Early acral melanoma in situ: correlation between the parallel ridge pattern on dermoscopy and microscopic features

In non-white populations, acral skin is the most prevalent site of malignant melanoma. Early melanomas of this anatomic site are often misdiagnosed as melanocytic nevi, which are not uncommon on acral skin. In fact, clinical and/or histopathological features of melanocytic nevi occasionally mimic those of early acral melanoma and vice versa, and thus differentiation of early acral melanoma from melanocytic nevus is sometimes very difficult for clinicians as well as for histopathologists. Our dermoscopic investigation has revealed that the parallel ridge pattern, a band-like pigmentation on the ridges of the skin markings, is highly specific to malignant melanoma in situ on acral volar skin. In the present study, we reviewed 22 acral melanocytic lesions that showed the parallel ridge pattern on dermoscopy but had very subtle clinical and/or histopathological presentations. We diagnosed 20 of them as early melanoma in situ by careful histopathological examination, which revealed histopathological features very similar to those seen in macular portions of overt acral melanoma, but fundamentally different from features found in melanocytic nevi on acral skin. In correspondence with their dermoscopic pattern, in these early lesions of acral melanomas, proliferation of solitary arranged melanocytes was mainly detected in the crista profunda intermedia, the epidermal rete ridge underlying the ridge of the skin marking. The two remaining lesions were diagnosed as possible cases of acquired melanocytic nevus because of the formation of well-demarcated nests of melanocytes in the epidermal rete ridges. We propose that a finding of preferential proliferation of solitary arranged melanocytes in the crista profunda intermedia is an important clue for the histopathological diagnosis of early phases of acral melanoma.