Dah! That should be the first approach as it seems quite obvious and probably not difficult at all as the virus doesn't do any harm to the individual. Of course it may also be the one that causes the drug companies to make the least money be far. Is there anything we all can do to not let this get deep sixed by the drug companies and their henchmen?
Maybe it will turn out that to get the virus to kill a large percentage of the cancers, or to do so with fewer unwanted side effects, it will have to be genetically modified, which is something the pharmaceutical companies can happily slap a patent on.