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Small cells that reflect neither epithelial nor stromal differentiation.
Embryonal epithelial cells resembling the liver epithelium at 6 to 8 weeks of gestation.
Well-differentiated fetal hepatocytes morphologically indistinguishable from normal fetal liver cells.
Most often the tumor consists of a mixture of epithelial hepatocyte precursors. About 20% of tumors have stromal derivatives such as osteoid, chondroid, and rhabdoid elements. Occasionally neuronal, melanocytic, squamous, and enteroendocrine elements are found. Two histologic subtypes have clinical relevance: pure fetal histology throughout the tumor and foci of small cell undifferentiated cells.
Pure fetal histology hepatoblastoma
Analysis of patients with initially resected hepatoblastoma tumors (prior to receiving chemotherapy) has suggested that those patients with pure fetal histology tumors have a better prognosis than those having an admixture of more primitive and rapidly dividing embryonal components or other undifferentiated tissues. In a study of patients with hepatoblastoma and pure fetal histology tumors, there was a 100% survival rate for patients who received four doses of single-agent doxorubicin. This suggested that patients with pure fetal histology tumors might not need chemotherapy after complete resection of a stage I tumor.[2,3] In the Children's Oncology Group (COG) study COG-P9645, 16 patients with stage I pure fetal histology hepatoblastoma with two or fewer mitoses per 10 high power fields were not treated with chemotherapy. Their retrospective PRETEXT stages were stage I (n = 4), stage II (n = 6), and stage III (n = 2). Survival was 100% with no chemotherapy given. All 16 patients entered on this study were alive with no evidence of disease at a median follow-up of 4.9 years (range, 9 months to 9.2 years). Thus, complete resection of a pure fetal hepatoblastoma may preclude the need for chemotherapy.
Small cell undifferentiated hepatoblastoma
Small cell undifferentiated hepatoblastoma is an uncommon hepatoblastoma variant that represents a few percent of all hepatoblastomas. It tends to occur at a younger age (6–10 months) compared with other cases of hepatoblastoma [5,6] and is associated with AFP normal for age at presentation.[5,7]
Histologically, small cell undifferentiated hepatoblastoma is typified by a diffuse population of small cells with scant cytoplasm resembling neuroblasts. The chromosomal abnormalities reported for small cell undifferentiated hepatoblastoma are distinct from those occurring in other hepatoblastoma subtypes and are more similar to those observed in malignant rhabdoid tumors. These abnormalities include translocations involving a breakpoint on chromosome 22q11 and homozygous deletion at the chromosome 22q12 region that harbors the SMARCB1/INI1 gene.[5,9] Lack of detection of INI1 by immunohistochemistry is another characteristic shared by some small cell undifferentiated hepatoblastomas and malignant rhabdoid tumors. A third characteristic shared between small cell undifferentiated hepatoblastomas and malignant rhabdoid tumors is the poor prognosis associated with each.[5,6,10] Patients with small cell undifferentiated hepatoblastoma whose tumors are unresectable have an especially poor prognosis. Patients with stage I tumors appear to have increased risk of treatment failure when small cell elements are present. For this reason, completely resected tumors composed of pure fetal histology or of mixed fetal and embryonal cells must have a thorough histologic examination as small foci of undifferentiated small cell histology indicates a need for aggressive chemotherapy. Aggressive treatment for this histology is under investigation in the current COG study, COG-AHEP0731. Hepatoblastoma that would otherwise be considered very low or low risk is upgraded to intermediate risk if any small cell undifferentiated elements are found (refer to the Stage Information section of this summary for more information).