Acquired fibrokeratoma presenting as multiple plantar nodules
Division of Dermatopathology, Mount Sinai School of Medicine, New York, New York. email@example.com
Acquired digital fibrokeratoma is a rare benign fibroepithelial tumor that typically presents as a solitary asymptomatic nodule on the finger or toe. Middle-aged adults are most commonly affected. Here we discuss an unusual case of acquired digital fibrokeratoma presenting as a cluster of multiple nodules on the sole of a 15-year-old boy.
The term, fibrokeratoma, was first coined by Paul Gerson Unna in 1896. It denotes a fibroepithelial tumor with a fibrous core and thick keratotic epithelium, generally found on the eyelid or neck of an older adult. The term acquired digital fibrokeratoma (ADF) was first used in 1968 by Bart et al. to describe a set of benign lesions most often located on the digits .
Clinically, ADF typically presents as a small, solitary, skin-colored papule on the finger or toe. It is often surrounded by a collarette of slightly raised skin called a “moat,” and is usually asymptomatic [2, 3]. Although the term “digital” is used to describe ADF, there have been multiple cases noted in the literature of ADF lesions that presented on the palm, sole, heel, proximal nail fold, dorsum of hand, dorsum of wrist, ankle, and prepatellar region [1, 3-8]. It has been hypothesized that ADF occurs as the result of trauma to the affected area, either via a single major event or multiple minor episodes [1, 3].
Histopathologically, ADF demonstrates an epithelium with marked orthokeratosis and acanthosis . The rete ridges are thickened and often branching. The enveloped dermis is composed of thick interwoven bundles of collagen, which are oriented parallel to the long axis of the lesion. Elastic fibers are thin and sparse, if not completely absent, and the tumor is often highly vascular [1, 3, 8]. Curative treatment is by complete surgical excision .
A 15-year-old male, with no significant past medical history, presented with a three-year history of painful fleshy papules on the sole of his left foot. The patient denied any history of trauma to the area. He noted that the lesions had been growing in size. Physical exam was notable for twelve clustered, flesh-colored, hyperkeratotic, verrucous papules on the left mid-sole. The overall affected area had the dimensions of 4 cm x 1.5 cm. The remainder of his exam was within normal limits.
Two of the excrescences were removed by shave biopsies. Histopathological analysis revealed a hyperkeratotic and acanthotic epidermis with thickened rete ridges. The dermis consisted of thickened bundles of collagen that were oriented parallel to the epidermis. The microscopic findings were consistent with those of acquired digital fibrokeratoma.
The patient underwent shave excisions of the remainder of his papules and has had no recurrence at five months.
This case of ADF is unique for several reasons. First, the patient presented with a cluster of lesions rather than one solitary lesion. Only two prior cases in the literature have described a patient presenting with more than one ADF. In the first case, however, the two ADF lesions were not concurrent, with one lesion appearing only after the first was treated . In the second case, the two ADF lesions appeared on the hallux in the setting of long-term cyclosporine use; it partially regressed once the cyclosporine was discontinued .
This case is also noteworthy for its location. ADFs appearing on the sole or heel of the foot are rare, though there have been several cases that have described this particular presentation both in association with trauma  or idiopathically [3, 5, 7].
Lastly, the age of the patient is also notable. The mean age of ADF onset is approximately forty, with most cases being between the ages of 31 and 60 [1, 8]. While ADF has been noted in younger patients, only two prior reports have described ADF in a patient under the age of fifteen [1, 7, 8].
The differential diagnosis for ADF includes verruca vulgaris, supernumerary digit, periungual fibroma (Koenen tumor), pyogenic granuloma, acrochordon, cutaneous horn, neuroma, fibroma, neurofibroma, and eccrine poroma [2, 7, 8]. Of these, only Koenen tumor will typically present with multiple lesions as observed in this patient. However, Koenen tumor is also associated with other dermatologic features of tuberous sclerosis, such as adenoma sebaceum, shagreen patches, and ash leaf macules. Our patient exhibited none of these. Therefore, this diagnosis is unlikely .
Several authors have suggested that ADF might be more accurately named “acral fibrokeratoma” or “acquired fibrokeratoma” since these lesions have been shown to occur in non-digital locations [2, 7]. This unusual presentation of ADF as a cluster of plantar nodules on the sole of the foot adds additional support to this argument.
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