Kidney cysts are a common finding on CT done for any reason. Most of the time they are benign and do not need treatment, Kidney cysts can be completely filled with fluid or have other more complex areas inside like thickening of the wall, nodules or septations. Kidney cysts can range in size from barely visible to enormous, pushing adjacent structures and causing pain. In rare cases, cysts can rupture.
Some kidney cysts are more concerning because they are more complex then simply containing fluid. A CT done without contrast injected through your vein does not completely evaluate a cyst. Since cysts are so common, many radiologists will diagnose a cyst on a CT without contrast and not require further testing unless they see something suspicious.
Kidney cysts are best evaluated on a CT with contrast injected through your vein. The radiologist will then see if parts of the cyst show complex areas or enhancement. Enhancement after contrast injected through your vein means that that part of the cyst is solid and not a cyst. This is more concerning for a cancer.
Cysts can also fall into a grey zone where we’re not sure if they are benign or malignant. Radiologists use a published Bosniak criteria to grade the cyst based on its suspicion, with lesions graded as 1 are benign and 4 as cancerous. Some lesions may show some features that are concerning but not clearly cancerous. These lesions may be followed (Bosniak 2F) or if more concerning (Bosniak 3) treated like a cancer with the knowledge that some will end up being benign.
The treatment of a Bosniak 3 or 4 graded cyst is partial or complete removal of the kidney. A Urology doctor will guide the management. Those cysts that fall into category 2F will be followed. Lesions graded as 1 or 2 are left alone. Sometimes an ultrasound or MRI may be suggested for certain cysts to further evaluate.