TRICHOBLASTOMA
A recent definition of trichoblastomas by Headington1 states: "Trichoblastomas are rare tumors of hair germ that are purely epithelial and therefore lack a potentially inductive stroma. They tend to present as discrete, circumscribed subcutaneous  nodules in non-sun-exposed areas." Though the prototypic trichoblastoma is subcutaneous, this definition does not exclude the possibility of such a discrete, circumscribed tumor of hair germ epithelium occurring in the dermis.

Ackerman, et al2, are more liberal in the definition and include a wide variety of tumors of hair germ epithelium and, sometimes, stroma under the term trichoblastoma as long as they have 'benign' profiles.  In their terminology the adjective large as in large nodular trichoblastoma refers to the size of the multiple subunits of the tumor. For purposes of distinction, I would propose spheronodular trichoblastoma for the tumor presented. How about uninodular trichoblastoma?

A tumor, such as the one presented here, might also be classified as a basal cell carcinoma (nodular) in that it is relatively monomorphic with regard to the follicular components represented. This is in contrast to multiple follicular components being expressed in tumors that are accepted to be benign such as traditional trichoepitheliomas and trichofolliculomas. Peripheral palisading of basaloid nuclei, minimal retraction of tumor nests, adenoid and cystic foci containing mucin, etc. would support a diagnosis of basal cell carcinoma. However, the lack of significant mitotic activity plus the growth pattern that only expresses expansile properties provide less than compelling evidence of aggressive biological behavior. To place this tumor in the same category as a basal cell carcinoma having widely scattered small nests of tumor would be inappropriate. If one were to come up with a Working Classification of Adnexal Tumors, this lesion might be designated as a 'simple, primordial, follicular tumor with benign or expansile properties'.

References:
1) Headington JT. Tumors of hair follicle differentiation. In: Farmer ER, Hood AF, eds. Pathology of the Skin 2nd Edition, East Norwalk, Conn: Appleton & Lange; 2000:1015 & 1016.

2) Ackerman AB, de Viragh PA, Chongchitnant N. Neoplasms With Follicular Differentiation. Philadelphia, Pa: Lea & Febiger;1993:359-420.

An indolent, pigmented tumor from the posterior aspect of the left shoulder of a 75 year old male:
 
Very low power view (direct scan of glass slide). The tumor is a solitary, discrete, circumscribed, expansile nodule restricted to the dermis.
A high power view from the superficial aspect of the specimen. An epidermal connection was not demonstrated. Note the peripheral palisading of nuclei and the minimal degree of  retraction of the tumor from the superficial dermis.
A medium power view. Adenoid patterns and the top of a cystic component are seen. Tissue mucin (stringy, basophilic material ) is in some of the spaces. A pigmented area is also present.
Another medium power view. Peripheral palisading of nuclei, tissue mucin accumulations, and minimal retraction of the tumor from the adjacent stroma are illustrated. A thin band of nondescript tumor stroma separates the epithelium from the dermal collagen.

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