Abstract

Trichilemmal (pilar) cysts are benign but locally aggressive dermal neoplasms that arise from the outer sheath of a hair follicle. They are the second most common type of skin cyst following epidermal inclusion cysts, affecting approximately 10% of the population. Proliferating trichilemmal cysts are the tumor form of pilar cysts and occur in less than 3% of all pilar cyst cases. These tumors have the rare potential to undergo malignant transformation, with only a few hundred cases reported in literature and fewer than 30 detailed reports of metastasis. As the benign cysts contain keratin and are outlined by stratified squamous epithelium, malignant transformation can often mimic squamous cell carcinoma (SCC). Distinguishing malignant proliferating pilar tumors (MPPT) from SCC is key as, unlike SCC, MPPTs are more unpredictable and have less established treatment algorithms. We report two cases of malignant proliferating pilar tumors originally classified as squamous cell carcinoma and highlight the key histologic differences to avoid misdiagnosis and mistreatment of MPPTs.

Financial Disclosure:
No current or relevant financial relationships exist.