The pilomatricoma (pilomatrixoma) is a peculiar, distinctive benign tumor. In this case of a 68-year-old woman, the clinician suspected a squamous cell carcinoma. This case is slightly unusual, because they typically appear in childhood. The head and neck is a common location, however. This lesion is thought to have a primitive follicular origin showing matrical differentiation. Malignant transformation is rare. These lesions are relatively straightforward to diagnose as few lesions show the combination of germinative epithelium admixed with "wet keratin", or keratinizing ghost cells. This phenomenon is also seen with craniopharyngiomas. In some lesions (usually with increasing age of the lesion), only the wet keratin is present, often with an associated foreign-body giant cell reaction. Pigment, calcification, and ossification can also be seen.
At higher power of the same lesion, one sees an interface between the cellular nodules and the keratin. The basophilic epithelium appears metabolically active, with open chromatin, conspicuous nucleoli, and scattered mitotic figures. Peripheral palisading is present. In some foci (not shown) the epithelium transitions after several layers into the eosinophilic "wet keratin". In this field they represent separate nodules. The wet keratin consists of "ghost cells", or "shadow cells", containing pale remnants of nuclei within an eosinophilic keratinous mass. The keratin is bordered by a rim of foreign-body giant cells.