Meredith,
Several donors on here have had tests that have revealed some type of unusual anatomical structure, which is why they do all the tests they do. As you have said, cysts really are very common and as an ultrasound tech I have seen many cysts on kidneys. Now there can also be some dense lesions on organs that are also not clinically significant like benign lesions or lipomas (contain fat). it may also be something related to your adrenal gland since he said it is on top of the kidney, not really sure. While it is easy for me to say, don't worry, i understand your concern. So try not to jump to any conclusions yet. See if you can ask the surgeon to get what's called a "wet read" or stat read so the radiologist will call the requesting physician immediately and then he can call you. This way you don't have the "waiting game". Once you have the information of what this lesion is they saw on CT (I am assuming it was a CT) then you can move to step 2. Worse case scenario, it is something that needs to be removed and you may not be eligible to donate, but may be a lifesaver for you. Best case scenario, it is a benign lesion or cyst and they may still consider you to be a good candidate to donate and they will leave you with the "clean" kidney and use the other one for donation.
Will keep you in my thoughts and prayers. Keep us posted.
Sherri