Hydatid cyst (echinococcosis) is a cystic lesion caused by the parasite Echinococcus granulosus in the liver and is frequently encountered worldwide, especially in endemic regions where sheep farming is common (Mediterranean, Middle East, South America, Australia). Dogs are the definitive host, humans are intermediate hosts; transmission occurs via fecal-oral route (contaminated water/food or dog contact). The parasite larva reaches the liver via the portal vein and localizes in the liver in 65-70% of cases. The cyst has a three-layered wall structure: outer laminar layer (pericyst — host fibrous reaction), middle chitinous layer, and inner germinal layer. Daughter cysts form by budding from the germinal layer, creating multiple cystic structures within the mother cyst. The Gharbi/WHO classification defines the evolutionary process of the cyst in 5 stages (CE1-CE5) and is critically important for treatment planning.
Age Range
10-60
Peak Age
35
Gender
Equal
Prevalence
Common
E. granulosus oncospheres pass from the intestine to the portal venous system and are trapped by capillary filtration in the liver — making the liver the most commonly affected organ. The oncosphere develops in the liver parenchyma into a hydatid cyst containing protoscoleces. The three-layered wall structure is pathognomonic: germinal layer (active parasite tissue — produces protoscoleces and daughter cysts), chitinous layer (laminar, acellular structure — barrier between parasite and host — source of 'double wall' sign on US), and pericyst (host fibrous-inflammatory reaction — source of delayed enhancement on CT). Budding of daughter cysts from the germinal layer creates the 'wheel-within-wheel' appearance. During cyst aging, the germinal layer degenerates and the endocyst (inner layers) separates from the pericyst and floats in cyst fluid — this creates the 'water-lily sign'. In advanced stages, the cyst becomes completely calcified (CE5) — calcification is the final stage of the chronic fibrous-inflammatory process in the pericyst and indicates cyst death/inactivation.
Detachment of the endocyst membrane from the pericyst floating in cyst fluid — pathognomonic finding of cyst degeneration and CE3b stage. The undulating movement of the membrane can be demonstrated with patient position change.
Multiple regularly arranged daughter cysts within the mother cyst — 'wheel-within-wheel' or 'rosette' pattern. Daughter cysts are anechoic cystic structures arranged at the periphery of the mother cyst. This finding is pathognomonic for echinococcosis. Defines WHO/Gharbi CE2 stage.
Report Sentence
A _x_ cm cystic lesion is identified in liver segment _ containing multiple peripherally arranged anechoic daughter cysts (rosette pattern — consistent with hydatid cyst CE2).
Undulating membranous structures floating within cyst fluid — detached endocyst membrane creates a 'water-lily' appearance. This finding indicates the beginning of cyst deactivation (CE3b stage). Membrane movement can be demonstrated with patient position change.
Report Sentence
Undulating membranous structures are seen floating within the cyst fluid (water-lily sign — consistent with hydatid cyst CE3b).
Well-defined, water-density cystic lesion on non-contrast CT. Thin or coarse calcification in the pericyst wall — partial (arc-like) or complete (eggshell-like). Complete calcification indicates cyst inactivation (CE5). Daughter cysts may be identified as small cystic structures of lower density than the mother cyst.
Report Sentence
A _x_ cm well-defined, water-density cystic lesion is identified in liver segment _ with partial/complete wall calcification.
On contrast-enhanced CT, cyst contents do not enhance (avascular parasitic cyst). The pericyst may show mild-moderate enhancement — particularly more pronounced in the delayed phase. Pericyst enhancement is more pronounced in active cysts; minimal or absent in inactive cysts.
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On contrast-enhanced CT, cyst contents show no enhancement with mild enhancement of the pericyst wall.
Cyst fluid is markedly hyperintense on T2-weighted imaging (free water signal). The pericyst (fibrous wall) appears as a T2 hypointense rim — creating a 'double ring'. Daughter cysts are identified as bright cystic structures on T2. Water-lily sign can also be demonstrated on MR — the floating membrane appears as a hypointense linear structure on T2.
Report Sentence
The cyst shows markedly hyperintense signal on T2-weighted sequence with T2 hypointense pericyst wall and multiple hyperintense daughter cysts.
On T1-weighted imaging, cyst fluid is typically hypointense. However, if the protein content of cyst fluid is high, T1 signal may increase. The pericyst appears as a thin hypointense rim on T1. On gadolinium-enhanced sequences, the pericyst shows thin enhancement; cyst contents do not enhance.
Report Sentence
Cyst fluid shows hypointense/mildly hyperintense signal on T1-weighted sequence with thin enhancement only at the pericyst wall on contrast-enhanced sequences.
Criteria
Homogeneous, anechoic, well-defined, unilocular cyst. Double wall sign may be seen. No daughter cysts. Active stage.
Distinct Features
May be difficult to differentiate from simple hepatic cyst — distinguished by double wall, endemic history, and serology. Treatment: PAIR or surgery.
Criteria
Multiple regularly arranged daughter cysts within mother cyst. Rosette/wheel-within-wheel pattern. Pathognomonic echinococcosis appearance. Active stage.
Distinct Features
Most diagnostic stage. PAIR contraindicated; surgery + albendazole recommended.
Criteria
CE3a: Water-lily sign — detached endocyst, floating membrane. CE3b: Daughter cysts + solid matrix area. Transitional stage — partially active.
Distinct Features
CE3a: PAIR applicable. CE3b: Surgery may be preferred. Partially active — requires monitoring.
Criteria
CE4: Heterogeneous, degenerative content ('ball of wool' pattern). CE5: Complete calcification. Both stages are inactive.
Distinct Features
Generally does not require treatment — surveillance ('watch and wait'). Surgery may be needed in complicated cases. CE5 complete calcification indicates cyst death.
Distinguishing Feature
Simple cyst is completely anechoic, thin-walled, no septation/daughter cysts/calcification/membrane detachment. In hydatid cyst, double wall, daughter cysts, water-lily sign, and wall calcification are distinguishing.
Distinguishing Feature
Pyogenic abscess shows thick, irregular wall enhancement, peripheral edema, and septa; no daughter cysts. Fever and leukocytosis dominate clinically. In hydatid cyst, regular wall structure and daughter cysts are distinguishing.
Distinguishing Feature
Biliary hamartomas are usually <15 mm multiple cystic lesions. Hydatid cyst is usually solitary and large (>5 cm). Daughter cyst architecture and calcification are not expected in biliary hamartomas.
Distinguishing Feature
ICC is a solid mass showing peripheral rim + delayed central enhancement; cystic component is secondary to necrosis. In hydatid cyst, cyst fluid does not enhance and daughter cysts create regular internal architecture.
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
6-monthTreatment is determined by WHO/Gharbi stage. CE1 and CE3a: PAIR procedure + albendazole. CE2 and CE3b: Surgery + pre/post-op albendazole. CE4-CE5: Usually surveillance ('watch and wait'). Albendazole (400 mg x2/day, min 3 months) forms the basis of antiparasitic treatment. Percutaneous biopsy is contraindicated — risk of anaphylaxis and dissemination. Cyst rupture, biliary communication, or superinfection are emergency surgical indications.
Rupture of hydatid cyst can cause anaphylactic shock. Pre-surgical Albendazole therapy is recommended. CE1-CE3a are active/transitional stages requiring treatment; CE4-CE5 are inactive and usually require follow-up only. PAIR (puncture-aspiration-injection-reaspiration) can be performed in selected cases.