I am 36 and told through ct scan that i have a "low density" 6mm lesion on my kidney that they believe is a cyst but too small to characterize. ultrasound after saw nothing. really worried. are kidney lesions common? is low density a good thing?

Depending on the purpose of getting the CT scan, sometimes findings are considered incidental, or found on accident. Simple or benign complex renal cysts are commonly found incidentally. If they are less dense and smaller in size, they are more likely to be benign. If there is an association with symptoms and abnormal kidney labs, a discussion should be made with the ordering provider.

This indicates a benign cyst which are incidentally found when doing the test for another reason. With your findings you need just a periodic followup at the discretion ogf your Urologist. Low desity means that it is filled with fluid and is not solid like a tumor