It is common to find too small to accurately characterize lesions in the kidneys on CT. These are spots in the kidney which are usually under a centimeter in size. They are too small to characterize or say what they are. Most are tiny cysts or other benign lesions. The ones which look dark on CT compared to the kidney are usually cysts or benign fat containing lesions.
If there is something suspicious about the tiny lesion, then I may recommend another test like an ultrasound. Usually, these more suspicious tiny spots will be brighter than the usual tiny dark spots I see with cysts. If a patient has blood in their urine, I may be more cautious and recommend another test or follow up. Blood in the urine can be seen with tumors of the kidney and urinary tract.
In many cases, these tiny lesions in the kidneys will be discovered on a CT done for some other reason. Sometimes the CT will be done without contrast given through your vein which limits evaluation A kidney CT, Ultrasound or MRI will be the tests most commonly recommended for these too small to accurately characterize lesions on CT.
A kidney CT will have thinner cuts through the kidneys and allow a more detailed view of the kidney lesion. It will also allow a look at the lesion before giving dye through the vein and after. The idea being that a kidney lesion which becomes brighter after giving dye is made of solid tissue and may be a tumor. A cyst will not get darker after giving dye through the vein because it has fluid inside.
An ultrasound is very good at telling if something is a cyst and filled with fluid, or solid and potentially cancerous. It’s limited by the skill of the operator and by the size of the patient. Patients who are large or obese can have limited ultrasound imaging. Also, sometimes ultrasound is not able to tell if a lesion is cystic or solid when it’s really small.
In some cases, no test will provide a precise diagnosis. In these cases, a follow up will be needed. The idea being that something that grows or develops more concerning features is more likely cancerous. Even in the worst case, following a small tumor is often not a big deal. These tumors often grow slowly and rarely spread when they are so small.
Kidney lesions which are too small to accurately characterize on CT are therefore very common and most likely benign. In some cases, additional testing will be suggested based on some suspicious features or a worrisome clinical history. This may include kidney CT, ultrasound or even MRI. In some cases, we will not be able to give a precise diagnosis. Follow up in these cases is the best course of action.