Dermatofibrosarcoma protuberans is CD117 negative even after extreme antigen retrieval conditions

Dermatofibrosarcoma protuberans (DFSP) is a malignant cutaneous tumor with a high rate of recurrence. Nevertheless, it responds to the drug Imatinib. The pathogenic mechanism why this drug works seems to be different to the inhibition of the Kit receptor of tyrosine kinase, since DFSP has repeatedly failed to express CD117 when immunohistochemically tested. Due to previous reports that claimed the presence of CD117 in DFSP, we designed a model of antigen retrieval under extreme conditions with two cases of DFSO from our archives. The slides were kept in the pressure cooker during 15 minutes (pH9). Moreover, the tissue was incubated with the primary antibody for 60 minutes. Three negative controls were also used. Even after the "over-retrieval" process, no convincing positivity was shown, although high background staining notably increased. We also discuss several alternatives that have been given in literature to explain the mechanism of how Imatinib works in DFSP.