivery in an in vivo liver tumor model led to embolization of the liver tumor blood vessels. Embolization was followed by disassembly of the composite. The nanoshells were then able to pass through the leaky tumor vasculature into the tumor tissue, thereby leading to slow and sustained release of the drug. As well as being relatively noncytotoxic, the composite was responsive to magnetic resonance imaging, thus making it a potentially useful theranostic agent.