Dr.s refuse to perform surgery, ot based on cost but life expenctancy outcome.?

My father has cancer and ontop of that has a perforated esophageus (sp?). THis perforation is making him dye much more quickly then he would otherwise. THe Dr.s are refusing to do the surgery because they arent even sure that his body is able to recover enough to survive. Or that he can even make it past the anestethia. Are they allowed to refuse just because he is a terminal patient? The insurance is going to pay for it that isn't the issue. My dad has indicated that he wants anything done that can possibly prolong his life (he is currently awake on a ventilator but able to communicate).