Infections involving the liver may result in abscess formation, cysts, and granulomatous or miliary lesions. In developed countries, most hepatic abscesses are bacterial in origin, whereas worldwide the most common cause of hepatic abscesses is amebic infection, usually with Entamoeba histolytica. Some fungi can also give rise to abscesses, particularly in immunocompromised patients. Hydatid cysts are caused by the larvae of the canid tapeworm Echinococcus granulosus, following ingestion of ova. Granulomatous and miliary lesions may be due to mycobacterial or fungal infections. Symptoms vary depending on the type of infection and location of the mass lesion within the liver and may include fever, nausea and vomiting, abdominal pain or fullness, hepatomegaly, jaundice, and weight loss. Leakage or rupture of hydatid cysts produces allergic reactions ranging from urticaria to anaphylaxis. Table 8.2 summarizes hepatic infections that result in mass lesions in children and adolescents.

Aspirates of bacterial and fungal abscesses typically yield turbid, thick, yellow fluid that is easy to smear. Microscopically, the smears are comprised of abundant neutrophils in a background of granular necroinflammatory debris. In some cases, microorganisms may be identified on routinely stained smears or with Gram, methenamine silver (GMS), or other special stains. Actinomyces has a distinctive filamentous appearance and may form large aggregates known as sulfur granules (Fig. 8.1). Amebic abscesses yield thick brown fluid, often likened to anchovy paste. Smears are characterized by mixed inflammation with eosinophils and Charcot-Leyden crystals in a background of necroinflammatory debris. Although rarely identified in fluid from the central cavity, trophozoites may be seen when the wall of the abscess is sampled and appear as round, blue bodies with well-defined cell borders, bubbly cytoplasm, single eccentric nuclei with a central karyosome, and engulfed erythrocytes. Aspirates of granulomatous lesions are characterized by clusters of epithelioid histiocytes, usually accompanied by lymphocytic inflammation in mycobacterial infections and by neutrophilic or mixed inflammation in fungal infections. Cavitary lesions may also yield necrotic debris. In aspirates from immunocompetent hosts, mycobacteria are usually rare or absent, whereas in those from immunocompromised hosts, mycobacteria may be numerous and appear as intracellular or extracellular negative images on modified Giemsa-type stains. Acid-fast or fluorescent Auramine O stains may be helpful for identifying the organisms. Fungal elements may be detected with routine GMS or periodic acid Schiff (PAS) stains. Hydatid cysts often have a characteristic appearance on imaging studies and, due to concern for anaphylaxis, are rarely aspirated for diagnostic purposes. When these cysts are aspirated, either accidentally or during a therapeutic PAIR (puncture, aspirate, inject [scolicidal solutions], re-aspirate) procedure, they typically yield sandy or grainy fluid with scoleces and hooklets from the tapeworm. Laminated membrane fragments may also be seen.

Special stains, such as Gram, GMS, PAS, and acid fast, performed on smears or sections from cell blocks, can be helpful for confirming an infectious etiology. In cases with limited material, an unstained smear from a diagnostic pass can be reserved for special staining in the event that the cell block has scanty or inadequate material. In addition, polymerase chain reaction (PCR) studies can be performed on material from a cell block or from frozen abscess fluid to detect the presence of microbial organisms and/or for speciation of mycobacteria. Microbial cultures may help to identify the causative microorganism(s) and determine sensitivities to antimicrobial agents.

Bacterial and fungal abscesses can have similar cytologic appearances, as can granulomatous lesions due to mycobacteria and fungi. In the absence of informative special stains, PCR, and/or cultures, the etiology of some abscesses and granulomatous lesions remains unknown. Necrosis and associated inflammation involving a primary or metastatic tumor may mimic an abscess both on imaging studies and cytologically. Malignant cells may be obscured, particularly when few in number, but are the key to the correct diagnosis. Pseudocysts and other cystic lesions in the peripancreatic and peribiliary region may also mimic an infectious cyst or abscess involving the liver.

When adequate material is available, microbial cultures should be submitted to identify the infectious agent and define antimicrobial sensitivities for treatment. An indirect hemagglutination test or other serologic tests can be done to confirm the diagnosis of an amebic abscess, given that the trophozoites are difficult to identify unless the wall is aspirated.

Focal nodular hyperplasia (FNH) is the most common cause of benign hepatocytic tumors in children over the age of 5 years. Although it is usually seen in young reproductive-age women, up to 15 % of cases occur in the pediatric age group. Usually FNH is asymptomatic given its slow growth. The lesion can be quite large when detected and on imaging appears as a well-circumscribed mass with a characteristic central stellate scar with vessels.

The aspirates show benign hepatocytes, in addition to some ductal epithelium, fibroblasts, and/or fibrous tissue fragments. The fibrous tissue fragments may make the aspirate difficult to smear and may also lead to nondiagnostic aspirates, necessitating a core needle biopsy for evaluation.

A cell block or unstained smears are helpful for performing routine histochemical stains, such as reticulin, iron, PAS, and PAS with diastase, to exclude other benign disorders of the liver, as well as well-differentiated or fibrolamellar hepatocellular carcinoma (HCC).

Definitive cytological diagnosis of FNH can be difficult or impossible given the fact that the aspirates show overlapping features with the normal liver, regenerative nodule, hepatic adenoma, well-differentiated and fibrolamellar HCC, and vascular lesions/neoplasms. If the cytopathologist is confident that the aspirate is representative of lesional tissue, the presence of ductal epithelium helps to exclude hepatic adenoma and HCC. Imaging studies should help to exclude sampling of the benign hepatic parenchyma adjacent to or admixed with a vascular lesion.

Correlation with findings on imaging studies, particularly the presence of a central stellate scar with vascularity, and associated rapid enhancement in the arterial phase and rapid washout in the portal venous phase, can be very helpful. Distinction between hepatic adenoma and FNH can be very difficult on aspirates, and thus, larger core needle biopsies are often performed.

Hepatic adenomas typically occur in adolescents and young adults and have been associated with oral contraceptive use (typically more than 5 years of use). They are usually solitary masses and may rupture giving rise to retroperitoneal hemorrhage. Upon discontinuation of oral contraceptives, the lesions may decrease in size or disappear.

Aspirate smears show benign hepatocytes with absent or exceedingly rare ductal epithelium and fibrous tissue, corresponding to the key histologic finding of benign hepatic tissue without conspicuous portal tracts or bile ducts. The intact reticulin meshwork may make aspirates difficult to smear. Central necrosis and/or calcifications may be present in these lesions and be seen in the background of smears or in sections from the cell block.

Reticulin stain should be performed on a smear or section of a cell block to exclude well-differentiated HCC.

Hepatic adenomas can be difficult or impossible to distinguish from the normal liver, regenerative nodules, FNH, and well-differentiated HCC on cytologic preparations. Correlation with clinical presentation, serum alpha-fetoprotein (AFP), and imaging studies, as well as reticulin stain and the presence or absence of ductal epithelium, can help to resolve the diagnosis. However, core biopsy may be required.

Given the difficulty in distinguishing hepatic adenoma, FNH, and well-differentiated hepatocellular carcinoma on aspirates and core biopsies, correlation with the imaging findings is crucial.

Hepatoblastoma (HB) typically occurs in children under 3 years of age and, although rare, is the most common malignant tumor of the liver in pediatric patients, comprising about half of hepatic malignancies in this population. These tumors can be congenital and usually are sporadic; however, some genetic diseases, particularly germline APC mutations (familial adenomatous polyposis) and Beckwith-Wiedemann syndrome, predispose to the development of HB. Patients typically present with abdominal enlargement and, less often, pain. On imaging, HB appears as a solitary or multifocal liver mass with or without stippled and/or coarse calcifications in a non-cirrhotic liver. Many tumors are considered unresectable at diagnosis due to large size and bilobar involvement. Metastases to the lung are present in up to 20 % of patients at diagnosis. Patients usually have elevated serum alpha-fetoprotein (AFP) and anemia , and may also have thrombocytosis.

The cytomorphology depends on whether the tumor is epithelial, mixed epithelial and mesenchymal, or small cell undifferentiated, as well as on the type(s) of epithelial component(s) present [2]. The majority of HB is of the epithelial type with fetal and/or embryonal components, and smears are usually highly cellular [2]. Fetal cells are arranged in orderly cohesive sheets, in three-dimensional clusters, and singly and may show a light and dark pattern due to the variable amounts of glycogen or fat in the cells. The cells are uniform and round to polygonal with moderate amounts of eosinophilic, vacuolated, or clear cytoplasm and round nuclei with fine chromatin and small nucleoli (Fig. 8.2). Extramedullary hematopoiesis can also be seen. Embryonal cells are arranged singly and in loosely cohesive, disorganized sheets and glandular or acinar arrays with crowded, overlapping nuclei. They are smaller than fetal cells and pleomorphic and have scant cytoplasm, oval to irregular hyperchromatic nuclei, inconspicuous nucleoli, and high nuclear-to-cytoplasmic ratios (Fig. 8.2). Recapitulating the histologic features of HB, the fetal and embryonal elements are often intermixed, and moreover, cells intermediate between fetal and embryonal may be present making the distinction between these cell types difficult. Stripped nuclei are usually present in the background. In mixed epithelial and mesenchymal HB, the mesenchymal component is usually focal and may not be represented in the aspirate smears. When a mesenchymal component is present, it usually appears as fragments of fibromyxoid tissue with stellate mesenchymal cells, osteoid, or cartilage. Small cell undifferentiated (SCUD) HB is rare and appears as discohesive, uniform, small round cells with round, hyperchromatic nuclei and high nuclear-to-cytoplasmic ratios. These cells are difficult or impossible to distinguish from those of other small round cell tumors of childhood without the assistance of immunoperoxidase stains, and recent evidence suggests that the majority of tumors classified as SCUD HB may represent extrarenal malignant rhabdoid tumors [3, 4].

Immunoperoxidase stains can be used to confirm the diagnosis of HB and help to distinguish fetal HB from well-differentiated HCC and embryonal HB from other poorly differentiated or blastemal tumors of childhood. The malignant cells in fetal and embryonal HB are positive for low molecular weight cytokeratins (Cam5.2, CK8/18), AFP, beta-catenin (nuclear ± cytoplasmic), glypican-3, and HepPar1. The SCUD variant of hepatoblastoma is positive for pankeratin, CK8/CK18, vimentin, and beta-catenin in a nuclear pattern, but is negative for glypican-3 and HepPar1 [3]. In addition, nuclear staining for INI-1 is lost the majority of SCUD HB. The immunophenotype and molecular evidence of INI-1/SMARCB1 deletion suggest that these tumors may actually represent extrarenal rhabdoid tumors [3, 4].

The other entities to consider when making the diagnosis of HB are well-differentiated HCC (more pleomorphism, lack of extramedullary hematopoiesis), yolk sac tumor (positive for germ cell markers such as SALL4), adrenal cortical neoplasm (positivity for adrenal cortical markers, such as Melan-A, calretinin, and inhibin), paraganglioma, and metastatic small round blue cell tumors.

This is the most common type of liver malignancy in children and should be considered before making a diagnosis of HCC in a young person. The SCUD type may actually represent extrarenal rhabdoid tumor and can mimic small cell carcinoma or any small round cell tumor of childhood.

The second most common liver tumor in children is HCC, which is typically seen in older children from 5 to 15 years of age. Risk factors include cirrhosis, viral hepatitis, cholestatic liver disease, such as type 2 progressive familial intrahepatic cholestasis, and inborn errors of metabolism, such as tyrosinemia and type 1 glycogen storage disease. In contrast to classic HCC, the fibrolamellar variant of HCC (FL-HCC) usually develops de novo in adolescence or young adulthood in the setting of a non-cirrhotic liver. Patients with HCC usually present with hepatomegaly, abdominal pain, and weight loss. Serum AFP is often elevated, but may be normal or only slightly elevated in well-differentiated HCC and FL-HCC. On imaging HCC typically appears as a hypervascular mass with or without necrosis and/or vascular invasion. Advanced HCC may be multinodular. FL-HCC is also characterized by a central scar, thereby mimicking FNH.

Aspirates from classic HCC are usually highly cellular with cells arranged in sheets, in clusters, in acinar arrays, and singly with numerous naked nuclei in the background (Fig. 8.3a). In well- and moderately differentiated HCCs, thickened trabeculae (greater than three cells thick) with endothelial wrapping, tissue fragments with transgressing vessels, and nuclear crowding and overlapping are characteristic and help to distinguish these malignancies from benign hepatocellular proliferations (Fig. 8.3). Well- and moderately differentiated HCCs are characterized by relatively monotonous populations of polygonal cells with moderate to abundant, eosinophilic cytoplasm and enlarged, round, centrally placed nuclei with prominent nucleoli. Cytoplasmic lipid vacuoles, bile, hyaline globules and/or Mallory bodies, and intranuclear pseudoinclusions can also be present. The nuclear-to-cytoplasmic ratio is usually increased, although this feature can be subtle in some well-differentiated tumors. In smears from poorly differentiated FL-HCC, the cells are often arranged in loosely cohesive clusters or singly and bear little or no resemblance to hepatocytes. Cytologic and nuclear atypia and pleomorphism can be marked, and bizarre spindle and giant cells may be present. In smears from FL-HCC, the cells are often arranged singly or in loosely cohesive clusters. The cells are larger and have lower nuclear-to-cytoplasmic ratios than normal hepatocytes and those of well-differentiated HCC due to the abundant, granular (oncocytic) cytoplasm, large round nuclei, and macronucleoli. Hyaline globules and/or pale bodies can be seen in some cells (Fig. 8.4). Fragments of dense fibrous stroma with parallel arrays of spindled nuclei are present and, in some cases, prominent and correspond to the lamellar bands of fibrous tissue seen histologically (Fig. 8.4). Extramedullary hematopoiesis is usually not seen in classic or FL-HCC, in contrast to HB.

Immunoperoxidase stains can help to confirm the diagnosis of HCC. The tumor cells show cytoplasmic positivity for AFP, glypican-3 (GPC-3), arginase-1 (ARG-1) and HepPar1 [5], and canalicular staining for CD10 and polyclonal CEA. Stains for AE1 and nuclear beta-catenin are typically negative. A histochemical stain for reticulin is very helpful for demonstrating the loss of normal hepatic cords two cells in thickness and the presence of thickened trabeculae, particularly on a cell block with tissue fragments or on a concurrent core needle biopsy (Fig. 8.3).