PPO plans do not require a referral, as long as the Dr or Hospital system accepts your insurance and is willing to take you on as a patient. HMO plans do require a referral. If you are on Medicare, a Medicare Advantage plan will require a referral, but if you have Medicare Part A and B and a supplemental plan, you don't need a referral. Good idea to check your RX provider's formulary as well if you will be going on ADT. Cancer drugs are insanely expensive.