My sister had a gastrointestinal sarcoma. There are apparently lots of different types of them, some malignant and some benign. If they use sarcoma to describe it, I believe it means it is malignant. She did not have PC, but had a tripple by pass after having a heart attack six months before they found the tumor. They think the stress of the heart attack may have set off the tumor growth and that perhaps the condition had been there for years, dormant. I can't think of anything more stressful than having gone through Whipple surgery.
My sister had surgery to remove the tumor. It was extensive surgery. Reason I am posting, they now have a treatment for stromal tumors. My sister was given oral chemotherapy, GLEEVEC, which she didn't tolerate well. Then they tried SUTENT and she did well on it. Even if your Father can't have additional surgery, maybe they can give him one of these drugs which suppresses tumor growth.
I'm very sorry for your Dad's troubles.