Reflectance confocal microscopy: a useful and non-invasive tool in the in vivo differentiation of benign pigmented skin lesions from malignant melanoma. Report of a case

The diagnosis of seborrheic keratoses (SK) undergoing regression can be challenging clinically and dermoscopically. We report a case of a SK with a history of change and equivocal dermoscopic features, thereby showing confocal features suggestive of solar lentigo/early SK. The present case emphasizes the potential value of reflectance confocal microscopy (RCM) in the differentiation of malignant from benign pigmented skin lesions.


Report of case
A 67-year-old man was referred to our department because of a pigmented skin lesion that was clinically suspicious for melanoma. The patient presented with several pigmented skin lesions on the back. He reported that they had been present for a few years and that he had noticed a change in color and slow enlargement in one pigmented skin lesion on his right shoulder.
Clinically, there was a light-to-dark brown patch approximately 1.5 cm in diameter. Dermoscopy showed a light-to-dark brown homogenous-reticular pattern with focal graybrown to gray-white areas ( Figure 1A). The clinical and dermoscopic features were most suggestive of a solar lentigo / early seborrheic keratosis (SK) with regression, but a melanoma on sun-damaged skin or a collision of a melanoma with an SK were considered in the differential diagnosis. With reflectance confocal microscopy (RCM), a slightly atypical honeycomb pattern was seen at the superficial epidermal layers. At the basal layer, multiple densely packed edged papillae of different sizes and shapes were observed ( Figure 1B).
The diagnosis of seborrheic keratoses (SK) undergoing regression can be challenging clinically and dermoscopically. We report a case of a SK with a history of change and equivocal dermoscopic features, thereby showing confocal features suggestive of solar lentigo/early SK. The present case emphasizes the potential value of reflectance confocal microscopy (RCM) in the differentiation of malignant from benign pigmented skin lesions. In addition, we observed multiple bright, round to triangular, non-nucleated cells, suggestive of aggregates of melanophages due to partial regression. Confocal criteria suggestive of a melanoma, such as bright, pagetoid nucleated cells, cerebriform cell clusters, or a disarrangement of the DEJ with non-edged papillae were not seen. These findings were in line with the RCM criteria of lichen planus-like keratosis reported by Bassoli et al [11].
In our opinion, the present case demonstrates the potential value of RCM in the diagnosis and differentiation of benign and malignant pigmented skin lesions. RCM may improve the diagnostic accuracy and spare unnecessary excisions of benign pigmented skin lesions with equivocal clinical and dermoscopic features.
The bright cells surrounding the dermal papillae were monomorphous in size and shape ( Figure 1C). In addition, bright branching tubular structures "cords" and bulbous projections were seen at the dermo-epidermal junction (DEJ) ( Figure 1D).
The superficial dermis displayed aggregated bright, round to triangular, non-nucleated cells, suggestive of melanophages ( Figure 1E). The overall RCM impression was of a seborrheic keratosis undergoing regression.
The lesion was surgically removed by shave biopsy due to the history of change and the presence of regression structures together with a pigment network in dermoscopy. Histology showed epidermal hyperplasia characterized by thin strands of basaloid cells, horn cysts and basal hyperpigmentation without increase of melanocytes. In the superficial dermis, a few melanophages and solar elastosis were present ( Figure 1B). The diagnosis of a reticulated seborrheic keratosis was made.

Discussion
Seborrheic keratoses are benign skin neoplasms that can usually be recognized either clinically or dermoscopically [1].
However, seborrheic keratoses displaying regression structures in dermoscopy can occasionally mimic melanomas. In addition, melanomas arising in association with seborrheic keratoses have been described in the literature [2][3][4].
RCM is a novel, non-invasive imaging technique that allows for examination of the epidermis and superficial dermis at a cellular resolution [5]. Confocal criteria for the diagnosis of benign and malignant melanocytic skin lesions have been extensively described in the literature [6][7]; however, there are only a few reports in the literature dealing with confocal features of solar lentigines/seborrheic keratoses [8][9][10].
The main RCM features of solar lentigo/seborrheic keratosis undergoing regression (lichen planus-like keratosis) and their relative frequencies found in the study by Bassoli et al were: (i) typical honeycomb pattern of the spinous layer (78.6%); (ii) elongated cords and/or bulbous projections at the dermalepidermal junction (75%); and (iii) numerous plump-bright cells and/or bright stellate spots in the superficial dermis (92.9%) [11]. These RCM features correlated with the following histopathological findings: (i) spinous-granular layers without significant atypia of keratinocytes; (ii) elongated, bulbous rete ridges; and (iii) dense infiltration of melanophages and lymphocytes in superficial dermis. With the use of these diagnostic criteria 71.4% of LPLK could be classified correctly, while misclassification of any of the skin cancers as lichen planus-like keratosis was avoided.
In our patient, we observed characteristic RCM features of SK, such as densely packed edged papillae, bright branching tubular structures, and bulbous projections at the DEJ.