Abstract

Pilar cysts typically present on the scalp of females and histologically are characterized by a sharp transition from an epithelial lining to an eosinophilic keratin layer without granular cell layer. Proliferative forms of pilar cysts exist on a spectrum from benign proliferating pilar tumors (BPPTs) to malignant proliferating pilar tumors (MPPTs) and features such as severe cytologic atypia, invasion/infiltration of surrounding stroma, increased mitotic activity, and necrosis are reported to be associated with MPPTs.  Only four MPPTs with sarcomatous transformation are reported in the literature, and detailed molecular analysis was not performed. We report a MPPT with epithelial and sarcomatous elements occurring on the scalp (present >20 years) of a 63-year-old male which underwent rapid growth after trauma. Partial resection of the mass revealed areas resembling a proliferating pilar tumor, areas of carcinoma with clear cell change, and sarcomatous areas composed of pleomorphic and spindled tumor cells. AE1/AE3 and p40 immunostains were positive in the carcinomatous areas and areas resembling a PPT, whereas there was negative staining in the sarcomatous areas. p53 immunostain demonstrated a mutant overexpression pattern in both epithelial and sarcomatous components, and Ki67 index was elevated (5-30%). Molecular studies showed chromosomal variations (15q gain and 6p/6q loss) that have been reported in both BPPT and MPPT. Shared mutations in carcinomatous and sarcomatous areas include TP53 (previously reported in MPPT), FLT1, HMBS, KIF1B, KMT2D, PML, and RIF1 mutations. CDKN2A nonsense mutation and a higher allelic fraction of a TP53 frameshift mutation are noted only in the sarcomatous component.