Proliferating trichilemmal tumors: Clinicopathologic evaluation is a guide to biologic behavior

Background: Trichilemmal (pilar) cysts are common skin lesions that usually occur on the scalp of elderly women. They differentiate towards the follicular outer root sheath epithelium and show trichilemmal keratinization. Proliferating trichilemmal tumor (PTT) shows features of typical pilaf cyst, but additionally shows extensive epithelial proliferation, variable cytologic atypia and mitotic activity. The malignant potential of PTT is controversial, as only a small number of histologically malignant PTTs and a smaller number of clinically malignant PTTs have been reported. Methods: We retrieved from our archives five PTTs that deviated from ordinary PTTs with regards to either cytology or architecture. We also reviewed all previous reports of histologically malignant PTTs, with the goal of delineating criteria for the diagnosis of malignant PTTs. Results: Five cases of PTT showing atypical cytoarchitectural features were retrieved from our archives and reviewed with respect to size, growth pattern, cellularity, cytologic atypia, and mitotic activity. The patients (four female, one male) ranged from 54 to 83 (mean 65) years. The tumors measured from 1 to 16 cm in diameter (mean 5 cm) and four out of five occurred on the scalp. All tumors showed at least focal areas of typical PTTs. Three cases were circumscribed but had areas of moderate to focally marked cytologic atypia. Two cases showed infiltrative growth, marked cytologic atypia and mitotic activity. Clinical follow-up was available for four of five cases and ranged from 6 to 84 (median 48) months. Follow-up showed two cases with local recurrence and one case with distant metastasis. This last patient died of disease; all other patients are disease-free. Conclusions: Review of our cases and the published literature suggests that the diagnosis of malignant PTT be given to PTT showing a combination of non-scalp location, recent rapid growth, size greater than 5 cm, infiltrative growth, and significant cytologic atypia with mitotic activity. At the present time the stratification of malignant PTT into low- and high-grade categories is not possible.