Simple renal cyst
are common after the age of 50 years incidentally detected on scans and need no further active treatment for 8mm cyst.
However you need to give further information to be sure that it is a simple cyst
1. wall of the cyst: thick or thin, regular or irregular margin
2. contents of the cyst : homogenous or heterogenous density.
3. presence or absence of septations in the cyst
4. Hounsfeld units of the cyst wall with any enhancement with contrast.
5. solitary or multiple and any cyst in the other kidney: with the information you have given , i believe it is a single cyst measuring 8 mm in the right kidney.
Based on the above details , the renal cysts can be classified according to Bosniak classification into 4 classes: further management depends on that. If it is a simple renal cyst ; it will be Bosniak Class 1 and would not require any treatment.
Reply with the above details , so that you will be able get a clear picture.