Depends. On the identity of the malignancy, e.g. Adenocarinoma, neuroendocrine , location and distribution. If it can be removed, surgery. Recommend evaluation by an experienced surgical or medical oncologist, preferably as a team.
Surgery, chemoRT. If disease limited to abdomen (not spread) then surgical removal if possible (whipple), which could be followed by chemo and/or radiation therapy (depending on pathologic findings). If surgical removal not possible, chemotherapy and radiation therapy. Radiation would be to abdomen. Chemo options include 5-fu based and/or gemzar (gemcitabine).