How to treat a slow-growing, malignant, pancreatic tumor?

Depends. The type of cancer in the pancreas will dictate the types of treatments available and the outcomes. Cystic neoplasms of the pancreas and neuroendocrine tumors are considered slow growing. If all disease can be removed, surgery is the best option.
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).