Conventional chemotherapy regimens continue to be important treatment options, sometimes as first-line therapy, for many tumor types. However, depending on the cancer, high rates of treatment failures may be observed. There are multiple reasons for this. In many cases, cancers are diagnosed after solid tumors have become well established and harder to treat. Solid tumor architecture can prevent drugs from penetrating into the deepest regions of the tumor. In addition, the tumor microenvironment may favor the development of drug- and radiation-resistant tumor cell types. Thus many tumors have cell populations that are difficult to reach and difficult treat, which presents a major challenge since the high chemotherapy doses needed for effective elimination of these tumor cell populations is prevented by severe toxicity.