INTRODUCTION
This observation describes the presentation, histopatho- logic diagnosis, and treatment of a malignant proliferating trichilemmal tumor.
Clinical Presentation
A 38-year old female presented with a several-year history of an enlarging, firm, and occasionally painful lesion on her left scalp (Figure A). The patient denied any bleeding or drainage from the nodule. There was no personal or family history of skin cancer. On physical exam, a 3 cm x 3 cm x 1.5 cm, well- demarcated, brown-colored multinodular, dermal lesion was noted on the patient’s left scalp. It was not ulcerated or fixed to underlying bone. A shave biopsy was performed, and histopathologic examination of the specimen revealed abrupt trichilemmal keratinization without a granular cell layer, cysts with homogenous keratin, and multiple, broad, anastomosing bands of nodules and lobules. Furthermore, pleomorphism with nuclear atypia, increased abnormal mitotic figures, hyper- chromatism, and necrotic cells led to the diagnosis of malignant proliferating trichilemmal cyst (Figure B).The patient elected for complete surgical excision given the histopathologic diagnosis. The patient continues to be closely monitored for recurrence and/or metastasis.
DISCUSSION
Wilson-Jones first described these tumors in 1966 as pro- liferating epidermoid cysts.1 Pilar cysts originate from the isthmus of the outer follicular hair root sheath, giving rise to the term trichilemmal cyst or tumor.2,3 Several years later, Saida et al introduced the classification of malignant proliferating trichilemmal cyst to describe a trichilemmal tumor that was infiltrative and metastatic with histopathologic fea- tures of atypia and increased mitotic activity.4,5 While benign pilar cysts are fairly common, affecting roughly 5-10% of the population, transformation into the malignant counterpart or de novo malignancies are extremely rare. Classically, these tumors are found on the scalp in females during their 8th to 9th decade of life.3,6 Ye et al examined 76 proliferating trichilemmal tumors (PTT) in order to outline histopathologic features that could predict clinical behavior and prognosis.7 Three distinct PTT variants were described based on malignant features such as nuclear atypical, mitotic activity, and presence of necrosis. Group I lesions are considered benign with recurrence not observed. Histopathologic examination reveals trichilemmal keratinization, focal nuclear atypia, mononuclear infiltrates, and absence of mitotic figures, perineural invasion, or vascular invasion. Group II lesions are low-grade malignant tumors with an increased risk of local recurrence. Histologically, these tumors are noted to have abrupt keratinization, areas of sin- gle cell necrosis, and desmoplastic stroma. Cytologic atypia is absent. Group III lesions are high-grade malignant tumors with a high risk of recurrence, lymph node involvement, and/ or distant metastasis. Group III tumors infiltrate into the der- mis and have abrupt keratinization with regions of multifocal necrosis. Atypical mitoses and a desmoplastic stroma are also present. Because histological examination of our patient’s le- sion demonstrated abnormal mitotic figures, cytologic atypia, and necrosis it was considered a Group 3 (high-grade) tumor based on the criteria proposed by Ye et al. Distinguishing the variant of PTT using histologic examina- tion is important, as no specific clinical criteria currently exists to differentiate benign and malignant tumors. Clinical signs that suggest a poor prognosis include rapidly enlarging tu-
