Case provided by Thuy
63 years old female from Eastern Europe with chronic shortness of breath and exhaustion. Elevated transaminases and eosinophilia were noted by the family doctor.
Ultrasound of the liver shows an ill-defined infiltrative mass with mixed echogenicity including areas of hyperechogenicity (A). Within this lesion there is a well-defined nodular solid lesion with canalicular intralesional architecture (B). T2-weighted MRI illustrates that the ill-defined lesion is predominantly composed of microvesicular areas. Occasionally there are larger cystoid areas filled with fluid of decreased intensity probably reflecting intralesional necrosis (C). The well-defined lesion in the medial aspect has a solid matrix with intralesional band-like structures (C). Contrast- enhanced T1-weighted MRI shows that despite some septal enhancement in the periphery of the infiltrative lesion both lesions do not exhibit contrast uptake (D).
The ill-defined liver mass is alveolar echinococcosis (Kodama type 1). The well-defined liver mass is cystic echinococcosis (WHO stage CE 4).
Cystic echinococcosis (CE) and alveolar echinococcosis (AE) are two different infectious diseases that have to be strictly distinguished from each other. Presence of both CE and AE in this patient is merely an exceptional coincidence. CE and AE differ from each other with regard to parasitology, epidemiology, treatment options, and finally imaging findings. A synopsis of important differences between CE and AE are given in table 1.
CE lesions are expansive cystic masses that predominantly occur in the liver (70%) and lung (20%). They exhibit a cyst evolution from active to inactive cyst stages. Cyst stages are defined by imaging features according to the WHO classification system (table 2). With increasing inactivation of CE cysts consolidation of cyst content is observable. In addition to the WHO classification, differentiation between complicated and uncomplicated CE cysts is of clinical relevance. CE may be complicated by cyst rupture, cyst infection, and cystobiliary or cystobronchial fistulas. As serological tests are frequently false negative (in CL, CE1, or inactive cysts), imaging is an important diagnostic tool. First choice imaging modality is ultrasound with which both cyst wall composition (double-lign sign in CE1 cysts) and cyst matrix composition can be assessed.
AE lesions are typically infiltrative liver masses mimicking cholangiocellular carcinoma or metastases. There is great morphological variability including the whole spectrum between completely cystic and completely solid lesions. The plethora of imaging features is reflected in the classification suggested by Kodama et al. subdividing AE lesions into five categories according to matrix composition. The typical AE lesion is an ill-defined mass composed of microvesicular cystic parts and fibrous solid parts. Presence of calcifications is a frequent finding and an important discriminator against malignancy. Due to the low grade of vascularization, which is an imaging feature of utmost importance as well, AE lesions tend to liquefaction necrosis eventually producing large cystoid areas filled with debris. The WHO proposed a so-called PNM staging system to categorize hepatic disease burden and involvement of extrahepatic sites in the style of the TNM staging system for malignant tumors. Serological test are generally positive for echincoccosis. However, biopsy may be required in ambiguous cases. At ultrasound the lesions have typically areas of hyperechogenicity. MRI is best to depict the typical imaging features of lesion composition but may not be sufficient to diagnose intralesional calcification.
- CE and AE are two separate parasitic diseases with distinct imaging findings.
- CE can be diagnosed by imaging based on pathognomonic findings and is classified according to the WHO stsging system reflecting parasitic activity.
- The imaging findings of AE are very variable and resemble malignancy. AE is relevant differential diagnosis for unclear liver tumors in endemic areas.
Hosch et al.
Imaging methods in the diagnosis and therapy of cystic echinococcosis
Rofo. 2004; 176(5), 679–687.
Kantarci et al.
Alveolar echinococcosis: spectrum of findings at cross-sectional imaging.
Radiographics. 2012; 32(7), 2053–2070.
Kodama et al.
Alveolar echinococcosis: MR findings in the liver.
Radiology. 2003; 228(1), 172–177.
Table 1. Synopsis of CE and AE features
|Causative organism||Echinococcus granulosus (cysticus)||Echinococcus multilocularis (alveolaris)|
|Growth pattern||Expansive growth||Infiltrative growth|
|Lesion matrix||Dependent on cyst stage, uni- or multilocular cysts with or without solid components or completely solid lesions||Typically combination of microcystic and fibrous components, amount of cystic and solid components vary|
|Classification||Cyst staging according to WHO with US or MRI reflects activity||PNM staging reflects disease burden for treatment stratification, PET-CT may help assess activity, Kodama classification reflects varying morphology|
|Effect of benzimidazoles||Parasitocide||Parasitostatic|
Table 2. CE staging according to WHO
|CL||Active||Unilocular cyst indistinguishable from simple dysontogenetic cyst|
|CE1||Active||Unilocular cyst with double-lign sign at ultrasound|
|CE2||Active||Multilocular cyst without solid components, honeycomb sign at imaging|
|CE3a||Transitional||Unilocular cyst with water-lilly sign (detached endocyst within mother cyst)|
|CE3b||Transitional||Multilocular cyst with solid components|
|CE4/5||Inactive||Consolidated cysts with canalicular matrix and without or with calcifications|