Dead is dead. If a patient is medically determined to be brain-dead, they are dead, and any life prolonging medical intervention should be stopped  immediately.

Any competent patient may refuse therapy. Not just terminal cancer patients, but an otherwise stable patient with renal failure can decide to stop dialysis or any person regardless of their reasons can decide to decline antibiotics or tube feeding or hydration, as long as they are judged to be competent.

Any therapy can be declined, not just chemotherapy or a respirator, but even antibiotics, food and water, or dialysis.

Withdrawing is the same as withholding. Just because a patient has elected to initially go on dialysis or a respirator does not mean they cannot change their mind and decide to stop the therapy

The person's autonomous right to make choices is transferred when they become incompetent. Since most people prior to death pass through a period of incompetence their rights cannot simply disappear, but should be continued or transferred to a guardian or a surrogate, through an advance directive/ living will or to a legally defined surrogate who will make decisions based on their expressed desires. If their is no one available to make choices or there is conflict about what the patient wants done then the courts should appoint someone (as long he is not a right wing politician.)  resuscitate