Proliferating Pilar Tumor: Two Cases and a Review of the Literature.

Trichilemmal cysts (TCs) are the most common cystic lesions arising on the scalp and up to 2% give rise to a proliferating trichilemmal tumor (PTT). A rare subset of PTTs are termed malignant due to their histologic characteristics, aggressive behavior and potential for metastasis. The histological hallmark of TCs and PTTs is the absence of a granular layer resulting in abrupt trichilemmal keritanization.