Pilomatrix carcinoma presenting as an extra axial mass: clinicopathological features

Pilomatrix carcinoma is the rare malignant counterpart of pilomatrixoma, a skin adnexal tumour originating from hair matrix cells. Pilomatrix carcinoma can arise as a solitary lesion de novo, or through transformation of a pilomatrixoma. Pilomatrixoma was first described erroneously as being of sebaceous gland origin but was later discovered to be derived from hair matrix cells. They are rare, slow growing tumours of the skin found in the lower dermis and subcutaneous fat and are predominantly found in the neck and the scalp. While known to be locally aggressive, no malignant form was thought to exist until it was described relatively recently. Since then, approximately ninety cases of pilomatrix carcinoma have been reported.