Clinical features

•Most arise on scalp

•Non-scalp location occasionally documented

•Exceptionally rare

•Sudden rapid growth

•>5 cm

Histological features

•Infiltrating tumor arising from the wall of a pilar cyst

•Malignant change within a proliferating pilar tumor

•Nuclear pleomorphism and nuclear hyperchromatism

•Marked mitotic activity

•Abnormal mitoses

•Necrosis

•Stroma may be desmoplastic

•Spindle cell subtype

•Variable CD34 & calretinin expression (outer root sheath)

•Ki67 and p53 expression may be over-expressed

•Copy number gains of 15q & losses of 6p & 6q

•Absent UV signature as seen in SCC