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Follicular Neoplasms


22.1

Trichofolliculoma

(see Fig. 22.1A–E )

Fig. 22.1, A Trichofolliculoma (low mag.).

Fig. 22.1, B Trichofolliculoma (high mag.).

Fig. 22.1, C Dilated pore of Winer.

Fig 22.1, D Pilar sheath acanthoma.

Fig. 22.1, E Folliculosebaceous cystic hamartoma.

Uncommon solitary papule on the face (1.44), sometimes with a plugged orifice in which vellus hairs are seen.

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    Large open or closed comedo-like lesion (sometimes resembling a cyst if there is no orifice) into which numerous small hair follicles with trichohyaline granules and vellus hairs open

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    Fibrotic stroma

Variations

  • 1.

    Dilated pore of Winer: solitary 1–3 mm orifice on face, buds of epithelium with no associated hair shafts connect into open comedo (1.24).

  • 2.

    Pilar sheath acanthoma: usually on upper lip , buds of epithelium from open comedo more massive , without hair shafts in the buds.

  • 3.

    Trichostasis spinulosa (10.4): common finding of multiple vellus hairs within a comedonal plug , either as an acneiform clinical condition of open comedones (1.24) or as an incidental histologic finding, not apparent clinically, in many different lesions, particularly melanocytic nevi.

  • 4.

    Folliculosebaceous cystic hamartoma (sebaceous trichofolliculoma): sebaceous glands associated with the comedo-like lesion, in adults.

Differential diagnosis

  • 1.

    Dermoid cyst (19.3): larger, congenital cyst, with larger hairs and hair shafts.

22.2

Trichoepithelioma (TE)

(see Fig. 22.2A–F )

Fig. 22.2, A Trichoepithelioma (low mag.).

Fig. 22.2, B Trichoepithelioma (medium mag.).

Fig. 22.2, C Trichoepithelioma (high mag.).

Fig. 22.2, D Desmoplastic trichoepithelioma.

Fig. 22.2, E Trichoadenoma (low mag.).

Fig. 22.2, F Trichoadenoma (medium mag.).

Somewhat common skin-colored papule or nodule (1.129), most common on the central face (1.44). Lesions more commonly are solitary , but multiple trichoepitheliomas with childhood onset may be associated with cylindromas (23.4) and spiradenomas (23.11), inherited as the autosomal dominant Brooke–Spiegler syndrome, related to the CYLD gene mutation. Trichoepithelioma is a benign neoplasm, even though the antiquated word epithelioma usually means low-grade carcinoma. It can sometimes be very difficult to distinguish trichoepithelioma from a basal cell carcinoma (basal cell epithelioma).

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    Circumscribed basaloid tumor islands , often in a reticulated pattern or cribriform pattern (interconnecting cords), sometimes resembling poorly developed hair follicles

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    Horn cysts common

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    Peripheral palisading of nuclei, but no artifactual retraction between tumor and stroma

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    Loose stroma with many fibroblasts surrounds basaloid islands

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    Papillary mesenchymal bodies (clusters of fibroblasts adjacent to epithelial buds as in the germinative portion of the normal hair papilla)

Variations

  • 1.

    Trichoadenoma of Nikolowski: tumor cells are not basaloid and are more squamous like normal epithelium, having more pink cytoplasm and a pale nucleus, horn cysts numerous .

  • 2.

    Desmoplastic trichoepithelioma: thinner strands of basaloid cells , horn cysts usually present, resembling a sclerosing basal cell carcinoma.

  • 3.

    Basaloid follicular hamartoma: small papules , often multiple, usually on the face, vertically-oriented basaloid cells centered upon a comedo-like follicle .

Differential diagnosis

  • 1.

    Multiple facial papules may cause clinical confusion, but are much different histologically: Cowden’s disease (22.5), Birt–Hogg–Dube syndrome (22.6), and tuberous sclerosis (27.3).

  • 2.

    Keratotic basal cell carcinoma (BCC, 20.14) or BCC with follicular differentiation is more likely to have a pearly translucent clinical appearance rather than a fleshy papule or nodule with minimal history of growth, solar elastosis, stromal retraction, ulceration, necrosis, fewer fibroblasts in the stroma, less hair papilla formation with papillary mesenchymal bodies, and bcl-2 expression more than just in the basal layer. CK20 positive Merkel cells are more likely to populate a trichoepithelioma than a BCC. CD34 is more likely to stain stroma immediately against the basaloid aggregates in trichoepithelioma, with a rim of sparing around basaloid aggregates of BCC, but this is highly overrated as helpful. Desmoplastic trichoepithelioma is more likely to be p75 +, PHLDA1 +, CK20 + for Merkel cells, and less likely to be CK7 +, CK15 + and androgen receptor positive than BCC. Androgen receptor can be positive in ordinary conventional trichoepitheliomas. Because it may be difficult to be sure of the distinction, a solitary trichoepithelioma in sun-damaged skin is usually best treated as if it were a BCC (when in doubt, cut it out).

  • 3.

    Trichoblastoma (22.7): The WHO classification of tumors lumps these lesions together with trichoepitheliomas. Trichoepithelioma can be considered by splitters to be a superficial type of trichoblastoma with a cribriform or germinative pattern.

  • 4.

    Microcystic adnexal carcinoma (23.13) is larger and more infiltrating, has sweat ducts in addition to the horn cysts, more likely to be CK7 + and CK15 +.

  • 5.

    Other basaloid neoplasms (1.11).

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