Inverted follicular keratosis (IFK) is a rare benign tumor that usually presents as a nonpigmented verrucous papule seen most often in middle-aged or elderly individuals.¹,² In 1954, Helwig³ described this tumor as an inverted cup-shaped crypt at the summit of an epithelial mass, which he named IFK. He felt that the apical crypt might be analogous to the opening of a pilosebaceous apparatus, which accounts for his use of the term “follicular” in the name. Several years later, Lund⁴ used the term basosquamous cell epidermal tumor (basosquamous cell acanthoma) as a synonym for this tumor,⁵ but that term is rarely used.

IFK is an epithelial neoplasm originating from the infundibular portion of the hair follicle.²,⁶,⁷ It manifests as an exo-endophytic tumor with large lobules or fingerlike projections of tumor cells that extend into the dermis.²,⁸ Several distinct histopathological variants exist, including a papillomatous wartlike pattern (exophytic), a keratoacanthoma-like pattern (exo-endophytic), and a solid nodular form (endophytic).

About 90% of IFK cases occur on the head and neck. Most previous larger series showed a male predominance ranging from 58% to 100%,²,⁵,⁸,⁹,¹⁰,¹¹,¹² but in a small series of 11 periocular IFKs,¹³ 64% were female.

Clinically, IFK is often misdiagnosed as a more common keratotic lesion. Some studies have estimated that fewer than 2% of IFK lesions are correctly diagnosed before surgical removal and histopathologic examination.¹⁴,¹⁵ The clinical diagnosis is most often as a viral wart, actinic keratosis, keratoacanthoma, or a variety of malignancies such as basal cell carcinoma, squamous cell carcinoma, and adnexal tumors.⁸,¹¹

IFK usually presents as a small solitary lesion, seen most often in adults at a mean age of 50 years,⁸ and is rarely seen in children or adults in the second and third decades.⁵,¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷ They are most often found on the face, particularly on the cheek, upper lip, chin, forehead, eyebrow, and nose. Occurrence on the trunk is uncommon, and involvement of the extremities is rare.¹¹ Lesions may have a history of slow growth over 2 to 6 months, but occasionally they may have been present for many years.¹⁸ Most of the lesions are small, ranging from 3 to 8 mm in diameter. They rarely involve the eyelids, with a predilection for the eyelid margins.¹⁹,²⁰,²¹ In general, IFK appears as an asymptomatic, firm, pink-to-white scaly papule and occasionally can be associated with a cutaneous horn.⁵,¹¹,²¹

Dermoscopy has been useful in the clinical diagnosis of IFK.⁹,²² Llambrich et al⁹ described the dermatoscopic features of 12 IFK and reported a distinct white structureless area in or near the center of the lesion present in all cases, and white scales were found in 75%. Vascular structures were also present in all cases, showing a monomorphic pattern in 58% and a polymorphic pattern in 42%. The most common vascular structure was hairpin vessels surrounded by a whitish halo, which was found in 83%. Keratin was observed in 67% of lesions, mostly in the central area.

Vascular patterns, in general, have been considered important in the differential diagnosis of many nonpigmented skin tumors.²³ Hairpin vessels surrounded by a whitish halo are a common dermoscopic vascular pattern,⁹,²⁴ indicating epithelial differentiation,²⁵ that can be seen in benign and malignant keratinizing lesions including squamous cell carcinoma, verruca vulgaris, poroma, and thick melanoma.²³,²⁶ In contrast to the arrangement of hairpin vessels in the keratoacanthoma type of squamous cell carcinoma,²⁶ those of IFK often show a central white or yellowish amorphous area.²⁴