What is exophytic renal cysts? my husband has one that is measured 8mm. could it be a precursor to kidney cancer?

No: A renal cyst that grows from inside to surface of kidney is called exophytic renal cyst is (are) benign single or multiple, if no symptoms are left alone.

Certainly may be: An "exophytic" mass is one that grows away from the epithelium (or membrane lining) and often heralds a pre-malignant or possibly malignant mass. These need evaluation and management by both a urologist (kidney surgeon) and a nephrologist.

No: Cysts are just fluid filled pockets, renal cysts are common and are unrelated to kidney cancer.

Unlikely: Exophytic renal cysts are simply cysts that extend outside the kidney capsule. You can envision a "pimple" on the surface of the kidney. Endophytic cysts are those that are buried within the kidney cortex. The 8 mm cyst is very small and difficult to characterize on noncontrast imaging. However, such a small lesion without complex features is highly unlikely to be cancerous. May deserve followup.