My information is that kidney cysts are not biopsied. If the radiologists or the urological surgeons think it may be a cancer they will observe the cyst/cysts and if they increase in size they will intervene. They do not biopsy the cysts because if it is a cancer they may spread the cancer in the retroperitoneum.
If there are doubts about a simple cyst or possible cancer they could do a dedicated renal CT examination consisting of thin-section (2.5-5 mm) helical imaging of the kidneys before the intravenous administration of contrast agent, followed by imaging 60-70 seconds and 3-5 minutes after administration of the contrast agent.
I have several simple renal cysts which have not increased in size except for one of them which eventually was diagnosed a hemorrhagic cyst.
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