Cancer prognosis information, please?

on my second anniversary of my lung resection for cancer of the lung, i was told that my cancer metastasized to my brain and a lymph node near my esophagus and possibly to my chest wall. surgery is not an option but i am receiving radiation to the brain and then a course of Tarseva (sp?) chemotherapy. over the past week or so i have been dealing with disappointment and am even able to resurrect some hope. when you ask for a prognosis of time and your physician says that she can say "9-12 months with treatment but of course that varies." are the dr.s being conservative and when do they know when to challenge the patient to fight for more life and when to go too far the other way to palliative therapy. i also have a strong faith in God and believe that the human ability to be positive in healing is very important. i would be interested in comments from those in the medical field or patients who are dealing with these issues. thanks in advance.