Pigmented tumor in the nostril

A 64-year-old man was noted to have a single pigmented lesion in the nostril of his nose. Clinical examination revealed a 5 mm nodular growth and brown lesion. With a presumed clinical diagnosis of malignant skin tumor, a biopsy was performed. The histological examination revealed the unexpected diagnosis of pigmented inverted follicular keratosis. The inverted follicular keratosis is an uncommon benign lesion that is usually diagnosed histologically rather than clinically. It commonly simulates other proliferative skin lesions.


Introduction
The inverted follicular keratosis is an uncommon benign lesion that is usually diagnosed histologically rather than clinically. We report on a singular case of a pigmented skin lesion, which was revealed on histological examination to be a pigmented inverted follicular keratosis.

Observation
We present a case of a white 64-year-old man, with no significant co-morbidities, who had a remote history of a pro-  [1,2] support the hair follicle as the origin of this lesion, while some authors believe that this lesion is best characterized as a variant of seborrheic keratosis (SK) that has been irritated, [3] and others consider it as distinct entity [4]. It is characterized by a squamous epithelial expansion of the infundibular portion of the hair follicle in an exophytic and endophytic pattern.
IFK is a benign skin lesion reported initially by Helwig [1] that occurs in middle aged or older patients (>50 years), with median age at presentation of 69 years. Men are affected about twice as often as women. In IFK the lesions are generally asymptomatic, firm, and pinkish papules, but it can present chromatic variants, typically from yellow to brown, in relation to the content of melanin [5]. The lesion most often arises as a solitary skin nodule on the face (85%) [4]; however, multiple IFK have been described in Cowden's syndrome [6]. The cheek and the upper lip are the sites of predilection; other sites affected are the chin, forehead, eyebrow, nose, and eyelid [7]. Most of the lesions are between 3 and 8 mm in maximum diameter, but a few reach a size of 10 mm. The duration of the lesion varies between six weeks and three years in most cases, and in some cases as long as 15 years has been reported [7]. Irritated seborrheic keratosis is the first histopathological differential diagnosis. Acceptance of the "irritation theory" implies that a lesion of seborrheic keratosis must be present before the irritation can occur. Since a primary characteristic of seborrheic keratosis is its lack of penetration into the dermis, the presence of a prominent downward growth pattern in IFK suggests a different theory of origin. The presence of surface crypts, horn cysts, a downward growth pattern, and frequent association with the hair follicle in IFK favors a follicular origin. Some authors considered IFK as distinct from SK [4]. In addition to exophytic and endophytic patterns, a few koilocytes may occur in IFK and never in SK.
Appearance of eosinophilic cytoplasm in IFK contrast with the appearance of basaloid cell cytoplasm in SK. Lastly, the squamous eddies are not pathognomonic of the IFK, but they are particularly numerous [9].
In our case the only close differential diagnosis is trichilemmoma. IFK shares some histologic features with trichil- In addition, some authors postulate that HPV is associated to trichilemmona lesions [10]. The PCR for HPV performed in our case was negative.
The histopathological diagnosis of IFK can mimic also verruca vulgaris, viral warts, and squamous cell carcinoma.
IFKs can contain cellular atypia, and squamous eddies bear some resemblance to those of squamous cell carcinoma [11,12]. The well-delineated base, broad acanthotic epithelial downgrowth, squamous eddy formation, and lack of epithelial dysplasia support a diagnosis of IFK here [2]. The pathologist should be most attentive whenever histopathologic features suggestive of IFK are present. Indeed, more than half of these lesions have been mistaken for squamous cell carcinoma on initial evaluation, and this can result in unnecessarily drastic treatment of a benign lesion.