Yes it can... there is more than one oncologist in the world. Competition exists.

As for if youre talking about the chemo drugs, you don't have to use the latest-and-greatest on-patent treatment. There are now decades of drugs that are off patent. They may not be as good, but then again, it is your life so many choose to pay for the new better ones. If it was illegal to pay more for better drugs than they would never exist (and don't talk to me about government funding; not a single socialized medicine country has innovated any drug of value in the past 50 years, and yes, they do steal from us in the USA by taking our IP and not paying for it).