How can I prevent unnecessary treatment from being applied to me?

Think about your questions before talking to the doctor. If necessary, discuss this with the doctor who referred you. It is best to write down your questions. For some areas, you will find lists for this on this website. You may print them.
Generally speaking, ask about the exact diagnosis, the course of treatment, the risks, the benefits of the therapy and whether the treatment is really necessary or there are alternatives. Also remember to possibly get a second opinion.