Hi Kaz6655 Im not a medical expert but I'm also suffering from Renal Cancer and had read some information about it. As far as i know, Renal Cell Carcinoma (RCC), the most common type of Kidney Cancer, can be easily determined (or differentiate from other mass) via Ultrasound due to its composition that have a define identifiable common signatures. I read that RCC are less likely to have macroscopic fats which differentiates it from benign mass like angiomyolipomas (though some Kidney Cancers are fatty) and fats are quite identifiable using Ultrasound. I did asked my doctors about biopsy since my initial ct-scan readings were unclear, one suggested that biopsy is a viable diagnostic procedure, while the more expert one said that biopsy is not needed and its just a waste of time if i already considered having the mass surgically removed (which by then they can identify the cancer on a more in-depth level). None-the-less if the mass is big enough to cause intense sudden pain (which is probably an acute hemorrhage) then they'll probably advise for surgical removal regardless of malignancy/non-malignancy as it could potentially lead into a life threatening emergency condition.