Info

Lobular architecture

Present

Present

±

+

Mitotic activity

Absent

Present

Prominent

Marked

Nuclear pleomorphism

Absent

Moderate

Prominent

Marked

Fibrillary matrix

Prominent

Moderate

Slight

Absent

Rosettes

H-W +

H-W +

Flexner +

Absent

Necrosis

Absent

Absent

Occasional

Common

Calcification

±

+

Absent

Absent

H-W Homer-Wright rosettes, + present or absent

H-W Homer-Wright rosettes, + present or absent

Fig. 2.13. Primitive neuroectodermal tumour (PNET). a Monotonous proliferation of small, round, undifferentiated cells. b CD99-positive immune reaction at the cellular membrane

depict different histological patterns that may be predominantly papillary, glandular, compact, mucinous or mixed [17, 23]. Papillary tumours mainly consist of elongated outgrowths lined by intestinal-type cells with markedly atypical pseudostratified nuclei (Fig. 2.14a). Although most of them are high-grade tumours, low-grade forms (Fig. 2.14b) mimicking colonic villous adenoma may occasionally occur [174]. The glandular pattern resembles common-type intestinal adenocarcinoma. Compact or solid forms show poorly differentiated nests of cells in which glandular formation is rarely seen. In the muci-nous pattern, more than 50% of the tumour is composed of dilated mucin-filled glands lined by columnar mu-cin-secreting epithelium, and lakes of mucin containing fragmented epithelial elements (Fig. 2.14c). Other muci-nous tumours show mucin-filled cells with the pattern of "signet-ring" cell carcinoma. Various attempts have been made to correlate histopathological grading and typing with clinical behaviour [78, 81, 140].

Features such as cytologic atypia, high mitotic rate and areas of necrosis, which are common findings in most intestinal-type adenocarcinomas, help to distinguish the high-grade variants from rare low-grade intestinal-type adenocarcinomas and from mucoceles. The lack of epi-dermoid and squamous differentiation separates these tumours from mucoepidermoid and adenosquamous carcinomas. Immunohistochemistry and electron microscopy have confirmed the enteric differentiation of the tumour cells [24]. These tumours are positive for a wide spectrum of cytokeratin markers, whereas they are only occasionally positive for carcinoembryonic antigen [166]. Intestinal-type adenocarcinomas are frequently but not always positive for cytokeratin 7, while most express cytokera-tin 20 and CDX-2, two markers related to intestinal differentiation [79]. The prognosis for high-grade intestinal-type adenocarcinoma is poor. Recurrences and subsequent deeply invasive local growth are frequent; however, lymph node and distant metastases are rare [17, 78, 142]. Treatment of choice is complete surgical resection followed by radiotherapy.

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