A case of a CD56-expressing ectomesenchymal chondromyxoid tumor of the tongue: potential diagnostic usefulness of commonly available CD56 over CD57.

Ectomesenchymal chondromyxoid tumors (ECTs) are rare. Only approximately 55 cases have been reported in the English literature. Distinguishing ECTs from soft tissue myoepithelioma (STM) is often difficult owing to morphological and immunohistochemical similarities. Here, we present a case of an ECT arising from the anterior dorsum of the tongue in a 24-year-old woman. Grossly, the tumor was soft, had a myxoid appearance, and measured 8×7×7 mm. Microscopically, it was well-demarcated, lacked a fibrous capsule, and predominantly consisted of short, spindle to ovoid cells in a myxoid background. Vimentin, glial fibrillary acidic protein (GFAP), and S-100 protein were strongly positive on immunohistochemical analysis. While CD56 was moderately immunopositive, cytokeratin (AE1/AE3) and alpha-smooth muscle actin (αSMA) showed focal weak positivity. Thus, the immunohistochemical findings suggested a diverse immunophenotype, indicating mesenchymal (vimentin and αSMA positive), neurogenic (S100, GFAP, and CD56 positive), and epithelial differentiation (cytokeratin positive). This reflected the fact that ECTs probably arise from uncommitted ectomesenchymal cells that have the potential for multilineage differentiation. The immunohistochemical staining pattern observed for ECTs slightly differs from that of STMs. Strongly positive staining for GFAP and weakly positive staining for cytokeratin are observed in ECTs, whereas the opposite is typically observed for STMs. These findings indicated that the patterns of expression on immunohistochemistry differ between ECTs and STMs, although inevitably, there was some overlap. Thus, CD56 expression in the case presented here is noteworthy, and it could potentially become an adjunct diagnostic marker for ECT instead of previously used CD57.