Pilar and sebaceous neoplasms

Pilar neoplasms

Pilar neoplasms differentiate towards (resemble) various parts of the normal hair follicle. They are named according to what they resemble. Before reading this chapter, review the discussion of hair anatomy in Chapter 1. Blue pilar tumors differentiate towards elements of the inferior segment of the hair follicle. Red pilar tumors differentiate towards the isthmus and infundibulum. Clear cell tumors differentiate towards the glycogenated outer root sheath.

Benign trichoblastomas are large basaloid follicular neoplasms. The tumor islands resemble basal cell carcinoma, but the stroma resembles the normal fibrous sheath of the hair follicle. Trichoepitheliomas and lymphadenomas are distinctive forms of benign trichoblastoma. Trichogerminomas are a type of trichoblastoma with differentiation towards the hair germ. Basal cell carcinoma is the most common malignant counterpart of a benign trichoblastoma. Some trichoblastic carcinomas arising in long-standing trichoblastomas have been very aggressive tumors with metastases.

Trichoepithelioma

Key features

• Distinctive type of trichoblastoma

• Blue tumor composed of basaloid cells

• At scan, finger-like projections and cribriform (Swiss-cheese) nodules

• Cells resemble those of basal cell carcinoma

• Stroma resembles the normal fibrous sheath of the hair follicle, with concentric collagen and many fibroblasts (as with any other trichoblastoma)

• Papillary mesenchymal bodies typically prominent

• Mucin may be present within cribriform tumor islands, but never in the stroma

• No retraction artifact

Trichoepitheliomas commonly present as multiple small papules in the nasolabial folds. The multiple type is inherited in an autosomal-dominant fashion. Each papule is composed of basaloid islands in a fibroblast-rich stroma with papillary mesenchymal bodies. Horn cysts and calcification are common. Small clefts may occur between collagen fibers of the tumor stroma, but not between the tumor epithelium and stroma. Papillary mesenchymal bodies are round collections of plump mesenchymal cells resembling those in the follicular papilla.

Table 4-1

Characteristics of trichoepithelioma versus basal cell carcinoma

Characteristic	Trichoepithelioma	Basal cell carcinoma
Basaloid cells	Yes	Yes
Peripheral palisading	Yes	Yes
Finger-like and cribriform	Yes	Sometimes
Stroma	Concentric, fibroblast-rich	Myxoid
Mucin	In tumor islands only, none in stroma	Metachromatic mucin in stroma
Papillary mesenchymal bodies	Common	Rare
Horn cysts	Common	Rare
Calcification	Common	Rare
Clefts	Between collagen fibers within stroma	Between epithelium and stroma
CD34 staining	Strong staining in stroma	+/−
BCL-2 staining	Periphery of islands	Strong, diffuse
CK20+ Merkel cells	Present in tumoral islands	Absent in tumoral islands