Answer: Pilar Sheath Acanthoma

Histologically, there is a vertically oriented epithelial proliferation with a large central cavity that opens to the surface with a narrow ostium. The walls of the cavity are lined with keratinocytes that produce a basket-weave cornified layer that is somewhat thickened. Numerous eosinophilic staining lobular aggregates of cells radiate from the cystic cavity into the surrounding dermis. A single lobule at the base of the lesion demonstrates a central tubule lined with orthokeratotic keratinocytes that communicates with the large central cavity.

Figure 2

Mehregan and Brownstein assigned the term "pilar sheath acanthoma" to 9 such lesions they described first in 1978. [1] Subsequently, several others have described this benign follicular hamartoma, including an in-depth analysis and review by Ackerman. [2-5] As depicted in this case, pilar sheath acanthoma resembles clinically a comedo or dilated pore of Winer, presenting as a small, skin-colored papule or nodule with a central depression or opening. The classic location described for these lesions is the upper lip, although their appearance on other areas of the head and face has been described.[4] They are usually asymptomatic.

Histologically, the differential diagnosis includes dilated pore of Winer and trichofolliculoma. Dilated pore of Winer (also usually a solitary lesion) has the histologic appearance of a large cystic cavity with a gaping ostium at the surface. The cavity is filled with basket weave cornified debris that simulates the normal stratum corneum although it may be thickened and more compact. The lesion extends deeply into the dermis or occasionally to the level of the subcutaneous fat. The most distinctive feature of dilated pore is the extension of thin retia and papillae from the cyst wall into the surrounding dermis. These are distinctly different from the lobulated masses which extend from the cavity of pilar sheath acanthoma, within which a central tubule lined with corneocytes is sometimes visible. Trichofolliculoma is another benign follicular hamartoma with a central keratin-filled crater. Many times there are hair shafts visible within it as well. In groups around the cavity, numerous secondary hair follicles radiate from the wall of the primary follicle. An outer root sheath, inner root sheath and trichohyaline granules are commonly visible in the secondary follicles of a trichofolliculoma, features not seen in pilar sheath acanthomas.

Pilar sheath acanthoma is an interesting, uncommon, benign neoplasm of the skin not associated with other disorders. No tendency toward malignancy has been described

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