Benign Biliary Neoplasms 731

Biliary Cystadenoma

Biliary cystadenoma is a rare cystic neoplasm that represents less than 5% of all intrahepatic cysts of biliary origin that arise from intra- and extrahep-atic bile ducts [27]. This neoplasm may occur anywhere along the intra- or extrahepatic bile ducts, although about 80% of lesions are found partly or completely within the liver. The cause of biliary cystadenoma is unknown, although it could be related to a congenital anomaly of the biliary primitive bud.

This neoplasm can be classified as either a) cys-tadenoma with ovarian-like stroma, or b) cystade-noma without ovarian-like stroma. The variant without ovarian-like stroma is observed primarily in males and is considered more aggressive and more inclined to malignant degeneration. The form that develops predominantly in females has an ovarian-like stroma and follows an indolent course. Most lesions are more than 10 cm in diameter at diagnosis, with internal septa, and without solid components.

Microscopically, biliary cystadenoma has a mucin-secreting columnar epithelium lining the cysts. The lining cells have a pale eosinophilic cytoplasm and basally-oriented nuclei, typical of biliary-type epithelium. The epithelium is supported by a mesenchymal stroma which is compact and cellular [16].

Biliary cystadenoma is regarded as a pre-malig-nant tumor. Malignant transformation into cys-tadenocarcinoma may occur in up to 15% of cases. In situ carcinoma with papillary growth into the cysts may be the only lesion present although invasive adenocarcinoma may also be seen [35,68].

Approximately 90% of these neoplasms occur in middle-aged women. When present, the symptoms are those of a growing abdominal mass. Right upper quadrant abdominal pain, occasionally irradiating to the scapula, is the main symptom [8].

On US, biliary cystadenoma is seen as a large hypoechoic, multiloculated cystic-like lesion with intralesional septa (Fig. 19). Occasionally mural nodules occur in benign cystadenoma, although these are more common within cystadenocarcino-ma, in which they sometimes form a mass. Generally the liquid content is anechoic and homogeneous although complications such as hemorrhage or inflammation can increase the liquid echogenecity [33].

Hiatus Diameter
Fig. 19a, b. Biliary cystadenoma. US scans (a, b) reveal hypo- to anechoic lesions with thin septa (arrows)
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Fig. 21a, b. Biliary cystadenoma on MR. T2-weighted HASTE images acquired in the axial plane (a) and True-FISP images acquired in the coronal plane (b) reveal large, lobulated cystic lesions in the right liver lobe. The lesions are homogeneously hyperintense due to the fluid component and thin septa are visible (arrowhead

Fig. 21a, b. Biliary cystadenoma on MR. T2-weighted HASTE images acquired in the axial plane (a) and True-FISP images acquired in the coronal plane (b) reveal large, lobulated cystic lesions in the right liver lobe. The lesions are homogeneously hyperintense due to the fluid component and thin septa are visible (arrowhead

On CT, these tumors are large, low-attenuating intrahepatic masses with lobulated margins and generally thin irregular walls with fibrous septa. Although the cystic parts of the lesions do not enhance following the intravenous administration of contrast material, the internal septations, mural nodules and papillary projections do show enhancement (Fig. 20) [1,33].

On MR imaging, biliary cystadenoma appears as a multiloculated septated mass, whose signal intensity on T1- and T2-weighted images depends on the presence of solid ovarian-like stroma and the composition of the cystic fluid, which may be serous, mucinous, bilious, hemorrhagic, or a combination of these fluids (Fig. 21,22). Low signal intensity within the wall on T2-weighted images may represent hemorrhage. Following the administration of intravenous contrast agents, the internal septations, mural nodules and papillary projections enhance [8]. On hepatobiliary phase images after administration of hepato-specific contrast agents, no contrast material is seen within the cystic cavities, and no significant accumulation is detected in the solid components. Thus the solid components are seen as hypointense areas compared with the surrounding liver parenchyma.

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Fig. 22a-g. Biliary cystadenoma on MR. On the unenhanced T2-weighted HASTE image (a), a hyperintense, cystic lesion with internal septa is visible. On the corresponding T1-weighted fat-suppressed (b) and Tl-weighted (c) images, the lesion shows homogeneous low signal intensity. On dynamic contrast-enhanced imaging (d, e) (Gd-BOPTA, 0.05 mmol/kg BW) the lesion does not show any vascularization and the surrounding liver parenchyma shows normal perfusion. On the Tl-weighted fat-suppressed image in the equilibrium phase (f), the septa and the capsule of the lesion (arrows show contrast enhancement. However, Gd-BOpTA is not visible within the lesion on the Tl-weighted image acquired during the hepatobiliary phase (g)

Bile Duct Adenoma

This is a rare benign epithelial liver tumor which is mainly found incidentally at laparotomy or autopsy. Its maximal size often does not exceed 1-2 cm. Bile duct adenoma represents about 1% of all primary liver tumors. They occur both in children and elderly people, and may be present as solitary nodules or as multiple nodules throughout the liver [62].

Bile duct adenomas are typically located on the surface of the liver. Microscopically, small bile ducts lined by mucin-producing cells are embedded in a fibrous stroma. Based on immunhisto-chemical studies, the most likely pathogenesis is a reaction to a focal bile ductular injury. Histologi-cally, bile duct adenomas comprise a mass of disorganized mature peribiliary gland acini and duc-tules within a variable amount of connective tissue stroma showing signs of chronic inflammation and collagenization. The composition of bile duct adenoma has resulted in it being termed a peribil-iary gland hamartoma [6]. Although the tumor is benign in nature, there has been the suspicion of malignant transformation. Pathological differential diagnoses to be considered include bile duct hamartoma, cholangiocellular carcinoma, metastasis and hepatic granuloma.

On US, bile duct adenoma appears as a hypere-choic area with an acoustic shadow sometimes surrounded by a hyperechoic rim. Similar small hyperechoic liver lesions are hemangioma, focal nodular hyperplasia (FNH), hepatocellular carcinoma (HCC) and metastasis.

On unenhanced CT, bile duct adenoma is usually hypodense. However, the presence of calcifications may give the lesion a hyperdense appearance. On delayed contrast-enhanced CT, the lesions usually demonstrate heterogeneous enhancement, although homogeneous enhancement has also been described. The presence of fibrous stroma within the tumor results in the lesion demonstrating prolonged enhancement on contrast-enhanced CT.

The tumor is typically hypointense on unen-hanced T1-weighted images and hyperintense on T2-weighted scans. Enhancement of bile duct adenomas on dynamic phase imaging after the injection of gadolinium contrast agent may be heterogeneous, ring-shaped or homogeneous. As in contrast-enhanced CT, the presence of fibrous stroma within the tumor leads to prolonged enhancement on post-contrast T1-weighted MR imaging.

Due to its small size and peripheral localization, bile duct adenoma is often difficult to detect. It should be included among the diseases to be differentiated from hyperechoic hepatic tumors on US and from hepatic tumors showing delayed en hancement on contrast-enhanced CT and MRI.

Bile duct adenomas can frequently be distinguished from other liver tumors by their smaller size, their localization beneath the liver capsule, and their prolonged enhancement [62].

Biliary Hamartoma

Biliary hamartoma is a benign neoplasm composed of a proliferation of small, round, normal-appearing ducts with cuboidal, slightly basophilic cells that have regular nuclei but lack any evidence of dysplasia or increased mitotic activity. In this lesion there is always a fibrous supporting stroma. This neoplasm occurs mainly in patients of older age, shows no sex predilection, and is often associated with adult polycystic kidney disease. The lesion may be up to 4 cm in diameter but most are 1 cm or less at diagnosis. An association between cholangiocellular carcinoma and multiple biliary hamartoma has been reported [43], and it has previously been considered a reactive process rather than a true neoplasm or malformation.

Biliary hamartoma is sometimes confused with bile duct adenoma but it is usually multiple and distributed throughout the liver, forming part of the spectrum of fibropolycystic diseases of the liver due to ductal plate malformation. Biliary hamartomas are asymptomatic and are therefore usually incidental findings at fine needle biopsy, laparotomy or autopsy.

Imaging findings are usually not specific since these lesions often mimic metastases or abscesses. Therefore biopsy is usually required for a definitive diagnosis.

On US, the typical form of the lesion is characterized by multiple, small, hypoechoic lesions that affect all segments of the liver giving a "honeycomb" pattern.

On pre-contrast CT images, numerous, round, small, hypodense lesions throughout the liver can be seen. These lesions usually do not show enhancement after contrast medium administration.

On pre-contrast T1- and T2-weighted MR images, lesions appear hypointense and hyperintense, respectively, and are generally well-defined (Fig. 23). The nodules do not show enhancement after administration of hepatospecific contrast agents, because the lesions are independent and do not communicate with the biliary system [50].

Biliary Papillomatosis

Biliary papillomatosis is an extremely rare condition characterized by the presence of multiple be-

Biliary Hamartoma

Fig. 23. Biliary hamartoma. The MRC image reveals multiple, well-defined hyperintense round lesions (arrowheads) in both lobes of the liver nign papillary adenomas in the bile ducts, that are similar to adenomas observed in the intestinal tract. Papillomas can be present in the intra- and extrahepatic bile ducts, including the common bile duct. The lesion can occasionally be found in the gallbladder and in the major pancreatic duct. The papillary excrescences are composed of mucus-secreting columnar epithelial cells supported by thin fibrovascular stalks. In some cases it is possible to observe variable degrees of structural and cyto-logical atypia. Clinically, patients have episodes of obstructive jaundice, sepsis, and hemobilia [61]. A variable degree of biliary duct dilatation can be observed on imaging studies; in some cases intra-ductal tissue masses are present which may be variable in size.

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