SATB2 Immunopositivity in Spindle Cell (sarcomatoid) Squamous Cell Carcinoma: A Potential Pitfall in Diagnosis.

Spindle cell squamous cell carcinomas (SpSCC) are aggressive neoplasms constituting 1% of oral cavity tumors. A proportion of SpSCC do not stain with epithelial markers, and frequently express mesenchymal markers, viz. Vimentin, smooth muscle actin, muscle specific actin, S100 and desmin, confounding the diagnosis. Immunoexpression of SATB2, a transcription factor indicating osteoblastic lineage, has not been evaluated in SpSCC previously. We therefore performed SATB2 immunohistochemistry in 15 cases of SpSCCs and scored them with respect to intensity and percentage of tumor cells stained. SATB2 immunopositivity was identified in 9/15 (60%) SpSCCs, with varying intensity and distribution. Eight cases (53.3%) showed nonfocal staining of moderate to strong intensity, and 1 case (6.7%) showed focal weak staining. Of these, 3 cases (3/9; 33.33%) did not stain with epithelial/squamous markers. Thus, a subset of SpSCC demonstrate SATB2 immunopositivity. In oral tumors with bone involvement, SATB2 positivity may lead away from the diagnosis of SpSCC. Knowledge of this aberrant immunostaining is, therefore, extremely relevant to guard against misdiagnosis as osteosarcoma, particularly on biopsies which lack adjacent dysplastic epithelium, in cases which are monophasic spindle cell, and in those that do not show immunopositivity for epithelial/ squamous markers. Our results emphasize that an appropriate panel and not a single immunomarker is required to distinguish SpSCC from mesenchymal tumors including osteosarcoma.