Biopsy would be the only way to ID the base material (I believe) and biopsies are done in advance of *surgery* and are invasive. They tried getting me to do one 3 months ago and my personal MD/ND and self refused it and the hospital did agreed with us! They gave me the strongest dose possible of contrast fluid this time for the CAT scan and now say there isn't any fat in it. I could see the outline of the kidney thru it - so - the only thing we know for sure, is it is filled with fluid. The last test shows that it is - for sure - a complicated cyst and there is a possibility it could turn into a form of RCC *however* it isn't growing or changing or acting like cancer. My first hurdle was protesting the surgery and showing why it is most likely benign and now it's a matter of getting rid of it w/out surgery. I've been using a Mel lamp, essiac tea, seaweeds (adding more of these in), tumeric, carcuma, renafood and other things. The only major stress was when this was discovered - the 1st MD said it *was* cancer and wanted to take my whole kidney out asap and I've since proven that he was very wrong. I have a very relaxed life and a fun one and I'd hardly call it repressed. Cysts are hereditary in my family - all kinds of them - all benign. Both sides of the family. This is something I could have been born with and in that event, I just *might* have a hard time getting rid of it and would have been better off not knowing about it. Cheers then.... Birdie