Gastric adenocarcinoma is best treated surgically. I can not find any research showing a benefit for chemotherapy and radiation therapy is not recommended due to complications in using radiation therapy for gastrointestinal problems. When the entire tumor is removed and there is no evidence of metastasis to surrounding lymph nodes or organs at the time of surgery the prognosis is still guarded, meaning that recurrence of the tumor is likely even in this case. The average life expectancy post surgery for this type of tumor is probably only six months to a year, but patients do seem to be comfortable most of that time, at least in the very small number of cases in which we have diagnosed this problem and attempted surgery. I found a couple of anecdotal reports in which remission periods of longer than a year were reported, though.
Surgeons tend to be optimistic by nature.
Vets who practice primarily medicine tend to be a little more pessimistic, especially about surgical outcomes. Oncologists tend to be somewhere between these extremes, so the fact that the oncologist feels that there is a 50 percent chance of a cure is pretty good. Most older dogs recover pretty well from surgery, so the surgery itself doesn't impair their enjoyment of the remaining time they have, whether it is a short time or a long time. So I think that most owners don't regret attempting surgery, as long as the anesthesia and operation go OK, even if it doesn't turn out that it gives a long term cure. The pathologist's report is very encouraging.