A 58-year-old man presents with early satiety, left upper quadrant fullness, and 12-pound weight loss over 4 months. Physical examination reveals massive splenomegaly. CT of the abdomen shows diffuse splenic enlargement without a discrete mass. Biopsy of the spleen reveals diffuse large B-cell lymphoma. The attending notes that the spleen and pancreas both receive blood supply from branches of the celiac trunk and asks the resident: "Why does primary cancer of the spleen present as lymphoma rather than the adenocarcinoma we see in the pancreas?"
Which of the following best explains this difference in malignancy type between the spleen and the pancreas?