Pleomorphic fibroma and dermal atypical lipomatous tumor: are they related?

Pleomorphic fibromas represent dome-shaped or polypoid cutaneous lesions characterized by a paucicellular and densely fibrotic background punctuated by scattered atypical to pleomorphic spindle and multinucleated giant cells. Some of these tumors will have incorporated adipose tissue, although these adipocytic areas lack distinct cytologic atypia and may represent entrapped normal periadnexal or subcutaneous adipose tissue. Nonetheless, owing to the similarity of some of the morphologic features of pleomorphic fibroma with cutaneous atypical lipomatous tumor, diagnostic confusion can ensue. The potential diagnostic challenges are further highlighted by a recent report of a lesion with histopathologic features of both. In response, we studied the presence of 12q15/ MDM2 amplification by fluorescence in situ hybridization and MDM2 expression by immunohistochemistry in a series of 15 pleomorphic fibromas to investigate whether these two entities share a common pathogenic origin. One case of cutaneous atypical lipomatous tumor was used as positive control for 12q15 amplification. All 15 cases were negative for MDM2 by immunohistochemistry with no demonstrable 12q15/MDM2 amplification by fluorescence in situ hybridization. Therefore, these two entities are best regarded as pathogenetically distinct. MDM2 immunohistochemistry or fluorescence in situ hybridization studies can be used to differentiate between the two if needed.