Pancreatic and peripancreatic cystic lesions, referred to as pancreatic cysts, can include different diseases from benign to malignant, and can be generally divided into true cysts, pseudocysts and cystic tumors, among which pseudocysts are the most common in clinical practice, while the incidence of cystic tumors has been on the rise in recent years. True cysts include congenital simple cysts, polycystic disease, dermatomal cysts, and retention cysts, etc. The inner wall of cysts is covered with epithelium. In pseudocysts, the wall of the cyst is composed of fibrous tissue and is not covered with epithelial tissue. Cystic tumors include cystic adenoma and cystic carcinoma. Although all the above lesions are mainly cystic changes, there are some differences in etiology, clinical manifestations, diagnosis and treatment due to their different pathological bases. (a) True cysts The more common type of true cysts is retention cysts, which are mostly formed due to extra-pancreatic ductal compression, pancreatic duct stones, inflammatory stenosis, etc., resulting in obstruction and high pressure of the pancreatic duct, causing cystic dilatation of the distal pancreatic duct or alveoli and retention of pancreatic fluid. True cysts may be caused by congenital abnormal development of pancreatic ducts, which are also called congenital cysts. The endothelial cells of true cysts still have certain secretory functions, forming a cyst lined with complete endothelium. When a true cyst is combined with chronic inflammation or infection, the epithelial layer may also be damaged and disappear. The size of such cysts varies widely, reaching more than 10 cm as shown under the microscope. The inner wall of the cyst is smooth, covered with flat or low columnar epithelium, and the cyst contains plasma, mucus, or occasionally turbid fluid formed by bleeding from infection. The cysts do not contain solid tissue components, septa, or tumor-like abnormalities. Cysts are usually classified into single cysts and multiple cysts based on the number of cysts. Multiple cysts of the pancreas combined with fibrotic cystic fibrosis or called pancreatic fibrosing cystic disease, which is a systemic genetic disease and is also less common clinically, often combined with multiple cysts of the kidney, liver, lung or central nervous system. (ii) Pseudocyst Pancreatic pseudocysts are cysts formed when blood and pancreatic fluid spill into the peripancreatic tissue after pancreatic trauma or inflammation or, in rare cases, into the small omental sac where encapsulation occurs. The difference between pseudocyst and true cyst is that the latter occurs in the pancreatic tissue, the cyst is inside the pancreas, and the inner layer of the cyst is composed of glandular ducts or alveolar epithelial cells; while the former is a cyst formed by the peripancreatic tissue such as peritoneum, omentum or inflammatory fibrous connective tissue to encapsulate the fluid in the cyst wall, and there are no epithelial cells lining the cyst wall, so it is called pseudocyst, and it accounts for more than 80% of all pancreatic cysts. According to the etiology of pseudocyst formation, pancreatic cysts are divided into: 1. post-inflammatory pseudocysts: seen in acute pancreatitis and chronic pancreatitis. 2, post-traumatic pseudocysts: seen in blunt trauma, penetrating trauma or surgical trauma. 3, pseudocysts due to tumors. 4. Parasitic pseudocysts: caused by roundworms or encapsulated cysts. 5.Idiopathic or unexplained. About 75% of pseudocyst cases are caused by acute pancreatitis, about 20% of cases occur after pancreatic trauma, and 5% of cases are caused by pancreatic cancer. Since the necrotic exudate in pancreatitis penetrates the envelope from the anterior renal space forward into the small omental sac, backward into the perinephric and posterior renal space, or along the parabasal aorta to the paraspinal column, upward to the subdiaphragm or even through the diaphragm, or along the small intestine and transverse colonic mesentery to the abdominal cavity, pseudocysts can form anywhere above, but of course the most common is around the pancreas. The wall of the pseudocyst has no epithelium and is composed of granulation tissue and fibrous tissue. The contents of the cyst are necrotic tissue, inflammatory exudate, blood and a large amount of pancreatic enzymes, so it can be a clear yellow fluid or a chocolate-like thick fluid, and some of them are also covered with flocculent necrotic tissue. Animal experiments have shown that it takes 4 weeks for pseudocyst walls to form, and at least 6 weeks in humans. Pseudocysts of the pancreas are about 80% single and vary in size, and typical pseudocysts traffic with the main pancreatic duct. (iii) Cystic tumor of the pancreas Cystic tumor of the pancreas is a special type of pancreatic tumor, accounting for about 1% of pancreatic tumors and 10-13% of cystic lesions of the pancreas. Benign tumors include pancreatic plasmacytoma, mucinous cystadenoma, benign or junctional intraductal papillary mucinous neoplasm of the pancreas (IPMNs) and solid pseudopapillary tumors, and malignant tumors include mucinous cystic adenocarcinoma and invasive IPMNs. Among them, pancreatic plasmacytoma and mucinous cystadenoma are the most common pancreatic cystic tumors. benign tumors, accounting for 32-39% and 10-45% of cystic tumors, respectively. Mucinous cystadenoma has a tendency to become malignant, and the literature reports that 80% of mucinous cystadenoma specimens can be found to have local atypical hyperplasia or malignant manifestations. Previously, it was thought that plasmacytoma had no tendency to become cancerous, but recently, cases of plasmacytic cystadenocarcinoma have been reported. Papillary mucinous tumors in the pancreatic duct account for about 21-33% of cases, and histologically the pancreatic duct is cystically dilated, and both the main and branch pancreatic ducts can be involved. The tumor tissue in the pancreatic duct is papillary in growth, varies in size, and can be multicentric or segmental in distribution. The pancreatic duct is often filled with mucus and contains exfoliated tumor cells. The typical pathological features are pseudopapillary structures with foci of hemorrhagic necrosis, which are low-grade malignant or junctional tumors. Cystic tumors have obvious boundaries with normal pancreatic tissues, most have intact membranes, cystic fluid amylase is normal, in mucinous cystadenoma and cystic adenocarcinoma cystic fluid carcinoembryonic antigen is seen to be significantly increased, the cystic lumen is generally not connected with the main pancreatic duct. Clinical manifestations: The clinical manifestations of different types of cysts are actually similar. In addition to the symptoms associated with the primary disease, small pancreatic cysts are mostly asymptomatic, while larger cysts may present with pain in the upper abdomen or back, digestive symptoms (fullness, nausea and vomiting, constipation, jaundice), and abdominal masses due to cyst compression, intracapsular and/or pancreatic ductal hypertension. If the cyst ruptures into the abdominal cavity, there are manifestations of acute peritonitis; rupture into the digestive tract may form an internal fistula and present with recurrent high fever, abdominal pain, and even upper gastrointestinal bleeding. Because of the lesion of pancreatic parenchyma, there can be pancreatic endocrine insufficiency such as diabetes and steatorrhea. Diagnosis and differential diagnosis: Pancreatic cysts are not difficult to diagnose with medical history, symptoms and signs, combined with ultrasound and other imaging examinations. Ultrasound, CT or ERCP examination can determine the location and size of the cyst, but it has little significance in differentiating between plasma and mucus, benign and malignant, while ultrasound endoscopy can help differentiate through puncture biopsy. （Ultrasound examination is a simple and effective means of diagnosing pancreatic cysts, which is non-invasive, economical, accurate and repeatable, and is the preferred method of examination. In typical cases, a liquid dark area with clear location and definite scope can be detected in the epigastrium, and the adjacent relationship between the cyst and the pancreas can be understood, with high accuracy of localization and diagnosis. In addition, under the guidance of ultrasound, a cyst puncture can be performed to extract the cystic fluid for biochemical and cytological examination. It is more difficult to distinguish the true cyst from the pseudocyst. 2.X-ray examination Barium meal X-ray examination has localization value for pancreatic cysts, which can exclude lesions in the gastrointestinal cavity and can also see signs of compression and displacement of the cyst on the surrounding organs. Depending on the location of the cyst, barium meal examination of the stomach, duodenum or colon can show compression and displacement, curved indentation, and widening of the duodenal ring. If a large pseudocyst is present behind the stomach, barium can show the stomach pushing forward and the gastric lesser curvature can be compressed. Pseudocysts in the head of the pancreas can widen the duodenal flexure and displace the transverse colon upward or downward. Occasionally, pancreatic calcification shadow can be found on abdominal plain film. CT examination can show the anatomical relationship between the cyst and the surrounding structures, and also help to detect cystic lesions in the pancreas, and identify pseudocysts and tumor cysts in terms of cyst morphology, cyst wall thickness, cyst chasing organisms in the cavity, etc. It can also detect lesions outside the pancreas. (1) True cyst: CT shows a watery density mass in the pancreas with thin and smooth wall, no septum and soft tissue nodules within the cyst, no traffic with branches of the pancreatic duct, no significant changes after enhancement, and sometimes the thin wall of the cyst can be shown. (2) Pseudocyst: CT examination is the most effective examination method, which shows a single-room or multi-room foveal cystic mass. The cyst wall is unevenly thick and thin, and enhancement scans may show varying degrees of enhancement, generally speaking the closer to the acute inflammatory phase and the shorter the time of formation, the more pronounced the enhancement. Due to the mixing of blood and inflammatory exudate, CT values often reach 20-30 Hu. If CT examination shows the presence of gas-fluid planes, it indicates the formation of infectious abscesses. (3) Cystic tumor unit: Although various cystic tumors have their characteristic CT manifestations, there are not many typical ones. The CT features of plasmacytic cystadenoma are the central scar of the tumor with honeycomb-like cysts. Mucinous tumors often appear as single- or multi-housed cysts with segregation and cyst wall nodule formation. If there is peripheral calcification it is highly suggestive of cystic adenocarcinoma. Solid pancreatic pseudopapillary tumors most often present as large cystic solid occupancies with occasional peripheral calcification. Intraductal papillary mucinous neoplasm of the pancreatic duct mostly presents as dilated pancreatic duct with nodules in the duct lumen. 4.ERCP and MRCP These two examinations are mainly to help determine whether the cyst is connected to the pancreatic duct, and whether there can be pancreatic duct stenosis, dilatation, stones, etc. ERCP can also extract pancreatic fluid for cytological examination. Retained cysts are usually seen to be connected to the main pancreatic duct, and 2/3 of patients with pseudocysts are seen to be connected to the main pancreatic duct on ERCP, while cystic tumors are usually not connected to the main pancreatic duct. If there are filling defects or multiple cystic cavities and communicating with the pancreatic duct with mucus draining from the papilla, there is a possibility of the presence of intraductal papillary mucinous tumor (IPMT). The presence and location of the cysts can also be determined by ERCP and can help to differentiate them from pancreatic cancer. In pseudocysts, ERCP shows cyst filling; obstruction of the main pancreatic duct with tapered or truncated obstructed ends; displacement of the common bile duct by compression; and compression of pancreatic duct branches and limited branch non-filling in non-communicating cysts. 5.Endoscopic ultrasound Endoscopic ultrasound examination can not only understand the internal structure and multifariousness of the cyst, but also extract cystic fluid for cytology, tumor marker and amylase detection, as well as fine needle aspiration biopsy, which can help in the differential diagnosis of pancreatic cysts. （Except for pancreatic pseudocysts, all other types of cysts, regardless of their location, should in principle be treated surgically with the ultimate goal of removing the cyst. Single cysts are usually treated by simple cystectomy or laparoscopic cystectomy, while multiple cysts in the tail of the pancreas are treated by tail of the pancreas. While preoperative and intraoperative cysts in the head of the pancreas are suspected to have cancerous tendency, pancreaticoduodenectomy should be performed decisively on the basis of the frozen pathology report. （In principle, cystectomy is used for true cysts, but cyst-jejunostomy is also feasible for some of the retained cysts if resection is difficult or the patient’s condition is poor, or stenting is placed at the site of pancreatic duct stenosis or obstruction through ERCP. (B) Pseudocysts 1. Treatment principles: Not all pancreatic pseudocysts require surgical treatment. Although the rate of natural regression of pseudocysts after acute pancreatitis reported in different literature varies greatly, most of them are considered to be above 50%. It is generally considered that if the cyst is larger than 6 cm in diameter and does not regress after 4-6 weeks of observation, surgical treatment should be considered. If the patient has mild symptoms and the diagnosis of pseudocyst is clear, the observation time or long-term follow-up can also be extended appropriately. (1) Cyst removal: Generally, it is only applicable to small cysts in the tail of the pancreas, and it is more difficult to operate on large cysts. In general, it is not widely used. (2) Cyst drainage: For larger pancreatic pseudocysts, cyst-jejunum Roux-en-y internal drainage is preferred, and cyst-gastric anastomosis has also been used more often, the advantages and disadvantages of which are still controversial. If the patient has extensive intra-abdominal adhesions or is in poor general condition and the cyst has more necrotic tissues and incomplete liquefaction, external drainage is also a better choice, but due to the high incidence of pancreatic fistula after external drainage, internal drainage is still the first choice for the treatment of pancreatic pseudocysts. (3) Pancreatic resection: Pancreatic resection is often performed when the pancreas has serious lesions or malignant tumors, and can be done as pancreaticoduodenectomy, pancreatic body caudal resection or total pancreatectomy. Other drainage methods (1) Percutaneous drainage Under the guidance of CT or B ultrasound, the cyst is punctured and a drainage tube is placed percutaneously, which is suitable for emergency treatment of pseudocysts with pancreatic duct traffic or some infected pseudocysts. When there is no fluid flow from the drainage tube, it often means the closure of the fistula and the drainage can be stopped. However, care should be taken to rule out drainage tube obstruction before extraction. The advantage of this method is that it is less invasive, but the controversy lies in the recurrence rate of pseudocysts after placement and drainage, which is still inconclusive. (2) Endoscopic drainage If the pseudocyst is in close proximity to the gastric wall, it can be drained via endoscopy. This is done by endoscopically applying endoscopic ultrasound guidance to penetrate the gastric or duodenal wall and the cyst wall, and then placing one or more stents into the cyst for continuous drainage. As a minimally invasive treatment, it has become more prevalent in recent years. Its long-term results and whether it can replace the traditional cyst-jejunum drainage have yet to be verified by large number of cases and long-term follow-up. (iii) Cystic tumors Benign tumors such as cystic adenoma in pancreatic cystic tumors have the possibility of malignant transformation, especially mucinous cystic adenoma, so cystic tumors of the pancreas, regardless of benign or malignant, should be removed surgically as early as possible, and the surgical method is to completely remove the part of the pancreas containing the mass. If the intraoperative freezing indicates malignancy, the corresponding extended resection and regional lymph node dissection can be performed. For local resection of tumor, it is still controversial because of the risk of pancreatic fistula. For cystic tumors of the pancreas that cannot be resected surgically, most of them are caused by huge tumors and dumping into the surrounding tissues, and their complications should be treated, such as biliary-intestinal anastomosis to relieve obstructive jaundice and gastrointestinal anastomosis to relieve gastric pylorus or duodenal obstruction caused by tumor compression.