Lichenoid dermatoses with pseudomelanocytic nests vs inflamed melanoma in situ: A comparative study

Aims Pseudomelanocytic nests or “pseudonests” arising in lichenoid dermatoses can be a diagnostic pitfall for melanoma in situ (MIS), especially on sun‐damaged skin. We sought to evaluate histopathological features that may be helpful in distinguishing this benign process from inflamed MIS. Methods Ten biopsy specimens containing pseudomelanocytic nests within lichenoid dermatoses and twenty cases of inflamed MIS were retrospectively reviewed. Cases with pseudomelanocytic nests represented either a rash (n = 6) or a discrete non‐melanocytic lesion, such as lichenoid keratosis (n = 4). Results All cases with pseudomelanocytic nests showed nests of microphthalmia‐associated transcription factor‐positive cells at the dermoepidermal junction (DEJ) with interface changes and lichenoid inflammation. Pagetoid scatter, confluence of solitary melanocytes at the DEJ and significant cytologic atypia was not seen in any of these cases. In contrast, all cases of inflamed MIS demonstrated confluence of single melanocytes at the DEJ with cytologic atypia ( P < 0.001) and 18/20 cases showed pagetoid scatter of melanocytes ( P = 0.001). Conclusions Our results show that, of the different histopathological features assessed, confluent growth and pagetoid scatter of atypical melanocytes were seen in most cases of inflamed MIS but were absent in all cases with pseudomelanocytic nests. Therefore, in addition to clinicopathological correlation, these features may be useful in differentiating pseudomelanocytic nests arising in lichenoid dermatoses from inflamed MIS.