3. Cancer cells can be hard to get to with drug therapy. Even if we have a drug that kills the cell in question we can't always get therapeutic concentrations

3. Cancer cells can be hard to get to with drug therapy. Even if we have a drug that kills the cell in question we can't always get therapeutic concentrations into the core of solid tumors, and we can't get many drugs past the blood brain barrier. 4. Cancer cells can be highly unstable, genetically, depending on what part of the DNA got mangled. Unstable cells that mutate rapidly follow the same Darwinian logic you see in microbes. A single tumor can contain thousands of different mutations of the original cell type, and if your therapy kills 99% of them the 1% that come roaring back will be highly resistant to whatever strategy you used the first time around.