Procollagen 1 and Melan-A expression in desmoplastic melanomas

The histopathologic diagnosis of desmoplastic melanoma (DM) is usually based on typical conventional microscopic findings coupled with expression of S100 protein by neoplastic cells. Important differential diagnostic considerations include atypical fibroxanthoma (AFX) and spindle cell squamous carcinoma. Spindle cell squamous cell carcinoma is characterized by positivity for cytokeratin, whereas the diagnosis of AFX has been one of exclusion. Procollagen 1 (PC1) has been identified as a relatively sensitive marker of AFX. In this study, we sought to analyze the expression of PC1, S100 protein, and Melan-A in a series of 37 DMs (27 males and 10 females; ages 36-92 years, mean age 74 years). All lesions displayed a spindle cell morphology with varying degrees of desmoplasia. The neoplastic cells avidly expressed S100 protein in 37 of 37 neoplasms. A complete lack of PC1 expression was noted in 24 of 37 (64.9%). There was a weak PC1 expression by 9 DMs (24.3%) and a moderate expression by 4 DMs (10.8%). Melan-A expression was found in 19 of 37 DMs (51.4%), but in 10 lesions, the expression was only faint and focal. We conclude that PC1 labeling of DM is not uncommon but poses little risk for misdiagnosis, provided the stain is performed as part of panel that includes S100 protein. Melan-A offers little for the diagnosis of DM, as less than a quarter of lesions exhibit a strong reaction with this antibody. It is critical to employ a broad panel of antibodies, including S100 protein, Melan-A, cytokeratin, PC1, and others, in the immunohistochemical evaluation of spindle cell neoplasms of the skin.