Trichilemmal Keratosis Manifesting as a Cutaneous Horn

Corresponding author: Che-Hao Hsu, Department of Dermatology, Cathay General Hospital, 280 Jen-Ai Rd., Sec. 4, Taipei, Taiwan TEL: 886-2-27082121 FAX: 886-2-27074949 E-mail: [email protected] Funding source: none Confl ict of interest: none declared CASE REPORT A 45-year-old man presented with an asymptomatic, slowly growing, keratotic nodule on his left fl ank for one year. Physical examination revealed a pea-sized, horn-like plaque protruding from a brownish nodule (Fig. 1 A, B). The tumor had been present for years as a cyst-like lesion before, but the tumor ruptured 1 year before this consultation. Later, a horn-like structure developed on the center of it. The lesion was excised en bloc under the impression of keratoacanthoma. Sections showed a cup-shaped epidermal proliferation with overlying compact hyperkeratosis and focal parakeratosis (Fig 2). The epithelium invaginates into dermis and form connecting lobules. Palisading basal cells were seen at the peripheral of the lobules (Fig. 3). The epithelium were composed of pale keratinocytes that transform into a keratinized layer without a granular layer formation (Fig. 4). There is a clear distinction between trichilemmal keratinization and epidermoid keratinization of the perilesional skin. Vascular dilatation and chronic infl ammatory cell infi ltrations are noted in the dermis. The CD34 staining in this case shows positive in the peripheral cells of the lesion (Fig. 5). DISCUSSION Headington first used the term trichilemmal keratosis in 1976 to describe a rare keratinizing tumor resembling a cutaneous horn or hyperkeratotic actinic keratosis. The pattern of keratinization is similar to that seen in follicular isthmus and trichilemmal cyst. However, very few cases were studied at that time. In 1979, Brownstein proposed the name trichilemmal horn to describe the same condition. In his observation, only 19 trichilemmal horn were found after he had examined 75000 specimens (incidence: 0.021%). Among them, eleven patients were female and six patients were male. The age ranged from 16 to 72 years with median age of 50 years. DiMaio reviewed the literature and declared that only 28 cases of trichilemmal keratosis were reported so far. Clinically trichilemmal keratosis are solitary, keratotic, horn-like structures. The most common affected areas are the extremities, head and back in decreased frequency. Histologically, there is marked hyperkeratosis and verrucous epidermal hyperplasia. The epithelium showed trichilemmal keratinization in which the keratinocytes abruptly transform into dense lamellar keratin without formation of a granular layer. The keratinocytes were large polygonal cells with abundant eosinophilic cytoplasm. The pattern of keratinization is