Proliferating trichilemmal cyst. Report of four cases, two with nondiploid DNA content and increased proliferation index.

Proliferating trichilemmal cyst (PTC) is an uncommon tumor that usually arises on the scalp of elderly women. Its biologic nature is that of a benign lesion with occasional local recurrences. PTC can be confused both grossly and microscopically with squamous cell carcinoma and malignant PTC. Distinction between these lesions has historically been made on a histologic basis. We present four cases of PTC. Histologically, the lesions consisted of lobules of basaloid cells admixed with larger pale-staining cells with abrupt trichilemmal type of keratinization and peripheral palisading embedded in a fibrous stroma. Flow cytometry performed on nuclear extracts of the lesions revealed two of the four lesions to have nondiploid DNA content. Proliferation index, measured by immunohistochemical staining with Ki-67 monoclonal antibody along with mitotic rate count, was higher in the two nondiploid lesions as compared with their diploid counterparts. The results raise the question of the significance of aneuploidy and increased proliferation indices in otherwise benign PTC.