Integrating clinical, dermoscopy, and reflectance confocal microscopy findings into correctly identifying a nevoid melanoma.

Introduction

Nevoid melanoma (NeM) is an uncommon subtype of melanoma that can mimic the morphology of an intradermal nevus both clinically and histopathologically. Similar to other melanoma subtypes, NeM is prone to local recurrence and metastasis, necessitating early diagnosis and prompt management. Recently, reflectance confocal microcopy (RCM) has shown to be a valuable noninvasive imaging device to help differentiate benign from malignant skin lesions with cellular resolution. Here we report a unique case of NeM that clinically mimicked an intradermal nevus. The appearance of the dermoscopic features of irregular globules and polymorphous vessels led to the expansion of the differential diagnosis, which included melanoma, irritated intradermal nevus, and an irritated clonal seborrheic keratosis. RCM imaging was performed to narrow the differential diagnosis and determine the true nature of the lesion. RCM revealed an atypical melanocytic neoplasm highly suspicious for melanoma, prompting a biopsy and averting a delay in diagnosis. This case highlights the importance of RCM as an adjunct tool that adds useful information to the clinical and dermoscopic findings. All of the gathered information (including features seen and features noticeably absent) were used in concert to arrive at the diagnosis of NeM via abductive reasoning or Bayesian inference.

Case report

A 75-year-old woman came to the clinic seeking an opinion regarding the management of 2 excised melanomas in situ (lentigo maligna type) on her face. On cutaneous examination, an isolated brownish-pink, well-demarcated, partly sessile, soft, and slightly mamillated 9-mm lesion (Fig 1, A) was identified on the patient's right medial knee. The patient was not aware of the lesion and was unable to provide any history pertaining to its duration or evolution.

Fig 1

A, Clinical image showing a 9-mm, brownish-pink, slightly mamillated lesion. B, Dermoscopy image showing atypical vessels (arrows) and irregular globules (arrowheads).

Dermoscopy (Fig 1, B) revealed a 2-component pattern with 2 distinct zones. One part of the lesion was pink and soft and had polymorphic vessels. The other area was brownish-pink with irregular globules. On the basis of the clinical morphology and presence of irregular globules and polymorphic vessels seen with dermoscopy, the differential diagnosis included melanoma, irritated intradermal nevus, and, less likely, an irritated clonal seborrheic keratosis. Because the lesion was soft and lacked any milia-like cysts, comedo openings, gyri, and sulci, the differential diagnosis shifted more in favor of a melanocytic lesion (melanoma vs intradermal nevus).

RCM (Fig 2) was performed to narrow the differential diagnosis by confirming the nature of the lesion. RCM revealed a well-circumscribed but asymmetric lesion. The epidermal surface appeared mamillated with some keratin-filled epidermal invaginations and an overall regular honeycomb pattern. No prominent pagetosis was identified. Multiple large, irregularly-sized, dense and sparse nests were identified at the dermal-epidermal junction. These nests were composed of pleomorphic atypical nucleated cells, raising the suspicion of melanoma. Features of clonal seborrheic keratosis, such as well-defined intraepidermal clusters of pigmented polygonal keratinocytes (Borst-Jadassohn appearances on histopathology) were not identified.5, 6 Histopathology confirmed the diagnosis of a 0.5-mm thick nevoid melanoma (Fig 3). On scanning magnification, the lesion appeared well circumscribed but asymmetrical, with both junctional and dermal nests, mimicking a compound nevus. At a higher magnification, these nests comprised atypical cells with nuclear pleomorphisms and prominent nucleoli and extended to the base of the lesion, indicating a lack of maturation. No prominent pagetosis was identified. The tumor had a low mitotic count of <1 mm2. With the exception of mitotic count and degree of cellular maturation, RCM revealed the cellular morphology of this NeM with good correlation to what was observed on histopathology.

Fig 2

Reflectance confocal microcopy mosaic (∼6.5 mm × 8 mm) of nevoid melanoma acquired at the stratum spinosum and dermoepidermal junction showing a slightly papillated, well-circumscribed, asymmetric lesion. The epidermis has a regular honeycomb pattern (green asterisks) without any prominent pagetosis. Multiple large, irregularly sized and shaped nests (red asterisks) are seen at the dermoepidermal junction. The inset in red (∼1 mm × 1 mm) shows the nonhomogenous, dense and sparse nature of these nests. The dotted-green box within the inset shows cytologic atypia (red arrowheads) within the melanocytic nests at a higher magnification. (Original magnifications: ∼×2; red inset, ∼×15.)

Fig 3

Histopathology image of a nevoid melanoma showing a well-circumscribed but asymmetric melanocytic lesion with junctional and dermal nests (red arrowheads). The inset shows cytologic and nuclear atypia in the melanocytic cells. (Hematoxylin-eosin stain; original magnifications: ×4; inset, ×20.)

Discussion

NeM can be diagnostically challenging clinically even after histopathologic examination.1, 2, 3 Therefore, it is not uncommon for these lesions to be diagnosed only after they have metastasized. Dermoscopic features of irregular globules and atypical vascular structures, though helpful for diagnosing melanoma, can also be seen with other entities, such as irritated intradermal nevus and clonal seborrheic keratosis. Although RCM has shown high sensitivity and specificity for diagnosing melanoma, NeM has been reported as a potential diagnostic pitfall of RCM.

This case highlights the importance of integrating information gathered by clinical examination and dermoscopy with the RCM findings to arrive at the correct diagnosis. The lesion had features (visually and on palpation) of an intradermal nevus, but the sharp demarcation and surface topography also raised the slim possibility of an irritated seborrheic keratosis. Dermoscopic features of irregular globules and atypical vascular structures raised concern for melanoma, but the differential diagnosis of an irritated clonal seborrheic keratosis could not be excluded. With the clinical and dermoscopic differential diagnosis of intradermal nevus, melanoma, and clonal seborrheic keratosis, the lesion was evaluated via RCM with the aim of narrowing the differential diagnosis. The lack of RCM features associated with clonal seborrheic keratosis narrowed the differential to intradermal nevus versus melanoma. The presence of cytologic atypia in the dermal nests increased concern for melanoma. On the basis of the clinical, dermoscopic, and RCM morphology, the most probable diagnosis of a nevoid melanoma was deduced. Unlike a previous study, in which 4 NeMs were missed on RCM, our case had cytologic atypia seen on RCM, which raised the suspicion for melanoma. The previous study was a retrospective study in which RCM diagnosis was made without knowledge of clinical history or dermoscopic findings. In the absence of clinical context, subtle RCM cytologic atypia might have been overlooked, potentially leading to the failure to correctly diagnose those 4 melanomas.