In order to kill all cancer cells in the body, it probably needs to be delivered to every single cell in the organism, and scan the nucleus of that cell. Viruses usually don't infect every single cell, just a small percentage.

So one needs to figure out a delivery method that is efficient enough, and that doesn't elicit an immune response. But I guess one can analyze the cancer in the lab and figure out which receptors it expresses, and then bind to those? We could have a toolkit of different delivery methods, tailored for each patient's cancer.