5.1.2　Acral melanoma

Acral melanoma is found in extremities（e.g.，palms，soles，nails）.It is a subtype of melanoma with a high morbidity（about 50％）in China and even in Asia.Therefore，it is particularly important to distinguish between benign and malignant pigmented tumors in extremities.Acral melanoma often manifests as brown spotted rash in the early stage，and it is difficult to differentiate between early acral melanoma and acral melanocytic nevi through naked eyes observation.It is difficult for dermatologists to diagnose when facing acral pigmented skin lesions，and dermoscope is extremely powerful to help dermatologists to improve accuracy rate of diagnosis.Acral melanocytic nevi has three main patterns upon dermoscope：parallel furrow pattern，pigmentation along the furrows of the skin markings（the most common dermoscopic pattern in acral melanocytic nevi）；lattice-like pattern，pigmentation along and across the furrows；fibrillar pattern，fine fibrillar pigmentation perpendicular to the furrows.

There is a significant difference between the features of acral melanoma and acral melanocytic nevi upon dermoscope.Parallel ridge pattern is the most common and specific manifestation of acral melanoma.This pattern is composed of pigmentation located on the ridges of the skin markings.The sensitivity and specificity are as high as 86％and 99％，respectively，which greatly improve the diagnosis accuracy of acral melanoma.Unlike the parallel furrow pattern of acral melanocytic nevi where the pigment is located along the furrows of the skin markings，the parallel ridge pattern of acral melanoma is where the pigment parallel line is located at the ridge.Besides，acral melanoma also has an irregular diffuse pigmentation pattern，which is more common in late skin lesions.The diffuse pigmentation pattern is manifested as a diffuse and unstructured dark brown pigmentation with color gradation.Other manifestations，such as irregular spots or globules，irregular stripes，blue-white veils，ulcer formation，atypical vascular morphology，and even the three common patterns of acral melanocytic nevi can be found in acral melanoma.In the case of melanoma skin lesions located on the extremities，Saida et al.suggested that when facing patients with suspected extremity melanoma in clinical practice，dermoscope should be firstly used to diagnose the parallel ridge pattern，and the positive patients should immediately undergo histopathology.For negative patients，it is necessary to further confirm whether they have a typical acral melanocytic nevi pattern（e.g.，parallel furrow，lattice-like，fibrillar）.If so，long-term clinical follow-up observation should be performed.If not，and the diameter of the skin lesion is larger than 7 mm，a histopathological examination is required.(https://www.daowen.com)