Melanosis on the fingers showing a parallel ridge pattern

Abstract

A 40-year-old man presented with a 20×13 mm brown macule on the right index finger pad, which had been noted and had gradually enlarged and darkened over a year. The macule was ill-defined and indistinct. Dermoscopy revealed a parallel ridge pattern with globules of various sizes and densities. The lesion was entirely excised and full-thickness skin grafting was performed. Histopathological examination showed proliferation of melanocytes with minimally atypical nuclei at the basal layer, mainly at the crista profunda intermedia. A new, similar macule appeared on the right middle finger pad 1 month later. As another melanoma seemed to be unlikely considering the short period, the new lesion was monitored without administering treatment. The new lesion spontaneously disappeared after 8 months and reappeared after 10 months. Although the parallel ridge pattern on dermoscopy strongly suggested the possibility of melanoma, benign lesions could also be included in the differential diagnoses. The lesion in the present case was thought to be a reactive lesion, but did not fit the characteristics of any known entity, and remained undiagnosed.[Skin Cancer (Japan) 2018 ; 33 : 61-65]