Renal cysts, or cystic kidney disease, as the name implies, is a general term for the presence of cystic lesions in the kidney that do not communicate with the outside world. Common renal cysts include simple renal cysts, parapelvic cysts, and polycystic kidneys, among others. With the popularization of physical examination and the wide application of ultrasound and CT, the detection rate of renal cyst disease has increased significantly and has become a more common kidney disease in clinical practice. Some kidney tumors (e.g. cystic kidney cancer) may appear like cystic structures due to the formation of necrosis and cavity inside the tumor, which should be differentiated from renal cysts. Disease classification: By etiology, most renal cysts are congenital (hereditary), while a few are acquired or uncharacterized. Simple renal cysts: Simple renal cysts are the most common lesion in human kidney disease, and are one or several cystic cavities of varying size, not connected to the outside world, filled with cystic fluid, in unilateral or bilateral kidneys. Disease characteristics: The etiology is unclear. The vast majority of patients are adults, and it is rare in children. There are more males than females. The incidence increases with age, and about 50% of people over 50 years of age have one or more simple renal cysts, and the prevalence is as high as 90% in people over 70 years of age. Simple renal cysts have a thin, transparent wall containing a yellowish, clear fluid, and if there has been inflammation, the wall may thicken, become fibrotic or even calcified. The cyst is not connected to the renal pelvis and the wall is lined with a single layer of flattened epithelial cells. Smaller simple renal cysts grow slowly and have little effect on renal function and little chance of malignancy, and only require regular observation; for simple renal cysts larger than 4 cm in diameter, cysts that increase rapidly, or cysts with suspected malignancy, surgery should be considered. Clinical manifestations: Usually asymptomatic, mostly found by chance during physical examination or imaging examination of other diseases. Some patients may feel the symptom of “back pain” on the affected side, which is often not directly caused by the kidney cyst. Some very large kidney cysts, especially those with intracapsular hemorrhage or infection, can cause significant pain and discomfort in the lower back and abdomen. Some kidney cysts happen to compress the ureter or the neck of the calyx, which will cause hydronephrosis and secondary infection, followed by symptoms of back pain, fever, and urinary tract infection. Individual simple renal cysts will develop cyst wall carcinoma, the rate of carcinoma is about 1%, and the possibility of carcinoma should be alerted when there is bleeding inside the cyst. Examination means and differentiation: Abdominal ultrasound (or color ultrasound) is preferred for routine physical examination, which can both detect kidney cysts and serve as a means of regular review. If the ultrasound result cannot accurately determine the nature of the cyst, abdominal enhancement CT should be checked. The focus of differentiation is to distinguish simple renal cysts from cystic tumors (such as cancerous cysts or internal necrotic tumors). In the former, there is a round homogeneous liquid dark area with no enhancement, thin wall, posterior wall echogenic enhancement and clear and smooth demarcation between the cyst and renal parenchyma under ultrasound and CT; in the latter, the cyst wall is irregular, there are tumor-like contents inside the cyst and there is enhancement inside the cyst. In the latter case, the cyst wall is irregular and there are tumor-like contents within the cyst, and there is enhancement within the cyst.