Mayo
http://www.mayoclinic.org/about/
Scott Okuno, MD is one oncologist who treats GIST patients at the Mayo Clinic.. his webpage. One patient says "He is always very caring and very informed."
Mayo Clinic physicians have written a paper about GIST of the bowel and rectum [1]
The Mayo Clinic does many Whipples:
http://www.mayoclinic.org/pancreatic-cancer/whippleprocedure.html