Scattered atypical melanocytes with hyperchromatic nuclei in the nail matrix: diagnostic clue for early subungual melanoma in situ

BACKGROUND: The lack of highly specific clinical and histopathological criteria has contributed to the delay in diagnosis of subungual melanoma in situ in its early stages. METHODS: Eighteen cases of subungual melanoma in situ, the largest series reported to date, were analyzed to characterize the clinical and histopathological findings of early stages of subungual melanoma in situ along with five cases of nail matrix nevus and five cases of subungual lentigo serving as histologic control. RESULTS: Clinically, longitudinal melanonychia was present in all 18 cases of subungual melanoma in situ, consisting of irregular dark brown to black streaks within a brown background with (11 cases) or without Hutchinson's sign. Histopathologically, variable shaped and sized, hyperchromatic nuclei surrounded by retraction artifact were present in all cases. Nine cases showed a significant increase in the number of atypical melanocytes with marked nuclear atypia, while the rest of the cases showed less noticeable changes in nail matrix including lower density of melanocytes and/or mild nuclear atypia. In 15 cases, the nuclear enlargement in some of the melanocytes was greater than two times that of the neighboring matrix cells. In the remaining three cases, the nuclei were enlarged to a much lesser degree. All cases displayed areas of haphazard and uneven distribution of solitary melanocytes and, although not observed in all cases, some degree of pagetoid spread was present in majority of the cases. In contrast, nail matrix nevi showed well-formed nests consisting of relatively monomorphous melanocytes with abundant cytoplasm and subungual lentigos consisted of subtle increase in the number of dendritic melanocytes in solitary units within the lower layers of the nail matrix. CONCLUSION: Increase in the number of scattered atypical melanocytes with large hyperchromatic nuclei in a partial nail matrix may provide a diagnostic clue to subungual melanoma in situ in concert with its clinical suspicion.