SOX10 expression distinguishes desmoplastic melanoma from its histologic mimics

Desmoplastic melanoma (DM) presents diagnostic challenges due to histologic mimics and limited immunohistochemical staining. Although S100 usually stains DM, other melanoma markers (HMB-45 and Melan-A) are often negative. Dermal/subcutaneous mimics of DM [spindle cell/poorly differentiated squamous cell carcinoma, atypical fibroxanthoma (AFX), and sarcoma] show negative or unreliable immunohistochemical staining. Recently, SOX10 expression has been shown to be a sensitive and specific marker of DM. However, there are no published studies comparing the sensitivity and specificity of SOX10 for DM compared with its most common histologic mimics of the dermis/subcutis. We examined 76 cases, including DM (n = 15), spindle cell/poorly differentiated carcinoma (n = 18), AFX (n = 13), sarcoma with spindled morphology (n = 20), and malignant peripheral nerve sheath tumor (MPNST) (n = 10). Most (75%, 15/20) of sarcomas were centered in the dermis/subcutis and included sarcoma not otherwise specified, DFSP with sarcomatous transformation and myxofibrosarcoma. SOX10 was diffusely positive in 100% (15/15) of DMs and showed focal staining in 30% (3/10) of MPNSTs. All other tumors were negative for SOX10 [0% (0/18) of carcinomas, 0% (0/13) of AFXs, 0% (0/20) of sarcomas]. In conclusion, SOX10 is a highly useful marker to confirm the diagnosis of DM. In our study, SOX10 showed 100% sensitivity for DM and SOX10 was negative in all histologic mimics of the dermis/subcutis, including spindle cell carcinoma, AFX and sarcomas. Similar to S-100 protein, some MPNSTs show scattered positivity but did not show diffuse positivity seen in DM.