I had a ct scan done for a kidney stone. it says there appears to be interval thinning of left cortex at the poles. what does that mean?

Renal scarring: The upper and lower poles of the kidney or more vulnerable to scarring secondary to reflux. This may have occurred earlier in life secondary to a febrile urinary track infection characteristic for pyelonephritis which during the healing process can lead to scarring. You cannot eliminate scarring once present but you can help to prevent further urinary tract infections specifically with fever.