A renal cyst is a round, fluid-filled sac that develops in or on the kidney. Simple renal cysts are extremely common, especially as people age, and are almost always benign. On ultrasound, a simple renal cyst has characteristic features: it is completely anechoic (black), has a thin, smooth wall, shows posterior acoustic enhancement, and contains no internal echoes, septations, or solid components. When all these criteria are met, the cyst is classified as Bosniak category I and requires no further workup. Complex cysts — those with septations, calcifications, thick walls, or solid components — are classified into higher Bosniak categories and may require additional imaging or even biopsy to rule out malignancy. Ultrasound is the primary tool for detecting and characterizing renal cysts.
Sonography Term