Keratin filled microcysts associated with thin strands of  epithelium embedded in dense stroma are features seen in desmoplastic trichoepitheliomas, microcystic adnexal carcinomas, and, less commonly, in syringomas and basal cell carcinomas. The type of epithelium and stroma seen in this particular tumor are not characteristic of a basal cell carcinoma, so that possibility will not be discussed here.

Microcystic adnexal carcinomas are larger and more deeply invasive than the tumor in this case.

 The keratin filled microcysts, when seen in syringomas, are restricted to the superficial part of the tumor. This localization may or may not be present in desmoplastic trichoepitheliomas. Empty tubules or tubules containing amorphous material are more characteristic of syringomas. Intracytoplasmic lumen formation within the epithelium in syringomas is an important differential point. The dense appearance of the stroma in syringomas results from the close spacing of moderately large collagen bundles. The circumferential arrangement of more widely spaced collagen bundles that are relatively small is characteristic of desmoplastic trichoepitheliomas.

Finally, syringoma is the only tumor among those mentioned that may present as numerous small papules.

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One of about fifty papules on the face near the eyes of an adult:

Microcysts (arrowheads) are found superficially in this lesion which has dense stroma (cf. the normal dermis in the right side of the picture). Some of the microcysts contain keratin.

A keratin-filled microcyst is attached by a thin strand of epithelium to a ductal structure having amorphous, granular material in the lumen. The ductal structure has syringomatous features.

Lumina that are almost empty. Note the dense collagenous stroma. This combination is very characteristic of a syringoma.

Intracytoplasmic lumen formation within epithelium.

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