A 55 year old male patient presents with backache and pain radiating down the left lower limb since 6 months. He also complains of anorexia and weight loss. For the last 3 months he complains of distension of abdomen. No history of therapeutic irradiation. No family history of swelling all over the abdomen.
On examination, the patient is afebrile, blood pressure is 140/86 mm of Hg. There is pitting edema of the left lower limb. On per abdominal examination, there is distension of abdomen which is more towards the left half. A big bosselated mass of 25cm(vertically) X 15cm(horizontally) is felt in the left half of the abdomen occupying the left hypochondrium, left lumbar region and left iliac fossa. The mass is firm in consistency and does not move with respiration. All the borders except lateral border is well defined. Medially it is not crossing the midline. It is bimanually palpable, but not ballotable. It is intraabdominal as demonstrated by cough test. The mass is not falling forward as demonstrated by examination in knee elbow position and therefore retroperitoneal. The mass has no intrinsic mobility. On percussion, there is resonance over the mass. The rest of the abdomen is normal. No free fluid in the abdomen and there is no organomegaly. Normal bowel sounds heard on auscultation. There is no evidence of varicocele. Hernial orifices are normal. Left lower limb pulsations are normal, no neurological deficits? in left lower limb. Chest examination is normal.
What is the most probable diagnosis in this case?