F1Ch39 (G7504): At its periphery, a proliferating pilar tumor presents a well-defined, expansile interface. The irregular interfaces, that develop within the confines of such a lesion as a consequence of individual cell dyskeratosis and sclerosing entrapment, generally do not distort the peripheral interface. On the other hand, a lesion that has developed into a destructive, invasive carcinoma will betray its nature at the periphery of the lesion. This type of invasion will be accompanied by cytologic atypia and mitotic activity. In the absence of patterns of destructive invasion at the periphery, cytologic atypia and mitotic activity is evidence of an atypical variant or a variant showing intraepithelial carcinoma, depending on the degree of atypia; such lesions should be thoroughly examined and sectioned. If destructive invasion at the periphery is not found , the atypical variants should be characterized as borderline lesions of uncertain malignant potential.
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