Keratoacanthoma : Histologic Features
There is marked proliferation of well-differentiated keratinized squamous epithelium with eosinophilic ground glass cytoplasm. Microabscesses with neutrophils are usually present. A typical case lacks atypia; however, some cases may show nuclear pleomorphism, necrosis, increased mitotic activity (limited to basal epithelial layers and no abnormal mitoses), invasion of underlying skeletal muscle, and even perineural invasion or vascular invasion. The deeper aspects of the tumor have pushing margins and a mixed inflammatory infiltrate frequently containing eosinophils. When the lesion undergoes involution, it loses its keratin plug, the epithelial proliferation markedly diminishes, and the floor of the lesion flattens out. There is chronic inflammation and fibrosis often accompanied by foreign body giant cell reaction to released keratin.