Question category: Nephrology
A 70 year old male with previous radiation therapy for colorectal carcinoma presents with non specific symptoms of weight loss, malaise, nausea and dull abdominal and back pain. He has also noticed increased frequency and hesitancy of urination. Bloods reveal a normochromic anaemia, increased ESR and renal failure.. His renal failure does not improve with fluids and an ultrasound is organised. This reveals a retroperitoneal mass. What is the most likely diagnosis?