Answer: Pilomatrixoma

Histology: The lesion is a multilobulated tumor situated within the dermis. The individual lobules are composed of a variable mixture of basaloid and “ghost” cells. The former predominate in early lesions and the latter are seen in older lesions along with calcification and occasionally ossification or stromal amyloid deposition. Basaloid cells are small and uniform with round vesicular nuclei and prominent nucleoli. Mitotic figures may be numerous but this feature is not an abnormality, but rather an indication of a rapid growth phase. As the tumor matures, the basaloid cells transform into ghost cells, acquiring abundant eosinophilic cytoplasm. Eventually, the nuclei are lost leaving sheets of intensely eosinophilic keratinous debris in which the ghost-like outlines of tumor cells are seen and among which giant cells are often present. Keratin formation in pilomatrixoma is predominantly pilar and lacks a granular layer. Occasionally melanin pigment may be found within basaloid cells and tumor histiocytes.

Discussion: Pilomatrixoma usually presents as a solitary lesion. On occasion, multiple tumors may be present as part of an autosomal dominant disorder and, rarely, pilomatrixoma may represent a dermatological marker of Gardner’s syndrome. The lesion presents as a slowly growing, firm nodule on the head, upper limbs, neck, trunk and lower limbs in decreasing order of frequency. It occurs predominantly in children and young adults. There may be a second peak of incidence among patients in their sixth and seventh decades of life. Pilomatrix carcinoma is a very rare lesion and shows a predilection for males. Histologic features that suggest malignancy are large size (> than 4 cm.), an infiltrating border with involvement of fascia or skeletal muscle, abnormal mitotic figures, stromal desmoplasia and lymphatic or perineural invasion. Proliferating trichilemmal (pilar) cyst is a rare lesion with marked female predominance. Like pilomatrixoma, they occur in the deep dermis. Histologically, however, they consist of a lobulated intradermal mass of squamous epithelium, which may simulate squamous carcinoma. The individual lobules, however, are sharply defined and have a regular, non-infiltrative border. Widespread keratinization with necrosis is characteristic but ghost cells are not a frequent feature. Lobules themselves are easily surrounded by an intensely eosinophilic thickened basement membrane.