PROLIFERATING TRICHILEMMAL TUMOR
This is also known as proliferating trichilemmal cyst, and, in the past, it has been described under a variety of other terms including pilar tumor of the scalp (in spite of the fact that this occurs in locations other than the scalp). Intradermal proliferation of follicular type epithelium with the production of amorphous masses of keratinous material and foci of trichilemmal keratinization characterize this tumor. Mitoses may be present. Lesions of this type are supposed to be benign, but locally aggressive and metastasizing variants have been described.
 
Scan. No epidermal connection with the dermal tumor was demonstrated in any of the sections. Ribbons and distorted cystic structures composed of epithelium of the type found in the zone of trichilemmal keratinization in the telogen phase follicle make up the proliferating component of the lesion.
Low power.
High power of above. Note the way broad, waxy masses of keratin interdigitate with the luminal surface of the keratinocytes. This is trichilemmal keratinization.
High power of another area showing abrupt spillage of amorphous keratinous material into the connective tissue. There is also a small nest of benign squamous epithelium that appears to be separate from the main mass of epithelium..
Low power of another area. 'fbgc' are foreign body type giant cells, and 'SE' is squamous epithelium. This type of reaction is common in areas of keratin spillage.
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