Low-grade sinonasal sarcoma with neural and myogenic features--diagnostic challenge and pathogenic insight.

A low-grade sinonasal sarcoma with neural and myogenic features has recently been defined and characterized. We present a case of this morphologic entity and discuss the differential diagnostic considerations, immunophenotypic character, electron microscopy (EM) findings and positron emission tomography (PET) appearance. We propose an alternative hypothesis of its origin on the basis of immunophenotypic and EM features. A 59-year-old Caucasian male was found to have a mass filling the right ethmoid sinus. On PET, the lesion had a maximum standardized uptake value of 2.9, which is of borderline intensity for sarcoma. Pathologic examination showed morphologic features similar to those reported for low-grade sinonasal sarcoma with neural and myogenic features. The tumor was positive for S100, β-catenin, caldesmon, and vimentin and negative for smooth muscle actin, muscle-specific actin, desmin, myogenin, and pankeratin. Positivity for β-catenin raises the alternative possibility of fibroblastic differentiation instead of the proposed myogenic differentiation. EM findings were also consistent with fibroblastic cells.