Immunophenotyping of dermal spindle cell tumors: diagnostic value of monocyte marker Ki-M1p and histogenetic considerations.

Various studies have reported the utility of anti-CD34 staining in the differential diagnosis of dermal spindle-cell tumors. To investigate whether monoclonal antibody Ki-M1p might add practical diagnostic information, we examined a total of 120 cutaneous spindle cell neoplasms using a panel of markers. Anti-CD34 antibody QBEnd/10 consistently stained dermatofibrosarcomas, Kaposi's sarcomas, neurofibromas, and, to a lesser extent, hemangiopericytomas. A positive reaction was also found in > 18% of the dermatofibromas. Ki-M1p staining showed an intense immunoreaction in all dermatofibromas, whereas no reactivity was observed in dermatofibrosarcomas. In addition, a subset of cells was labeled in atypical fibroxanthomas and Kaposi's sarcomas. Neurofibromas, spindle-cell hemangioendotheliomas, and hemangiopericytomas were negative. Dermatofibrosarcomas and atypical fibroxanthomas also moderately expressed smooth muscle-specific actin. Immunohistochemically, a discrimination between dermatofibrosarcomas and neurofibromas was possible only by means of an antibody against the nerve growth factor receptor. We conclude that the combination of several antibodies, in particular anti-CD34 and Ki-M1p, may improve the accuracy of diagnostic immunohistochemistry in the field of cutaneous spindle cell tumors. We speculate that dermatofibroma is primarily a macrophage-rich inflammatory lesion in which cytokine secretion induces a secondary proliferation of fibroblasts, whereas dermatofibrosarcoma is likely to issue from primitive dermal cells of uncertain origin.