What is the most likely diagnosis?
Considering a fat-attenuation renal mass, angiomyolipoma is the primary diagnosis. In view of subcapsular collection and surrounding heterogeneity, hemorrhage (also termed Wunderlich syndrome) is also present.
Does presence of fat always means angiomyolipoma? Are there any other fat-containing tumour of kidney?
Although this case is typical, without a reasonable differential diagnosis, one should consider a retroperitoneal liposarcoma invading into the kidney. Presence of fat may be occasionally seen in oncocytomas, renal cell carcinomas and Wilms' tumours, although it usually only constitutes a minority of the whole mass.
What typical feature of angiomyolipomas predispose them to spontaneous hemorrhage?
Angiomyolipomas typically have large abnormal arterial vessels. Spontaneous hemorrhage is typically seen in larger (>4cm) tumours.
CT of the abdomen demonstrates a large fat density mass arising from the lower pole of the left kidney. It is associated with significant surrounding fluid which extends down towards the pelvis, and up behind the kidney.