Chapter 16: Hair Follicle Tumors

OVERVIEW

INTRODUCTION

Hair follicle tumors can be classified by both their level of maturation (the degree to which they resemble mature hair follicles) and by the specific portion of the hair follicle that the tumor resembles. The majority of the hair follicle tumors discussed in this chapter are benign. Their diagnostic significance often lies in their differentiation from malignant epithelial tumors. A representative biopsy specimen is necessary for the differentiation of benign follicular neoplasms from malignant epithelial tumors such as basal cell carcinoma and squamous cell carcinoma. Most follicular tumors form well-circumscribed symmetric neoplasms that are located in the upper to mid-dermis. In a smaller, superficial biopsy specimen, circumscription, symmetry, and maturation with depth cannot be adequately addressed. For this reason, a punch biopsy or excisional biopsy specimen is recommended for diagnosis. Malignant transformation in follicular neoplasms presenting with rapid growth, pain, ulceration, or bleeding has been described.¹ Excision of these lesions is recommended for diagnostic purposes and to ensure completeness of removal.

EPIDEMIOLOGY

Benign follicular tumors are uncommon but not rare in clinical practice. Their recognition is important to prevent an inappropriate diagnosis of basal cell carcinoma, which would result in inappropriately aggressive surgical techniques. The most common follicular tumors include trichilemmoma and inverted follicular keratosis. In a review of over 100,000 specimens received in a private dermatopathology laboratory in 2005, these tumors accounted for 0.1% of all specimens received. Pilomatricomas and trichoepitheliomas were slightly less common, representing 0.07% and 0.03% of all specimens received, respectively. Other follicular tumors, including trichoadenoma and trichofolliculoma, are much less common. In contrast, malignant follicular tumors are quite rare. In a similar review of over 100,000 surgical specimens received in a private dermatopathology laboratory in 2005, malignant follicular tumors represented one specimen per ...