Pathologic Features

Macroscopically, the tumour usually appears as a white, firm, poorly defined, exophytic, polypoid, and centrally ulcerated mass with peripheral submucosal infiltration [21].

Microscopically, BSCCs are composed of small, closely packed basaloid cells, with hyperchromatic nuclei with or without nucleoli, and scant cytoplasm (Fig. 1.17a). The tumour grows in a solid pattern with a lobular configuration, with a frequent peripheral palisading of nuclei. Large central necroses of the comedo type are frequent. Distinctive features of BSCCs that are not found in conventional SCCs are small cystic spaces containing para aminosalicylate (PAS)- and Alcian blue-positive material and focal stromal hyalinisation [19, 372].

BSCCs are always associated with an SCC component, which can present either as an in situ or invasive SCC. The invasive SCC is usually located superficially, and is typically well- to moderately differentiated. It may also present as focal squamous differentiation within the basaloid tumour islands. The transition between the squamous cells and the basaloid cells is often abrupt (Fig. 1.17b), or there may be a narrow zone of transition.

If there is extensive ulceration, only dysplastic changes may be identifiable in the intact surface epithelium [19, 21]. Rarely, BSCCs exhibit a malignant spindle cell component [21, 250]. Metastases may demonstrate basaloid carcinoma, squamous carcinoma, or both [21].

By electron microscopy, desmosomes and tonofila-ments were demonstrated in basaloid cells and in squa-mous cells. There were no neurosecretory granules, myofilaments or secretory granules [154, 372].

Immunohistochemically, BSCCs express keratin and epithelial membrane antigen, but the percentage of positive cells varies among different reports. It is advised to use a cocktail of keratin antibodies (i.e. CAM 5.2, AE-1-AE3) to avoid false-negative results [21]. Some cases express carcinoembryonic antigen and neuron-specific enolase [19, 195, 318], while expression of S-100 protein, vimentin and muscle-specific actin varied among different reports. Vimentin was negative in some studies [69, 195], while Barnes et al. [21] described positive staining in the majority of basaloid cells, with a peculiar pattern of staining, forming a delicate perinuclear rim. Varying results have also been reported for S-100 immu-noreactivity. Some authors described focal immunore-activity in a few cases [19, 21], while others did not find any S-100-positive tumour cells [69, 195, 248]. However, most cases displayed numerous S-100-positive dendritic cells intermingled with the tumour cells [9, 21, 195, 248]. BSCCs do not express chromogranin, synaptophy-sin and GFAP [19, 21, 195].

Fig. 1.17. Basaloid squamous cell carcinoma. a Closely packed basaloid cells with hy-perchromatic nuclei and scant cytoplasm, with focal peripheral palisading of nuclei. b Abrupt transition between squamous and basaloid cells. c Focal squamous differentiation in adenoid cystic carcinoma. Courtesy of Dr. Pieter J. Slootweg

Fig. 1.17. Basaloid squamous cell carcinoma. a Closely packed basaloid cells with hy-perchromatic nuclei and scant cytoplasm, with focal peripheral palisading of nuclei. b Abrupt transition between squamous and basaloid cells. c Focal squamous differentiation in adenoid cystic carcinoma. Courtesy of Dr. Pieter J. Slootweg

1.3.6.3 Differential Diagnosis

Differential diagnosis includes neuroendocrine carcinoma, adenoid cystic carcinoma, adenocarcinoma and adenosquamous carcinoma.

Neuroendocrine carcinomas express various neuroendocrine markers that help to distinguish neuroendocrine carcinomas from BSCCs. However, as 60-75% of cases of BSCC have been reported to express neurone-specific enolase [19, 65, 318] the application of other neuroendocrine markers, including chromogranin, CD56, and synaptophysin, is advised [19, 65].

Adenoid cystic carcinomas, especially the solid variant, may resemble BSCCs but adenoid cystic carcinomas rarely show squamous differentiation (Fig. 1.17C). Im-munohistochemistry may also be helpful: tumour cells in adenoid cystic carcinomas express S-100 protein and vimentin, while tumour cells in BSCCs usually do not express either of the two markers [21, 195].

Adenocarcinomas and adenosquamous carcinomas can be distinguished from BSCCs by the presence of gland formation and mucin secretion within the tumour cells.

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