Hydatid cyst, with an atypical appearance on CT
Computed tomography (CT) of the liver was obtained before and after the administration of 120 mL of nonionic contrast (Omnipaque 350, GE Healthcare, Princeton, NJ) on a Sensation 16 scanner (Siemens Medical Solutions, Malvern, PA) using 0.75-mm collimation, 120 KVp, 300mAs, 0.75 slice thickness with reconstruction of data every 0.5 mm. The axial images showed a well-defined, avascular 3-cm ovoid lesion in the left lobe (Figure 1). The lesion measured 51 HU before the administration of contrast, with no evidence of enhancement on either thearterial- or venous-phase images. The lesion was poorly visualized on both noncontrast and arterial-phase contrast-enhanced CT but was clearly seen as a well-defined, low-density mass on venous-phase imaging.
Based on the CT findings, the possibility of an atypical hepatic adenoma or focal nodular hyperplasia was initially considered but was considered unlikely. In light of the recent history of weight loss, there was a concern for neoplasm or abscess, and so a biopsy was performed. Under ultrasound guidance, 2 passes with a 22-gauge Chiba needle were performed in the lesion. Evaluation of the aspirated material revealed the presence of echinococcal hooklets, yielding a final diagnosis of hydatid cyst (Figure 2). The patient was placed on albendazole therapy and referred to a gastrointestinal parasitologist for further treatment.
After the biopsy, it was discovered that the patient had taken 2 trips to Mexico within the past 2 years and that her symptoms had begun just after her first trip.
Hepatic cystic echinococcosis is caused by infection with the metacestode stage of the tapeworm Echinococcosis granulosus. The disease remains endemic in many parts of the world, including the Mediterranean, the Middle East, and parts of South America. Initial infection is always asymptomatic, and subsequent clinical manifestations depend on cyst size and the site of infection. Although hydatid cysts may be found in almost any area of the body, the liver is the most frequent site of involvement, with the majority of cysts affecting the right lobe. In a study of 368 patients with hydatid disease, nearly 75% had liver involvement and nearly half of those had cysts solely in the liver.1Hepatic cysts can remain asymptomatic for years, but growth and rupture of hydatid cysts can have serious sequelae. Cyst growth can produce a mass effect that restricts bile orvenous drainage through the portal system, resulting in cholestasis or portal hypertension. Secondary bacterial infection may lead to the development of liver abscesses. Rupture of cyst contents into the biliary tree can cause cholangitis, obstructive jaundice, or pancreatitis.2
The differential diagnosis of a solitary hepatic cystic mass is broad and includes simple (bile duct) cyst, benign adenoma, focal nodular hyperplasia, metastatic lesion, biliary cystadenoma or cystadenocarcinoma, primary hepatoma, pyogenic or amebic abscess, and echinococcal cyst. Imaging findings on CT can help to narrow the diagnosis.
Simple cysts result from congenital defects in bile-duct formation.3 Radiographically, they are well-defined, measure water density, and do not exhibit enhancement after the administration of intravenous contrast. This lesion did not measure water density and did not exhibit enhancement (either early or delayed).
Hepatic adenomas and focal nodular hyperplasia are usually hypervascular and, unlike the lesion in this case, usually enhance, especially during the arterial phase. The possibility of atypical avascular adenoma remained on the differential until the time of biopsy, as did focal nodular hyperplasia. Primary hepatoma and metastasis were also included in the differential diagnosis, although in sequential imaging these generally enhance to a greater degree than as seen in this case. Cystadenomas appear as low-attenuation intrahepatic masses on CT as well. They are typically more cystic in nature than hydatid cysts, with thick nodular walls and septations. Pyogenic abscesses are variable in appearance radiographically and can often be confused with amebic abscesses. However, both can generally be distinguished from hydatid cyst (and from each other) by correlation with the clinical picture and laboratory data.4
An echinococcal cyst is most often seen radiographically as a well-defined area of low attenuation on CT, often with several classic diagnostic findings. These include focal or segmental cyst wall thickening, crescentic calcification of cyst borders, and the presence of daughter cysts.5 None of these CT findings were apparent in this case.
Classically, the diagnostic imaging method of choice for assessing cystic lesions of the liver has been ultrasound, as it is noninvasive, accessible, and sensitive.6 However, as the use of CT as a primary imaging modality continues to rise, and as CT has become a preferred modality in the investigation of liver lesions, it is important to consider hydatid cyst in the differential diagnosis when presented with unusual cystic lesions in the liver, regardless of whether classic CT features are present. This case illustrates that a detailed clinical history is essential in narrowing down a differential diagnosis-especially when imaging findings are nonspecific.
Hydatid disease remains an endemic problem in many parts of the world. Despite attempts to identify imaging findings that distinguish hepatic echinococcal cysts from other hepatic cystic lesions, echinococcosis is still a challenging disease to diagnose radiographically. This case of hydatid cyst in the liver with atypical CT findings demonstrates the importance of a broad differential diagnosis that includes hydatid cyst when investigating any unusual hepatic cystic lesions.
Hydatid cyst. Appl Radiol.