Complex cysts are fluid-containing hepatic lesions with one or more of the following features: wall thickening or irregularity, septation, internal nodularity, enhancement, calcification, and hemorrhagic or proteinaceous contents (Fig.1).

Fig. 1: Characteristics of simple cyst and complex cyst.

Because a broad range of disease processes can result in complex cystic liver lesions, they may be further grouped as neoplastic, inflammatory or infectious, and other miscellaneous entities (Fig. 2).

Fig. 2: Classification of complex cysts.

A careful evaluation of particular imaging features as well as associated radiologic and clinical and laboratory findings is necessary to suggest a specific diagnosis.

Neoplastic

Biliary cystadenoma and biliary cystadenocarcinoma— are premalignant

and malignant cystic biliary ductal neoplasms, respectively, that account for fewer than 5% of intrahepatic cystic lesions of biliary origin. They arise mainly from the intrahepatic ducts and rarely from the extrahepatic ducts or gallbladder. These neoplasms are most frequently found within the right lobe of the liver (55%). Biliary cystadenoma presents predominantly in middle-aged white women with abdominal pain, nausea, vomiting, and obstructive jaundice.

The characteristic CT appearance is a solitary complex cystic mass with a well-defined thick fibrous capsule, internal septations, and mural nodularity (Fig. 3).

Fig. 3: Biliary cystadenoma. Axial CT images show a solitary complex cystic mass with a well-defined thick fibrous capsule and internal septations.

The key difference between biliary cystadenoma or biliary cystadenocarcinoma and a hemorrhagic or infected hepatic cyst is that the capsule, internal septations, and mural nodules show contrast enhancement in the former and do not in the latter. It is generally taught that mural nodules or polypoid, pedunculated excrescences are more common in biliary cystadenocarcinoma than in biliary cystadenoma.

Imaging characteristics cannot definitely distinguish biliary cystadenoma from biliary cystadenocarcinoma. Therefore, the optimal management of these masses is surgical resection.

Cystic metastases— Hepatic metastases may appear cystic either due to necrosis and cystic degeneration of rapidly growing hypervascular tumors (sarcoma, melanoma, carcinoid, neuroendocrine tumors, and some lung and breast tumors) or as a manifestation of mucinous colonic or ovarian adenocarcinomas.

On image findings, cystic metastases appear as solitary or, more commonly, multifocal lesions with complex features, such as thick, irregular, enhancing walls; thick or nodular septations; mural nodularity; or internal debris (Fig. 4).

Fig. 4: Hepatic cystic metastases. Multiple hypodense hepatic masses represent colon metastases with internal cystic change due to necrosis.

A clinical history of a known primary malignancy, particularly in the setting of multifocal lesions, may help to suggest the diagnosis of cystic hepatic metastases, which can be confirmed with imaging-guided biopsy.

Cavernous hemangioma— Giant cavernous hemangioma is another primary hepatic neoplasm that can outgrow its blood supply and show central cystic degeneration. This tumor frequently occurs in middle-aged women.

At ultrasound, giant cavernous hemangioma with central cystic necrosis may share some features with more typical hemangiomas, such as a well-circumscribed echogenic periphery with a hypoechoic center (“reverse target” sign). However, the appearance may not be diagnostic.

The central cystic component appears hypodense on unenhanced CT and on all phases of contrast-enhanced dynamic examination (Fig. 5).

Fig. 5: Cavernous hemangioma.Initial CT scan after bolus injection of contrast material shows low-attenuation lesion in posterior segment of right lobe of liver. Delayed CT scans show progressive peripheral nodular enhancement of lesion.

On contrast-enhanced CT and MRI, even hemangiomas with predominantly cystic components continue to show the characteristic peripheral nodular enhancement pattern that helps make the diagnosis. Symptomatic large lesions may require surgical resection.

Inflammatory or Infectious Cysts

Abscess— A hepatic abscess is a localized collection of pus in the liver, with associated destruction of the hepatic parenchyma and stroma. Hepatic abscesses may be further classified as pyogenic, amebic, or fungal. Because amebic abscesses do not require drainage, it is important to distinguish them from pyogenic abscess, which can be established on the basis of clinical, radiologic, and serologic data.

Pyogenic abscesses most commonly occur as complications of ascending cholangitis or portal phlebitis. Common causative organisms include Escherichia coli, Clostridia species, Staphylococcus aureus, and Bacteroides species. Clinically, pyogenic abscesses usually manifest in middle-aged or elderly patients who present with fever, right upper and lower quadrant pain, tender hepatomegaly, and elevated WBC counts. On CT, these abscesses appear as well-defined hypoattenuating masses (0–45 HU) with peripheral rim enhancement after the administration of IV contrast material. A characteristic CT finding is the “cluster of grapes” sign, which represents the coalescence of small pyogenic abscesses into a single large multiloculated cavity (Fig. 6).

Fig. 6: Hepatic abscess. US and axial CT images shoe the “cluster of grapes” sign.

The presence of gas within an abscess may be due to infection by gas-forming organisms such as Clostridia species and is strong evidence for pyogenic rather than amebic abscess. The “double target” sign (hypodense rim, isodense periphery, and decreased attenuation in the center) is also characteristic of complex pyogenic abscess. The treatment of pyogenic abscesses includes antibiotic therapy and percutaneous drainage.

Amebic liver abscesses, caused by Entameba histolytica, are the most frequent extracolonic complication of amebiasis. Clinically, in addition to tender hepatomegaly, right upper quadrant abdominal pain, and diarrhea, patients with amebic abscesses often have a history of travel to an endemic area and positive amebic serology. The radiologic features of amebic and pyogenic abscesses often overlap, necessitating clinical and serologic data for diagnosis.

Fungal abscesses due to Candida species are seen in immune-compromised patients. CT shows multiple low-attenuation lesions, which typically have rim enhancement and often also involve the spleen.

Echinococcal cysts— Hepatic echinococcosis, or hydatid disease, is caused by the larval stage of the tapeworm Echinococcus granulosis (more common) or E. multilocularis (more aggressive). After the patient ingests eggs of E. granulosis or E. multilocularis by eating contaminated food or by contact with dog excrement, the larvae invade the intestinal wall and gain access to the liver (via the portal vein), where they develop into hepatic hydatid cysts.

Each hydatid cyst consists of an outer pericyst (compressed and fibrotic host liver tissue), middle laminated membrane or ectocyst, and inner germinal layer. Together, the middle laminated membrane and inner germinal layer are referred to as the endocyst. Daughter cysts develop on the periphery as a result of germinal layer invagination (Fig. 7).

Fig. 7: Biologic cycle of Echinococcus granulosus and histopathologic analysis of hidatid cyst.

Clinically, echinococcal cysts are predominantly seen in middle-aged patients who present with right upper quadrant abdominal pain and jaundice. Key laboratory and clinical features that can distinguish an echinococcal cyst from other cystic liver lesions are eosinophilia, positive serology and Casoni skin test (seen in 25% of patients), and a history of travel within an endemic areas (Mediterranean basin and sheep-raising countries).

On CT, hydatid cysts appear as large unilocular or multilocular hypoattenuating liver cysts. One half of them have crescentic mural calcifications. Daughter cysts are seen as round peripheral structures that may have lower attenuation than fluid within the mother cyst. In the absence of daughter cysts, it may be difficult to differentiate an echinococcal cyst from a cystic metastasis or pyogenic abscess radiographically without clinical and serologic data.

On ultrasound, E. granulosis infection typically appears as a multiseptate cyst with daughter cysts and echogenic material between them (Fig. 8).

Fig. 8: Hidatid cyst. US images show a multiseptate cyst with daughter cysts and echogenic material between them. Axial CT images demostrate two unilocular hypoattenuating liver cysts with peripheral calcifications.

Complications of hydatid cysts include bile duct compression and rupture into the biliary tree with resulting cholangitis. Treatment strategies include medical therapy (albendazole or mebendazole), drainage or surgical resection, or even liver transplantation.

Postraumatic and Miscellaneous Cysts

Hepatic hematoma—Intrahepatic or perihepatic hematomas usually develop secondary to surgery, trauma, or hemorrhage within a solid liver neoplasm (especially HCC). Clinically, these lesions may produce signs and symptoms related to blood loss, peritoneal irritation, right upper quadrant tenderness, and guarding.

On CT, a hepatic hematoma is a fluid collection within the liver that has a higher attenuation value than pure fluid in the acute or subacute setting but an attenuation value identical to pure fluid in chronic cases (Fig. 9).

Fig. 9: Intrahepatic hematoma in 36-year-old man who sustained blunt trauma to abdomen 2 weeks previously. CT scan shows well-circumscribed area of hypodensity in right lobe of liver.

Biloma—A biloma is an encapsulated collection of bile outside the biliary tree. It can develop spontaneously, be secondary to trauma, or represent an iatrogenic complication after an interventional procedure or surgery. Leakage of bile within the liver parenchyma induces an inflammatory response, which may result in the formation of a well-defined pseudocapsule.

On CT and MRI, a biloma appears as a well-defined or slightly irregular cystic lesion without septations, calcifications, or a true capsule (Fig. 10).

Fig. 10: Intrahepatic biloma in 40-year-old woman who had sustained blunt abdominal trauma. Axial and coronal CT images show well-circumscribed fluid-density collection within liver.

On ultrasound, a biloma is seen as a focal fluid collection within the liver that is situated close to the biliary tree and has no vascularity within the lesion.

The management of bilomas includes percutaneous drainage of the fluid collection and ERCP with stent placement to improve biliary drainage and prevent further bile leakage.