INTRODUCTION The trichilemmal cyst is also referred to as a sebaceous or pilar cyst. It is derived from the outer root sheath of the deeper parts of a hair follicle and consists of a well-keratinized epidermal wall surrounding semi-solid hair keratin and cholesterol-rich debris, rather than just sebaceous material. Trichilemmal cysts are more common in females and most cases occur in the sixth and seventh decades of life. They differ from epidermoid cysts in that they lack a granular layer in the lining epithelium. Foci of proliferating cells and mitoses can occasionally be seen producing the proliferating trichilemmal cyst or pilar tumor. Some authors regard these as malignant tumors.
CLINICAL PRESENTATION Clinically the trichilemmal cyst is indistinguishable from the epidermoid cyst.
Unlike the epidermoid cyst no central punctum is seen. They occur on hair-bearing skin including the scalp and face as single or multiple round smooth, mobile cysts within the dermis that may rupture and discharge a thick stringy, cheesy white material. Occasionally they may form a cutaneous horn. The wall can rupture the contents into the dermis resulting in an inflammatory reaction or infection, becoming red and tender with the formation of granulation tissue which at times can resemble a well-differentiated squamous cell carcinoma.
HISTOPATHOLOGY Trichilemmal cysts have a fibrous capsule on which rests a layer of darkly staining basal cells. The basal cells merge with stratified squamous epithelium whose cells increase in size and vertical diameter as they mature and abruptly change into the eosinophilic-staining keratin within the lumen. The epithelial lining lacks a granular cell layer. The keratinous material in the lumen commonly has cholesterol clefts.
DIFFERENTIAL DIAGNOSIS The differential diagnosis includes dermoid cyst, epidermoid cyst, pilomatrixoma, acne comedones, dermatofibroma, lipoma, milia, and cystic squamous cell and basal cell carcinoma.
TREATMENT Treatment of trichilemmal cysts is not necessary. When of cosmetic concern they are removed surgically with excision of the intact cyst wall. The rare proliferating trichilemmal cyst can be locally aggressive and rare cases of malignant transformation have bee reported. It is important to distinguish them from squamous cell carcinoma. They should be completely excised with clear margins.
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