Alveolar echinococcosis

Alveolar echinococcosis, also referred as hepatic alveolar echinococcosis or E. alveolaris, is a more aggressive and invasive form of hepatic hydatid disease caused by Echinococcus multilocularis. It mimics a slow-growing tumor, as in contrast to E. granulosus it does not form a well defined encapsulated mass, but rather infiltrates the liver and its surrounding structures, especially at the porta hepatis (portal vein, hepatic veins, inferior vena cava (IVC), and the biliary tree).

For a general discussion, particularly epidemiology and pathology, but also for links to other system-specific manifestations, please refer to the article on hydatid disease. 

Terminology

Although the condition is also named as Echinococcus alveolaris or E. alveolaris, note that this is not the species name.

Radiographic features

It commonly presents as large irregular cystic/necrotic masses with a characteristic lobar involvement, peripheral calcifications, and no significant enhancement. Extension into the biliary tree, portal vein, hepatic vein, or IVC is characteristic of this pattern of hydatid disease. 

Ultrasound 

• large irregular mostly hypoechoic mass with central anechoic areas and scattered peripheral foci of calcification
• peripheral thick hyperechogenicity may be present representing fibrous tissue
• less common presentations representing up to a third of the cases:
  • multiple clustered hyperechoic nodules (hailstorm pattern ¹)
  • pseudocyst characterized by a massive necrotic area
• IVC, hepatic and portal vein invasion may be present
• biliary duct involvement usually characterized by asymmetric/focal intrahepatic biliary tree dilatation

CT

• large (~10 cm) multiloculated/confluent necrotic mass
  • irregular margins
  • peripheral scattered calcifications
  • central hypoattenuating necrotic areas
• diffuse lobar involvement
• usually, no relevant enhancement is observed after intravenous contrast administration
  • surrounding fibroinflammatory enhancement may be present on a delayed phase ¹

MRI

• T1: intermediate signal intensity 
• T2
  • high T2 signal intensity within the multiple small cystic lesions and central necrotic components
  • marked low T2 signal intensity at the peripheral fibrotic tissue ¹
  • MRCP: useful to assess the biliary distortion, compression, and dilatation, as well as communication and extension of the parenchymal cystic/necrotic lesions into the biliary tree ¹
• T1 C+ (Gd): no enhancement within the mass lesion

Differential diagnosis

• cystic hepatic metastases
• hepatocellular carcinoma (HCC)
• biliary cystadenocarcinoma or cystadenoma ¹