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WHO Classification of Tumours
Trichoepithelioma
Skin


Definition

Trichoepithelioma (trichoblastoma) is a benign hair follicle neoplasm with trichoblastic differentiation. The tumour typically presents as a small (<1cm; rare cases up to several cm), solitary papule, or, particularly in association with Brooke-Fordyce and Brooke-Spiegler disease, as multiple centrofacial papules or nodules. Trichoepithelioma most frequently affects head and neck, but it may arise on any location bearing hair follicles. Characteristic features of trichoepitheliomas are trichoblastic epithelial components associated with stereotyped stroma. Five histological patterns are distinguished:
> Large and small nodular trichoblastomas are circumscribed lesions and contain uniformly distributed solid trichoblasts with follicular papillae. The epithelial cells are uniform, basophilic, and typically overlap each other. In some cases, lymphocyte-rich nodules are observed.
> Retiform trichoblastomas are reticulated; large fenestrations contain follicular stroma.
> Cribriform trichoblastoma is the most common pattern in multiple lesions. The trichoblastic fenestrations are smaller than those of the retiform pattern.
> Racemiform trichoblastoma contains nests of epithelial cells (“clusters of grapes”). Stromal components are connected with the surrounding stroma rather than being isolated in fenestrations.
> Columnar trichoblastoma (desmoplastic “trichoepithelioma”) typically occurs as a solitary depression on the face of young women. These lesions are generally confined to the superficial dermis. They contain thin strands of epithelial cells compressed by dense stroma. Small trichoblasts may be observed, but they are less common than in the other forms of trichoblastoma.
Trichoepithelioma may arise in all age groups. Because of its benign nature, treatment usually is not required, provided that the diagnosis has been established with certainty

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LeBoit PE, Burg G, Weedon D, Sarasin A (Eds.)
World Health Organization Classification of Tumours. Pathology and Genetics of Skin Tumours
3rd Edition
IARC Press: Lyon 2005



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