Figure 2.

Histologic findings of malignant proliferating trichilemmal cyst. An excisional biopsy showing a fragmented collapsed cystic lesion in the dermis. The cystic lesion is lined with acanthotic stratified squamous epithelium with a central necrotic keratin material. The lining of the squamous epithelium shows abrupt keratinization without a granular cell layer (A, H&E, ×40). In contrast, irregular epithelial nests are found in an area adjacent to the proliferating trichilemmal cyst. The irregular epithelial nests reveal an infiltrative growth pattern and display pilar-type keratinization in the absence of granular cells, which indicates malignant transformation of proliferating trichilemmal cysts (B. H&E, ×100).