You always have the right to refuse treatment. It may not be in your best interest, and the doctor also has the right to refuse you as a patient (except in emergency situations), but you do have that right. Any treatments are/should be open to discussion/questions about necessity, reason, alternatives, etc.

Some doctors used to take offense at questioning as challenging their competence/authority. This is changing, and now most welcome (or at least do not get huffy) at questions, as they recognize patients' desire to become more proactive/involved in decisions about their medical care.

Doc