A study of CD117 expression in dermatofibrosarcoma protuberans and cellular dermatofibroma

BACKGROUND: Dermatofibrosarcoma protuberans (DFSP) is a relatively uncommon spindle cell tumor of the skin. It is locally aggressive and can be a therapeutic challenge. There are case reports of partial response of DFSP to the tyrosine kinase inhibitor STI571 (Imatinib), despite the reported negativity of the tumor cells for CD117. At least one publication reported focal CD117 positivity of DFSP cells, and we would like to clarify the issue. Cellular dermatofibroma (CDF) can mimic DFSP, but typical cases are easily differentiated from DFSP by their staining pattern for CD34 and factor 13a. We also report our experience with CD117 staining of typical CDFs. METHODS: Thirty-seven cases of clear-cut DFSP and 13 cases of clear-cut CDF were retrieved from the archives of Sunnybrook Health Sciences Center between 2000 and 2005. RESULTS: All DFSPs were CD34 (+), factor 13a (-) and CD117 (-). All CDFs were factor 13a (+), CD34 (-) and CD117 (-). CONCLUSIONS: Our study on a relatively large number of cases confirms the negativity of DFSP and CDF for CD117. Therefore, if adjuvant therapy is attempted with drugs such as STI571 (Imatinib), the eligibility of patients should not be based on immunohistochemical assessment of CD117 expression.