Basaloid epithelial cells, which vary from small, dark cells to larger, paler stained cells, form histomorphologic patterns that are described as solid, membranous, trabecular and tubular. A solid pattern, in which variable sized and shaped nests are separated by thin septa or thick bands of collagenous stroma, is most frequent. In the membranous type, tumours produce excessive amounts of eosinophilic, hyalinized basal lamina material that forms intercellular droplets and peripheral membranes. Interconnecting bands of basaloid cells characterizes the trabecular growth pattern. In the tubular type, there are luminal spaces among the basaloid cells. There are foci of squamous differentiation in some tumours. The nuclei of tumour cells along the interface with the collagenous stroma are often palisaded. The degree of cytologic atypia and the number of mitotic figures varies from one tumour to another but is often quite minimal. Infiltration of tumour cells into parotid parenchyma, dermis, skeletal muscle, or periglandular fat distinguishes basal cell adenocarcinoma from basal cell adenoma. Vessel or peripheral nerve invasion is evident in about a fourth of the tumours.


Immunohistochemical staining is variable among tumours. Tumour cells are reactive for cytokeratins and often focally reactive for S100 protein, epithelial membrane antigen, and carcinoembryonic antigen. Limited reactivity for smooth muscle actin and vimentin supports myoepithelial differentiation of some cells {2097,2793}.

Fig. 5.15 Basal cell adenocarcinoma. A Basal cell adenocarcinoma, parotid gland. Invasive growth. B Abundant, prominently eosinophilic basal lamina material within and around nests of tumour is characteristic of the membranous pattern of basal cell adenocarcinoma.
Fig. 5.16 Basal cell adenocarcinoma. A Basal cell adenocarcinoma of the parotid gland with solid pattern of growth. B Small, circular areas of squamous cells, sometimes with keratinization (bottom centre) occur in some basal cell adenocarcinomas.
Fig. 5.17 Basal cell adenocarcinoma. A Alignment of the nuclei of the tumour cells along the stromal interface creates a palisaded appearance. B Nests of tumour cells infiltrate into adjacent parotid gland parenchyma.

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