Verrucous melanoma simulating melanoacanthoma: Dermoscopic, reflectance confocal microscopic and high-definition optical coherence tomography presentation of a rare melanoma variant.

Melanoma is characterised by a variety of clinical and histopathological presentations. Dermoscopy is a fast, non-invasive technique that increases diagnostic accuracy for both melanocytic and non-melanocytic skin tumours. One example of a potential pitfall in melanoma diagnosis using dermoscopy is represented by seborrheic keratosis (SK) that looks like melanomas, which may show overlapping features of both tumours. Reflectance confocal microscopy (RCM) and high-definition optical coherence tomography (HD-OCT) are non-invasive imaging techniques that are considered second or tertiary level diagnostic tools for the diagnosis of clinically and dermoscopically selected equivocal skin lesions. Herein we report the case of a 56-year-old Caucasian woman who presented with a 12-month history of a new, growing, asymptomatic, pigmented plaque on her left ankle. A physical examination disclosed a sharply demarcated, heavily pigmented black plaque with a verrucous surface and a maximum diameter of 2 cm (Fig. 1a). Based on the clinical history and dermoscopic features (details in Fig. 1b) a diagnosis of verrucous melanoma versus SK, melanoacanthoma type, was made. The lesion was further examined by RCM and HD-OCT (details in Figs 2,3) that favoured the diagnosis of melanoma. Therefore, surgical excision of the lesion with narrow margins was performed. A histopathological examination (details in Fig. 4) showed features of an invasive melanoma, verrucous variant. From a histopathological perspective, a melanoma clinically resembling SK could correspond to three rare variants: verrucous-hyperkeratotic melanoma, follicular melanoma and discohesive (acantholytic-like) melanoma. Kuehnl-Petzoldt and colleagues described a series of the verrucous-hyperkeratotic variant of melanoma, showing marked verrucous hyperplasia and hyperkeratosis. To date, only rare additional cases of its clinical and histopathological presentation have been reported. Melanoacanthoma is a rare, heavily pigmented variant of SK showing a marked increase in the concentration of melanocytes. Our case could be clinically mistaken for melanoacanthoma due to the presence of a sharply demarcated, hyperpigmented plaque with a verrucous surface. However, the clinical history and unusual location for a SK raised doubts about the diagnosis. Moreover, dermoscopy disclosed equivocal structures of peripheral streaks arising from a heavily pigmented centre, which are commonly Figure 1 (a) Sharply demarcated, heavily pigmented black plaque with a verrucous surface on the left ankle. (b) Dermoscopy enabled the observation of peripheral streaks displaying variable nuances of blue and grey colouration arising from adjacent, confluent black, structureless areas surrounding a central aggregation of multiple, large, confluent comedo-like openings and keratotic plugs; a few milia-like cysts were also noted.