Proliferating trichilemmal tumor is one of rare benign tumors of the skin appendages,
considering as hamartoma of the terminal hair follicle, isthmic segment of the outer root
sheath.
We report a case of numerous proliferating trichilemmal tumors admixed with ordinary
trichilemmal cysts with familial history in 64·year-old man. He has total 157 lesions,
which are chiefly located in scalp (48), back (32), and also face, neck, trunk, extremities.
Histologically, the tumor is composed of irregularly arranged and anastomosed
trabeculae, lobules, or sheets of proliferated trichilemmal squamous epithelium with
peripheral palisading of the basaloid cells. Several layers of squamoid or ovoid cells have
PAS-positive clear cytoplasm. And it also shows abrupt amorphous, trichilemmal
keratinizaiton. Some squamous eddies with mild cellular atypism are associated, but
definite invasion or other evidence of the malignancy is not found. Usual multiple
trichilemmal cysts are admixed with tumor and occasionally exhibit connection between
them.
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