Extramedullary hematopoiesis as a 'clue' to diagnosis of hepatoblastoma on fine needle aspiration cytology: A report of two cases.

Extramedullary hematopoiesis (EMH) evidenced by erythropoietic cells and megakaryocytes is a characteristic feature of hepatoblastoma (HB). The typical cytomorphology, the presence of EMH and associated clinical and radiological findings offer a reliable diagnosis of hepatoblastoma by fine-needle aspiration cytology (FNAC). We describe the cytologic features of hepatoblastomas and discuss the differential diagnosis in two children, aged 53 days and 19 years. The usefulness of EMH in differentiating HB from other small round cell tumors and well differentiated hepatocellular carcinoma (HCC) on cytology is highlighted.

Introduction

Hepatoblastoma (HB) is an uncommon primary malignant hepatic tumor in children, primarily affecting infants. Isolated cases are described in older children and adults. It shows association with a variety of congenital abnormalities. Usual presentation is an abdominal mass and rarely as virilization due to ectopic sex hormone production. Serum alfa-fetoprotein (AFP) level is often elevated. The cytological and architectural criteria for the diagnosis have been studied extensively. Different histological types like pure epithelial, mixed, macrotrabecular, and small cell anaplastic are described. Foci of extramedullary hematopoiesis (EMH) characterized by erythroblasts and megakaryocytes are seen on conventional histology and by immunohistochemistry.[12] Although EMH is an important ‘clue’ on fine needle aspiration cytology (FNAC) for the diagnosis of HB and its differentiation from other small round cell tumors of childhood and also from hepatocellular carcinoma (HCC), its importance has not been adequately stressed.[13]

Case Reports

Case 1

A 53-day-old male baby presented with an abdominal mass at birth. Routine investigations showed hemoglobin of 5.6 g% and platelet count of 3.2 lakhs/cu.mm. Computed tomography (CT) scan showed a mixed nonhomogenous mass in the abdomen extending to the abdominal wall, compressing the liver, and reaching up to the bladder [Figure 1a]. With a possibility of neuroblastoma, FNAC was performed. Cellular smears showed cells in sheets, trabecular pattern, and occasional rosette formation. The cells exhibited moderate cytoplasm and round nuclei with condensed chromatin; a few cells displayed intracytoplasmic yellowish-green pigment indicating bile. Also seen were a few scattered megakaryocytes and erythroblasts suggesting the evidence of EMH [Figure 1b]. In view of these cytologic findings a diagnosis of ‘small round cell neoplasm suggestive of HB was offered, with an advice to correlate with the serum alpha-fetoprotein (AFP) levels. Serum AFP level was elevated to 2000 ng/mL (normal <10 ng).

Figure 1

(a) CT scan of case 1 showing non-homogenous mass in the liver. (b) Photomicrograph showing malignant cells in sheets and trabeculae (Pap, ×100); inset shows a megakaryocyte (Giemsa, ×400). (c) CT scan of case 2 showing massive hepatomegaly with a large hypodense lesion in right lobe and multiple small lesions. (d) Photomicrograph of malignant cells arranged in sheets and trabecular pattern (H and E, ×100); inset shows megakaryocyte (Pap, ×400)

Case 2

A 19-year-old male presented with a hypochondrial mass. Investigations showed a hemoglobin of 7.9g%; platelet count of 8.9 lakhs/cu mm; Liver function test was within normal limits, and serum AFP level was 1888 ng/mL. CT scan showed massive hepatomegaly with large hypodense lesion along with multiple small lesions in the right lobe [Figure 1c]. No associated cirrhosis or intra-abdominal lymphadenopathy was noted. FNAC smears were markedly cellular with sheets and clusters of cells having moderate to abundant granular/clear cytoplasm and central round nuclei with dense chromatin. Cells showed minimal nuclear pleomorphism and inconspicuous nucleoli. No intranuclear inclusions or bare nuclei were seen. A few singly scattered megakaryocytes and erythroblasts were seen [Figure 1d]. A diagnosis of HB was offered.

Discussion

HB is a malignant hepatic tumor predominantly of infants with isolated cases described in older children and adults.[4] Four percent of tumors are present at birth, 68% in the first 2 years, and 3% over 15 years of age.[5] The median age at presentation is 18 months. It is an embryonal neoplasm arising from multipotential blastomatous cells, capable of differentiating into epithelial (resembling fetal hepatocytes to embryonal cells) and mesenchymal cell lines. Beckwith-Wiedemann syndrome, trisomy-18, trisomy-21, Prader — Willi syndrome, familial adenomatous polyposis, and extremely low birth weight are some of the conditions reported in association with HB.[25] The other conditions include Wilms tumor, gonadoblastoma, and glycogen storage disease.[25] Clinically, 80% cases occur as single liver mass (involving the right lobe in 57%, left lobe in 15%) and 20% cases are present as multiple masses.[5] Metastases from HB occur almost always to the lungs. Anemia and thrombocytosis may occur as paraneoplastic syndromes.[5] Serum AFP levels are frequently elevated.[4] Hepatic angiography and CT scan are useful in the preoperative assessment of the location and extent of the tumor. Both our cases were anemic; one of them presented with thrombocytosis and multiple nodules in the liver.

Cytology of HB depends on the histologic subtype and shows cells arranged in three dimensional clusters, loose sheets, cords, rosette-like structures, and occasionally pseudopapillae. The epithelial type of HB is the most common subtype; it shows uniform cells with abundant cytoplasm, well-defined cell membranes, and large hyperchromatic nuclei. Mesenchymal type is characterized by spindle cells exhibiting hyperchromatic nuclei and poorly defined cell membranes. The small-cell subtype (anaplastic) shows discohesive cells with scarce cytoplasm and hyperchromatic nuclei.[35] Foci of EMH is a characteristic histologic feature of HB.[1234] They are consistently associated with epithelial histology; more in the fetal epithelial and less differentiated embryonal subtypes.[1] Erythropoietic cells and megakaryocytes are seen, but granulocytic precursors are seldom encountered. Hematopoiesis in HB is located in tumor sinusoids, or in intimate association with the tumor cells suggesting that the microenvironment is an important contributory factor for hematopoiesis in HB.[1]

The molecular mechanisms of EMH in HB are still undefined. It is suggested that HB tissue retains circulating CD 34 positive multipotential progenitors of bone marrow origin, which proliferate and differentiate to more mature erythroid precursors and megakaryocytes.[1] HB cells as such are clearly differentiated, so it is unlikely that they themselves undergo a transition to hematopoietic stem cells. Various cytokines such as erythropoietin, stem cell factor, and IL-1β are expressed by the epithelial HB cells. By paracrine stimulation, these cytokines can initiate stromal cell production of other hematopoietic cytokines including IL-6, G-CSF, GM-CSF, M-CSF and leukemia inhibitory factor. These may have a role in EMH.[1]

The differential diagnosis for HB in children include small round cell tumors, metastatic Wilms’ tumor, neuroblastoma, rhabdomyosarcoma and well differentiated HCC.[345] In our first case the clinical diagnosis was neuroblastoma. The embryonal cytology and occasional rosette formation added to the diagnostic dilemma. But the cells exhibiting moderate cytoplasm with bile pigment and EMH on smears prompted us advising serum AFP, and an elevated AFP levels assisted in the diagnosis of HB. The finding of EMH in liver of infants is not unusual; however, EMH seen along with neoplastic round cells on radiologically-guided liver FNAC helps to diagnose HB and differentiates it from neuroblastoma. In the second case, differentiated neoplastic cells suggested well differentiated HCC and HB in the differential diagnosis.[24] The presence of EMH and the absence of typical features of HCC allowed the correct diagnosis of HB.

Treatment of choice for HB is surgical resection and adjuvant chemotherapy. Preoperative chemotherapy is indicated in unresectable lesions and liver transplantation is useful. Prognosis depends on serum AFP levels and the ability to resect the lesion entirely. Overall, 5-year survival has improved from 30% to over 70%, and is now approximately 90% for the majority of children who present with standard risk disease. After the cytologic diagnosis, our first case was lost to follow up while the second case underwent surgical resection in another hospital. The diagnosis of HB was confirmed in the latter patient who is currently being followed up.

To conclude, HBs are rare lesions and their preoperative identification helps in an early management of patients. Extramedullary hematopoeisis and the presence of intracytoplasmic bile pigment need more emphasis as ‘useful clues’ for their accurate cytologic diagnosis.