Nothing to worry. Renal cortical cysts are usually age related changes. Since radiologist has mentioned that it is simple cyst so you doesn't need to panic or worry about much. These simple cyst converting in to renal cancer is very rare. You can do the following
Next Steps
Get your usg repeated at 6 months interval to monitor the cyst size and contents. If you are too worried you can go for contrast CT. it will give you exact picture
Health Tips
You need not worry much about this