Acral lentiginous melanoma in the Turkish population and a new dermoscopic clue for the diagnosis

Background The incidence of acral lentiginous melanoma (ALM) in the white population is low. Dermoscopy enhances diagnosis of ALM; however, diagnostic accuracy may sometimes be poor due to the considerable proportion of amelanotic ALM variants. Objectives To calculate the proportion of ALM among all melanoma subtypes and to determine the frequency of dermoscopic features of ALM in the Turkish population. Methods Out of 612 melanomas, there were 70 cases of ALM, of which 46 showed sufficient image quality for retrospective study of dermoscopic features. Data from patients and their lesions was classified according to clinical features and histopathologic parameters. The dermoscopic variables evaluated were based on pertinent literature on dermoscopy of acral melanocytic neoplasms. Results The prevalence of ALM among all melanoma subtypes was 11.4%. Parallel-ridge pattern (PRP) was detected in 60.8% of cases and irregular diffuse pigmentation (IDP) in 28.3%. The ALMs were amelanotic in 24%, showing an atypical vascular pattern in all cases; a new dermoscopic pattern, named “vascularized parallel-ridge pattern” (VPRP), was detected in 13% of ALMs. Irregular lines were observed in 81.8% of subungual melanomas and were often associated with a multicolored background. Conclusions ALM has site-specific dermoscopic patterns, with PRP being the most prevalent pattern. The newly described VPRP pattern may be an additional clue for ALM diagnosis, especially in thin amelanotic melanomas.

Due to the low incidence of ALM, clinical experience is limited to specialized centers. Moreover, the atypical clinical presentation and relatively high frequency of amelanotic variants are seen among ALMs. As a result, delayed diagnosis or misdiagnosis may lead to detection of ALM at a more advanced stage with poor prognosis [15].
The aim of the present study was to calculate the proportion of ALM among all melanoma subtypes and to determine the frequency of dermoscopic features of ALM in the Turkish population.

Methods
All melanoma cases between 2005 and 2014 were identified retrospectively from the archive of the Dermato-Oncology Unit, Department of Dermatology of Ege University, Izmir, Turkey. Our institution did not require an Ethics Committee approval for this retrospective study of dermoscopic images.
For all cases included in the study, the diagnosis of ALM was based on the histopathologic report. Images of all cases of ALM were reviewed; we excluded cases that lacked a dermoscopic image of the primary lesion or when image quality was insufficient for evaluation of pattern.

Discussion
In our series, the prevalence of ALM among the melanomas was 11.4%, in contrast to the reported prevalence among Caucasians of 2-8% [1-3]; this slightly higher prevalence may be related to Turkish ethnicity.
The most prevalent dermoscopic pattern in our series was the PRP, seen in 60.8% of ALMs, and in line with previous studies. Saida et al, who first described PRP [25], found this pattern in 86% of cases, with sensitivity and specificity of PRP for ALM diagnosis being 86.4% and 99%, respectively [23].
Likewise, Thomas et al [31] and Argenziano et al [29] have reported that PRP was detectable in 53% and 65.3% of their cases. IDP has been reported as the second most common dermoscopic pattern of ALM; it is more suggestive of invasive melanoma [19,21,23,31]. In our study, IDP was found in 28.3%. The prevalence of IDP reported in the literature is variable: Argenziano et al reported 13.6% [29], Braun et al reported 20.5% [30], Thomas et al 60% [31] and Saida et al 85% [23]. We speculate that IDP is seen in lower frequency in thicker melanomas, as notably 60% of ALMs in our series reached Clark's level IV and V. With regard to other acral patterns, a benign pattern such as PFP can be observed at times [30] but only focally and always together with melanomaspecific criteria. Multicomponent pattern (47.8%) and IFP (34.8%) were the other most prevalent patterns, as expected, in the literature [19]. Additional melanoma-specific criteria, seen in melanomas on non-glabrous skin [32,33]-including abrupt edge, irregular dots and globules (D/G), streaks, and blue-whitish veil (BWV)-were observed in one-third of our series; in addition, the high frequency of ulceration (40%), atypical vessels (>50%), and notably multiple colors (76%) are again attributable to the high proportion of thick melanomas in our series [19].
About one-fifth of cases were nail unit melanomas mainly exhibiting an irregular MSL pattern, with variability in lines, color, thickness, spacing and parallelism, in addition to Hutchinson's sign seen in all cases. Notably, the usual finding of brown background, as previously reported by Thomas et al [31], was replaced by a multicolored background in most of our cases (73%); we find this to be a useful diagnostic clue.
About one-quarter were amelanotic melanomas, in line with the previous reported frequencies by Thomas et al (28%-34%) [31], and Kato and coworkers (19.6%) in the Japanese population [34]. Furthermore, all melanomas exhibited atypical vessels, and in fact, the lack or scarcity of pigmentation  (Figures 7-8). Nail plate dystrophy was seen in 7 cases (64%, Figure 9). The most prevalent dermoscopic feature corresponded to irregular lines (81.8%) and multicolored background (72.7%) ( Table 2).         Melanoma cells, mostly among in situ melanomas, tend to cluster around the crista intermedia, leading to pigmentation on the ridges, seen as PRP on dermoscopy [22]. This may be due to the tendency, shown by melanocytic stem cells generating melanoma, to reside, at the beginning, near the crista intermedia or the eccrine ducts. On the other hand, in deeply invasive melanomas, tumor cells tend to diffusely proliferate with a similar intensity in both crista intermedia and crista limitans [22]. Since vascularization and inflammation is likely to be greater where the neoplastic melanocytes proliferate, we reason that in amelanotic cases, the increased vasculature will be most prominent along the same region, namely, the crista intermedia ( Figure 10). This will manifest under dermoscopy as erythema and dotted vessels on the ridges, along the crista intermedia areas, accounting for the dermoscopic feature described herein as VPRP ( Figure 11). In pigmented ALM, melanin obscures the increased vascularization leading to the PRP pattern. This is best seen in hypopigmented melanomas, whereby PRP in the lightly pigmented and VPRP in the amelanotic areas can both be seen.

Conclusion
In conclusion, we found that ALMs in Turkish patients display similar dermoscopic patterns as reported in other popu- Micro-Hutchinson's sign 0 0