Selenium dioxide is formed by the reaction of elemental selenium with oxygen:

  
    
      
        
          
            Se
            
              8
            
            
              
            
          
          +
          8
          
          
            O
            
              2
            
            
              
            
          
          ⟶
          8
          
          
            SeO
            
              2
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {Se8 + 8 O2 -> 8 SeO2}}}
  It is a polymeric solid that forms monomeric SeO2 molecules in the gas phase.
Type I: Most common variety (80-90%) involving saccular or fusiform dilatation of a portion or entire common bile duct (CBD) with normal intrahepatic duct. Type II: These cysts are present as an isolated diverticulum protruding from the CBD. Type III or Choledochocele: Arise from dilatation of duodenal portion of CBD or where pancreatic duct meets. Type IVa: Characterized by multiple dilatations of the intrahepatic and extrahepatic biliary tree. Type IVb:  Multiple dilatations involving only the extrahepatic bile ducts. Type V: Cystic dilatation of intrahepatic biliary ducts without extrahepatic duct disease. The presence of multiple saccular or cystic dilations of the intrahepatic ducts is known as Carolis disease. Type VI: An isolated cyst of the cystic duct is an extremely rare lesion. Only single case reports are documented in the literature. The most accepted classification system of biliary cysts, the Todani classification, does not include this lesion. Cholecystectomy with cystic duct ligation near the common bile duct is curative. Treatment
Choledochal cysts are treated by surgical excision of the cyst with the formation of a roux-en-Y anastomosis hepaticojejunostomy/ choledochojejunostomy to the biliary duct. Future complications include cholangitis and a 2% risk of malignancy, which may develop in any part of the biliary tree. A recent article published in the Journal of Surgery suggested that choledochal cysts could also be treated with single-incision laparoscopic hepaticojejunostomy with comparable results and less scarring.