Desmoplastic nevus of chronically sun-damaged skin: an entity to be distinguished from desmoplastic melanoma.

Desmoplastic (sclerotic) nevus (DSN) can often be difficult to differentiate from desmoplastic melanoma (DM). This can be especially difficult when DSNs occur in a background of heavy solar elastosis. We have observed numerous examples of DSNs occurring in chronically sun-damaged (CSD) skin. In a subset of these cases, we have observed notable pleomorphism and nuclear atypia raising concern for the possibility of DM. In this study, we evaluated the clinical, histopathologic, and immunohistochemical findings in 23 cases of DSN occurring in CSD skin and compared them with 10 cases of DM. DSN on CSD skin is seen in adults (mean, 53.2 years) with a female predominance (70%) and upper (57%) and lower (17%) extremity anatomic locations. Most DSNs present as small flesh-colored macules or papules. Typical histologic features include symmetry, limited junctional growth, presence of a lentiginous component often with focal and limited pagetoid spread (extension across only a few rete ridges), and lack of deep extension. DSN and DM had a statistically significant difference in immunohistochemical staining for Melan-A and p75. Melan-A was positive in 18 of 20 DSNs and only 2 out of 10 DMs, whereas p75 was positive in all DMs (10/10) and was weakly positive in 11 of 20 DSN cases. We believe that our study offers some useful clinical, histologic, and immunohistochemical clues to help differentiate DSNs on CSD skin from DMs.