The epithelial elements of pancreatoblastomas are highly cellular and arranged in well-defined islands separated by stromal bands, producing a 'geographic' low power appearance. Solid, hypercellular areas composed of nests of polygonal cells alternate with regions showing more obvious acinar differentiation, with polarized cells surrounding small luminal spaces. In rare tumours, larger glandular spaces lined by mucin-containing cells may be seen {939}. Nuclear atypia is generally minimal. Squamoid corpuscles. One of the most characteristic features of pancreatoblastoma is the 'squamoid corpuscle'. These enigmatic structures vary from large islands of plump, epithelioid cells to whorled nests of spindled cells to frankly keratinizing squamous islands. The nuclei of the squamoid corpuscles are larger and more oval than those of the surrounding cells; nuclear clearing due to the accumulation of biotin may be seen {1895}. The frequency and composition of the squamoid corpuscles varies in different regions of the tumour and between different cases. Stroma. Especially in pediatric cases, the stroma of pancreatoblastomas is often hypercellular, in some instances achieving a neoplastic appearance. Rarely, the presence of heterologous stromal elements, including neoplastic bone and cartilage, has been reported {127, 939}.

Histochemistry and immunohistochemistry

Over 90% of pancreatoblastomas exhibit evidence of acinar differentiation in the form of PAS-positive, diastase resistant cytoplasmic granules as well as immuno-histochemical staining for pancreatic enzymes, including trypsin, chymo-trypsin, and lipase {939, 1282, 1400}. The staining may be focal, often limited to the apical cytoplasm in areas of the tumour with acinar formations. At least focal

Fig. 10.33 Pancreatoblastoma with squamoid corpuscule (arrowhead), surrounded by solid (left) and tubular (right) structures.

immunoreactivity for markers of endocrine differentiation (chromogranin or synapto-physin) is found in over two-thirds of cases, and expression of markers of ductal differentiation such as CEA, DUPAN-2, or B72.3 is found in more than half of cases {939}. In most instances, the proportion of cells expressing acinar markers outnumbers the proportion expressing endocrine or ductal markers. In cases associated with elevations in the serum levels of alpha-fetoprotein, immunohisto-chemical positivity for AFP has been detectable {802, 939}. Immunohistochemical evaluation of the squamoid corpuscles has failed to define a reproducible line of differentiation for this component {939}.

Relationship to acinar cell carcinoma

Both pancreatoblastomas and acinar cell carcinomas consistently exhibit acinar differentiation and may exhibit lesser degrees of endocrine and ductal differentiation. {936, 939}. Histologically, aci-nar formations are characteristic of pan-creatoblastoma, and the solid areas resemble the solid pattern of acinar cell carcinoma. Biologically, the two tumours are also similar, with a relatively favorable prognosis in childhood, but a very poor prognosis in adulthood. For these reasons, some observers have suggested that pancreatoblastoma represents the paediatric counterpart of acinar cell carcinoma. Although this proposal is attractive in many ways, pancreatoblastoma remains a separately definable neoplasm with characteristic histologic, immunohis-tochemical, and clinical features.

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