A 62-year-old man complains of experiencing intermittent, but increasing abdominal pain, and has had a 9 kg weight loss in the last month. On physical examination, he seems to be slightly jaundiced. Alkaline phosphatase and bilirubin are increased. CT scan demonstrates a mass in the head of the pancreas. Which of the following surgical methods is used most often to treat patients with small tumors of the head of the pancreas with no evidence of metastasis?