Understanding Pseudoepitheliomatous Hyperplasia

Pseudoepitheliomatous hyperplasia is a benign proliferation of the epidermis into irregular squamous strands extending down into the dermis. Pseudoepitheliomatous hyperplasia can arise either from the epidermis or from adnexal epithelium, and is almost always associated with persistent inflammation of the subjacent dermis due to a chronic wound, ulcer, infection, malignancy, retained foreign material or an inflammatory dermatitis. It can closely mimic, abut, or degenerate into a squamous cell carcinoma. Unlike a squamous cell carcinoma, pseudoepitheliomatous hyperplasia never has atypical mitotic figures, rarely has dyskeratosis or atypical nuclei, and is never involved in vascular, lymphatic or perineural invasion. The histogenesis of pseudoepitheliomatous hyperplasia is speculated to be due to the proliferative effect of cytokines released by inflammatory or tumor cells within the subjacent dermis. The term “atypical pseudoepitheliomatous hyperplasia” is sometimes used to describe lesions similar to pseudoepitheliomatous hyperplasia, but which invade deeply into soft tissue and bone. However, atypical pseudoepitheliomatous hyperplasia lacks the cytologic atypia of a squamous cell carcinoma. There are reports of metastasis of these lesions; therefore, most consider atypical pseudoepitheliomatous hyperplasia to be a well-differentiated squamous cell carcinoma. Theories as to the histogenesis of malignant degeneration of pseudoepitheliomatous hyperplasia include that of activated keratinocytes, in which a few keratinocytes within the epidermis are intrinsically mitotically active and therefore easily proliferate when stimulated.