A renal cyst is a fluid collection in the kidney. There are several types based on the Bosniak classification. The majority are benign, simple cysts that can be monitored and not intervened upon. However, some are cancerous or are suspicious for cancer and are commonly removed in a surgical procedure called nephrectomy.
Numerous renal cysts are seen in the cystic kidney diseases, which include polycystic kidney disease and medullary sponge kidney.
Up to 27% of individuals greater than 50 years of age may have simple renal cysts that cause no symptoms.
The Bosniak classification categorizes renal cysts into five groups.
Benign simple cyst with thin wall without septae, calcifications, or solid components. It does not enhance with contrast, and has a density equal to that of water.
Benign cyst with a few thin septae, which may contain fine calcifications or a small segment of mildly thickened calcification. This includes homogenous, high-attenuation lesions less than 3 cm with sharp margins but without enhancement.
Well marginated cysts with a number of thin septae, with or without mild enhancement or thickening of septae. Calcifications may be present; these may be thick and nodular. There are no enhancing soft tissue components. This also includes nonehnancing high-attenuation lesions that are completely contained within the kidney and are 3 cm are larger.
Indeterminate cystic masses with thickened irregular septae with enhancement.
Malignant cystic masses with all the characteristics of category III lesions but also with enhancing soft tissue components independent of but adjacent to the septae.
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