DESMOPLASTIC TRICHILEMMOMA
Desmoplastic trichilemmoma is a variant of trichilemmoma within which there is an area of pseudoinfiltration. This focus of pseudoinfiltration is found near the center of the trichilemmoma in some lesions and in the periphery of others. The differential diagnosis includes basal cell carcinoma with outer root sheath differentiation and basal cell carcinoma arising from a trichilemmoma. Either of those diagnoses requires the presence of unequivocal basal cell carcinoma in the tumor.

This tumor is described in recent editions of most textbooks of dermatopathology.


From the face of an adult:
 
Scanning power view. The tumor connects with the epidermis at two points. This type of broadly based connection is characteristically found in trichilemmomas.
Medium power view of the area in the red rectangle. Note the thickened basement membrane (BM). Basement membrane thickening and/or clear cell differentiation of outer root sheath type are/is characteristic of a trichilemmoma.
Medium power view of the area in the black rectangle. The epithelial cells are not as clear as those noted in the picture above. A slightly thickened basement membrane is present. Many trichilemmomas have less clear cell differentiation than classical examples.
Medium power view of the area in the blue rectangle. Peripheral palisading of the slightly clear cells is seen around the connective tissue surrounding the blood vessel and a few lymphocytes. Peripheral palisading is more often found around the periphery of the tumor in trichilemmomas.
Medium power view of the area in the yellow rectangle. This intermingling of epithelium and stroma is characteristic of the pseudoinfiltration that distinguishes this variant of trichilemmoma.

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