Dermoscopic and confocal features of an axillary “special site” nevus

“Nevi of special sites” is a term that denotes melanocytic nevi presenting in specific anatomic locations including the scalp, genital area, flexural sites, and acral sites [1]. Nevi from these anatomic sites display at times histopathologic features that may lead the reading pathologist to recommend re-excision of these benign nevi. Reflectance confocal microscopy (RCM) is a noninvasive imaging tool that allows for visualization of epidermal, dermal-epidermal junctional (DEJ), and superficial dermal tissue structures at cellular level resolution. RCM features of special site nevi have not been previously described in the literature. Defining the RCM characteristics of special site nevi may increase diagnostic accuracy and assist in ruling out melanoma. Here, we report a case of a pigmented lesion appearing in the axilla of a patient with a recently diagnosed melanoma. Dermoscopic and histopathologic results were consistent with the diagnosis of nevus in flexural anatomic sites. In this case, RCM showed a regular honeycomb pattern of epidermal keratinocytes and enlarged, non-homogenous, discohesive nests at the DEJ, a pattern that corresponded well with the histopathologic findings. Larger studies are needed to establish RCM features of special site nevi in order to reliably rule out melanoma and lower the rate of unnecessary excisions of these benign nevi.

"Nevi of special sites" is a term that denotes melanocytic nevi presenting in specific anatomic locations including the scalp, genital area, flexural sites, and acral sites [1]. Nevi from these anatomic sites display at times histopathologic features that may lead the reading pathologist to recommend re-excision of these benign nevi. Reflectance confocal microscopy (RCM) is a noninvasive imaging tool that allows for visualization of epidermal, dermal-epidermal junctional (DEJ), and superficial dermal tissue structures at cellular level resolution. RCM features of special site nevi have not been previously described in the literature. Defining the RCM characteristics of special site nevi may increase diagnostic accuracy and assist in ruling out melanoma.
Here, we report a case of a pigmented lesion appearing in the axilla of a patient with a recently diagnosed melanoma. Dermoscopic and histopathologic results were consistent with the diagnosis of nevus in flexural anatomic sites. In this case, RCM showed a regular honeycomb pattern of epidermal keratinocytes and enlarged, non-homogenous, discohesive nests at the DEJ, a pattern that corresponded well with the histopathologic findings. Larger studies are needed to establish RCM features of special site nevi in order to reliably rule out melanoma and lower the rate of unnecessary excisions of these benign nevi.

Discussion
Flexural nevi that fit criteria for "special site" nevi may be clinically indistinguishable from other types of melanocytic nevi [2]. They are usually greater than 6 mm and have an irregular border [3].    57 cytologic atypia that rarely raises concern, and another with irregular nests and uniform junctional cytologic atypia that the pathologist may, at times, find to be more concerning for melanoma [12]. This is in contrast to studies that were not "site-specific," in which atypical pigment network and irregular globules were sensitive and specific for melanoma [6,7]. This discrepancy indicates that the when evaluating the dermoscopic characteristics needed to differentiate nevi from melanoma, the anatomic site should be taken into consideration. Thus, ancillary diagnostic methods that can assist to exclude, reliably and reproducibly, melanoma in "special" anatomic locations are warranted.
RCM is a noninvasive tool that utilizes differences in refractivity of skin structures to visualize the epidermis, dermal-epidermal junction, and dermis at the cellular level [8]. While there exists a considerable amount of data on RCM features of melanoma and nevi [9,10] RCM features specific to flexural or other special site nevi have not yet been elucidated.
The well-conserved honeycomb pattern of keratinocytes visualized at the spinous and granular levels in this case is more consistent with RCM findings of nevi [11]. In contrast, the RCM finding of enlarged, discohesive junctional nests with variability in shape, size, and spacing may elicit concern for melanoma, but have also been described in an RCM study of nevi, denoted by the authors as dysplastic nevi, from nonspecial anatomic sites [9]. Absence of large, bright pagetoid cells in the epidermis and cytologic atypia at the basal layer were criteria supporting the diagnosis of a nevus [11].
Two histopathologic patterns have been observed in flexural nevi; one with a papillomatous epidermis and mild