Pilomatricoma: A tumor with hidden depths

Background: Pilomatricoma is a benign tumor of hair matrix differentiation and has been classically described as comprising of basaloid and shadow cells admixed with multinucleated giant cells and areas of calcification. However, there are a diverse range of histologic features this tumor displays that are often unrecognized. Aims: This study was undertaken to record the histopathologic features of pilomatricoma with an emphasis on the occurrence of other forms of differentiation. Methods: The study included all skin biopsy specimens over a 13-year period from 1995 to 2007 that had a histologic diagnosis of pilomatricoma. Hematoxylin and eosin-stained slides were reviewed. Results: This study included 21 cases of pilomatricoma. Supramatrical differentiation was seen in all cases and three-quarters of the cases showed matrical differentiation. Also observed in some of the cases were clear cell differentiation toward the outer root sheath, infundibular differentiation, calcification, ossification and secondary inflammation with a foreign body giant cell reaction. Epidermal induction in the form of a downward plate-like growth of the epidermis was seen in a few cases. Conclusion: Pilomatricoma, although considered a tumor of hair matrix differentiation, can show cellular evolution toward the other parts of the hair follicle, such as the outer and inner root sheaths, sebaceous and infundibular components and, therefore, can be considered a panfollicular neoplasm.