An uncommon splenic mass

A 37-year-old man presented with pain and progressively increasing fullness in the left upper quadrant for three months. He also complained of loss of appetite and significant weight loss for two months. There was no associated fever, nausea or vomiting. On examination, there was a large, firm, non-tender mass in the left upper quadrant. The laboratory data indicated mild anemia with hemoglobin at 10.6 g/dL (normal, 12-14 g/dL). His liver function tests, renal function tests and blood tumor markers were normal. His past history was significant for total left parotidectomy for a tumor 3 years back. Ultrasonography (US) showed a large heterogeneous splenic mass. A biphasic computed tomography revealed a large 18X12 cm heterogeneously enhancing mass involving the entire splenic parenchyma with large central necrotic areas (Fig. 1A, arrow). Liver (Fig. 1A, short arrow) and lung nodules (Fig. 1B, arrow) were also detected. What is your diagnosis?