Cutaneous CD34+ spindle cell neoplasms: Histopathologic features distinguish spindle cell lipoma, solitary fibrous tumor, and dermatofibrosarcoma protuberans.

Spindle cell lipoma (SCL), dermatofibrosarcoma protuberans (DFSP), and solitary fibrous tumors (SFT) are cutaneous CD34+ spindle cell tumors that may exhibit histopathologic and immunophenotypic overlap. We sought ways to reliably distinguish among these lesions even in small or superficial biopsies. Ten morphologic characteristics were analyzed in a group of 5 SCLs, 6 cutaneous SFTs, and 12 DFSPs. SFT and DFSP exhibited extensive histopathologic overlap in small or partial biopsies. However, adnexal entrapment, defined as diffuse proliferation of tumor cells around pilosebaceous and eccrine structures with minimal disruption or expansion of the dermis, was a feature seen in 10 of the 12 DFSPs and in none of the SFTs or SCLs. Even when only superficial portions of a lesion were present, this feature was identifiable. Spindle cell lipomas posed little diagnostic difficulty, in part because excisional biopsies were performed in all cases of SCL. The number of samples included in the study is relatively small, in part due to the rarity of cutaneous solitary fibrous tumors. We conclude that careful attention to these histopathologic features enables reliable distinction among these tumors.