13.3.1.7The Folliculocentric Basaloid Proliferations (FBP)

FBP or Òfunny looking folliclesÓ are irregular strands of basaloid cells emanating from the upper portion of the hair follicles (Fig. 13.22a, b). They are most likely a reactive phenomenon to irritation and can be seen in normal skin, margins of BCC, or in a myriad of neoplastic and non-neoplastic skin conditions [22]. Moreover, they are most commonly associated with tumors of skin with abundant hair follicles, such as nasal and perinasal skin. They were originally thought to be hyperplasia of the bulge epithelium, but are believed now to be hyperplasia of the mantle epithelium [18]. FBPs can sometimes be difÞcult to distinguish from BCCs. Factors that help differentiate FBPs from BCCs are: (1) vertical orientation in relation to a hair follicle, (2) radial pin wheel or lattice-like orientation along an axis, (3) superÞcial location with or without epidermal attachment, (4) presence of basement membrane material around them, and

(5) absent retraction artifacts, stromal mucinous material, cytologic, and nuclear atypia, or atypical mitoses.

Moreover, some BCCs show overt follicle-like structures. These types are BCC with follicular differentiation, infundibulocystic BCC, and BCC with matrical differentiation. In this situation, and when in doubt, whether a particular basaloid cell formation is actually a part of BCC with follicular differentiation or a benign follicular structure, the Mohs surgeon should take an extra stage to be conÞdent that all of the tumor is removed. Factors that help identify benign follicular differentiation include: (1) follicular papillae and papillary mesenchymal bodies, (2) follicular bulbs, (3) presence of outer root sheath cells (pale cells), (4) trichohyaline granules, (5) shadow cells, and (6) hair shafts. Finally, follicular induction or basaloid cellular hyperplasia can be seen overlying some skin lesions, such as dermatoÞbroma or nevus sebaceus. This usually represents a reactive process and is not a true BCC. Some rare cases of true neoplasms, however, were reported in this setting [23]. The original clinical diagnosis should be very helpful when evaluating frozen sections in these cases.